Long-distance movement of phosphate starvation-responsive microRNAs in Arabidopsis

Plant microRNAs are small RNAs that are important for genetic regulation of processes such as plant development or environmental responses. Specific microRNAs accumulate in the phloem during phosphate starvation, and may act as long-distance signalling molecules. We performed quantitative PCR on Arabidopsis hypocotyl micrograft tissues of wild-type and hen1-6 mutants to assess the mobility of several phosphate starvation-responsive microRNA species. In addition to the previously confirmed mobile species miR399d, the corresponding microRNA* (miR399d*) was identified for the first time as mobile between shoots and roots. Translocation by phosphate-responsive microRNAs miR827 and miR2111a between shoots and roots during phosphate starvation was evident, while their respective microRNA*s were not mobile. The results suggest that long-distance mobility of microRNA species is selective and can occur without the corresponding duplex strand. Movement of miR399d* and root-localised accumulation of miR2111a* opens the potential for persisting microRNA*s to be mobile and functional in novel pathways during phosphate starvation responses.

Dual intracellular localization and targeting of aminoimidazole ribonucleotide synthetase in cowpea

De novo purine biosynthesis is localized to both mitochondria and plastids isolated from Bradyrhizobium sp.-infected cells of cowpea (Vigna unguiculata L. Walp) nodules, but several of the pathway enzymes, including aminoimidazole ribonucleotide synthetase (AIRS [EC 6.3.3.1], encoded by Vupur5), are encoded by single genes. Immunolocalization confirmed the presence of AIRS protein in both organelles. Enzymatically active AIRS was purified separately from nodule mitochondria and plastids. N-terminal sequencing showed that these two isoforms matched the Vupur5 cDNA sequence but were processed at different sites following import; the mitochondrial isoform was five amino acids longer than the plastid isoform. Electrospray tandem mass spectrometry of a trypsin digest of mitochondrial AIRS identified two internal peptides identical with the amino acid sequence deduced from Vupur5 cDNA. Western blots of proteins from mitochondria and plastids isolated from root tips showed a single AIRS protein present at low levels in both organelles. (35)S-AIRS protein translated from a Vupur5 cDNA was imported into isolated pea (Pisum sativum) leaf chloroplasts in vitro by an ATP-dependent process but not into import-competent mitochondria from several plant and non-plant sources. Components of the mature protein are likely to be important for import because the N-terminal targeting sequence was unable to target green fluorescent protein to either chloroplasts or mitochondria in Arabidopsis leaves. The data confirm localization of the protein translated from the AIRS gene in cowpea to both plastids and mitochondria and that it is cotargeted to both organelles, but the mechanism underlying import into mitochondria has features that are yet to be identified.

The forms and sources of cytokinins in developing white lupine seeds and fruits

A comprehensive range of cytokinins (CK) was identified and quantified by gas chromatography-mass spectrometry in tissues of and in xylem and phloem serving developing white lupine (Lupinus albus) fruits. Analyses were initiated at anthesis and included stages of podset, embryogenesis, and seed filling up to physiological maturation 77 d post anthesis (DPA). In the first 10 DPA, fertilized ovaries destined to set pods accumulated CK. The proportion of cis-CK:trans-CK isomers was initially 10:1 but declined to less than 1:1. In ovaries destined to abort, the ratio of cis-isomers to trans-isomers remained high. During early podset, accumulation of CK (30-40 pmol ovary(-1)) was accounted for by xylem and phloem translocation, both containing more than 90% cis-isomers. During embryogenesis and early seed filling (40-46 DPA), translocation accounted for 1% to 14% of the increases of CK in endosperm (20 nmol fruit(-1)) and seed coat (15 nmol fruit(-1)), indicating synthesis in situ. High CK concentrations in seeds (0.6 micromol g(-1) fresh weight) were transient, declining rapidly to less than 1% of maximum levels by physiological maturity. These data pose new questions about the localization and timing of CK synthesis, the significance of translocation, and the role(s) of CK forms in reproductive development.

A simplified approach for modeling diffusion into cells

Regulation of the intracellular concentration of substrates is essential for the maintenance of a stable cellular environment. Diffusion and reaction processes supply and consume substrates within cells and determine their steady-state concentrations. To realistically represent these processes by computer simulation they must be modeled in three dimensions. Yet three-dimensional models are inherently computing intensive. This study describes a method, which substantially simplifies the modeling of diffusion into a polyhedral body (a cube), that was used as a model representation of a cell. The method is applied to a case study of oxygen diffusion into nitrogen-fixing, rhizobia-infected cells in legume nodules. The method involved generating a one-dimensional representation of the three-dimensional problem to provide a "surface area profile" of three-dimensional diffusion. The one-dimensional models were significantly easier to program, several orders of magnitude faster to solve and in this study were validated by assessing their results against those of comparable three-dimensional models of diffusion into the same body. The results show the one-dimensional method to be a close approximation of a three-dimensional source-sink problem with systematic differences below 10% for fractional oxygenation of leghemoglobin, cell respiration and nitrogenase activity. Larger differences between models (up to 45%) in the predicted average and innermost O(2)concentrations had no effects on the physiological conclusions of the study, but were attributed to the poorer resolution of the three- than the one-dimensional model, and to an inherent simplification in the derivation of the one-dimensional surface area profiles. The one-dimensional modeling approach was found to be a simple, yet powerful tool for the study of diffusion and reaction in biological systems.

The site of oxygen limitation in soybean nodules

In legume nodules the [O2] in the infected cells limits respiration and nitrogenase activity, becoming more severe if nodules are exposed to subambient O2 levels. To identify the site of O2 limitation, adenylate pools were measured in soybean (Glycine max) nodules that were frozen in liquid N2 before being ground, lyophilized, sonicated, and separated on density gradients of nonaqueous solvents (heptane/tetrachloroethylene) to yield fractions enriched in bacteroid or plant components. In nodules maintained in air, the adenylate energy charge (AEC = [ATP + 0.5 ADP]/[ATP + ADP + AMP]) was lower in the plant compartment (0.65 +/- 0.04) than in the bacteroids (0.76 +/- 0.095), but did not change when the nodulated root system was exposed to 10% O2. In contrast, 10% O2 decreased the bacteroid AEC to 0.56 +/- 0.06, leading to the conclusion that they are the primary site of O2 limitation in nodules. To account for the low but unchanged AEC in the plant compartment and for the evidence that mitochondria are localized in O2-enriched microenvironments adjacent to intercellular spaces, we propose that steep adenylate gradients may exist between the site of ATP synthesis (and ADP use) in the mitochondria and the extra-mitochondrial sites of ATP use (and ADP production) throughout the large, infected cells.

AIR synthetase in cowpea nodules: a single gene product targeted to two organelles?

A cDNA (VUpur5) encoding phosphoribosyl aminoimidazole (AIR) synthetase, the fifth enzyme of the de novo purine biosynthesis pathway has been isolated from a cowpea nodule cDNA library. It encodes a 388 amino acid protein with a predicted molecular mass of 40.4 kDa. The deduced amino acid sequence has significant homology with AIR synthetase from other organisms. AIR synthetase is present in both mitochondria and plastids of cowpea nodules. A signal sequence encoded by the VUpur5 cDNA has properties associated with plastid transit sequences but there is no consensus cleavage site as would be expected for a plastid targeted protein. Although the signal sequence does not have the structural features of a mitochondrial targeted protein, it has a mitochondrial cleavage site motif (RX/XS) close to the predicted N-terminus of the mature protein. Southern analysis suggests that AIR synthetase is encoded by a single gene raising questions as to how the product of this gene is targeted to the two organelles. VUpur5 is expressed at much higher levels in nodules compared to other cowpea tissues and the gene is active before nitrogen fixation begins. These results suggest that products of nitrogen fixation do not play a role in the initial induction of gene expression. VUpur5 was expressed in Escherichia coli and the recombinant protein used to raise antibodies. These antibodies recognize two forms of AIR synthetase which differ in molecular size. Both forms are present in mitochondria, although the larger protein is more abundant. Only the smaller protein was detected in plastids.

Application of cytokinins to flowers to increase pod set in Lupinus angustifolius L

Exogenous application of a 2 mol m-3 buffered solution of N6 benzylaminopurine (BAP) to flowers on the main stem inflorescence of Lupinus angustifolius L, cv. Danja profoundly altered reproductive development by reducing post-anthesis abscission of flowers and small pods. The same effect of BAP was recorded for a mutant (abs-) of cv. Danja, in which organ abscission was completely absent, indicating that localized application of cytokinin enhanced reproductive development rather than reduced pedicel abscission per se in the parent line. Application to pedicel and sepals at the open flower stage completely eliminated flower abortion on the main inflorescence, compared with less than 50% pod initiation on untreated inflorescences, more than doubled final pod yield on the main inflorescence and increased the number of mature pods on the whole plant by 33%. A single dose of BAP, to an inflorescence which bore flowers ranging in their stage of development from post-anthesis to immature flower buds, significantly increased the number of pods initiated and at final harvest, measured on a per plant basis. A number of synthetic and naturally occurring cytokinins, including zeatin riboside and dihydrozeatin riboside, were also effective. BAP application induced a longer period of flowering and resulted in a considerably thickened raceme. This was most marked at the distal end which showed enhanced cambial development and secondary vascularization compared with untreated controls. The positive effects of BAP application on pod initiation were not restricted to cv. Danja but were found also for cv. Warrah and cv. Gungurru, both of which have enhanced pod set compared with Danja. Enhanced pod initiation on the main inflorescence generally reduced the number of pods developing on branch inflorescences. Additional application of BAP to flowers on branches, even at the most opportune time and at the most effective site, did not enhance pod initiation and, in some cases, significantly reduced pod set on these branches. The data indicate that it would be very difficult to exploit the positive effect of exogenous cytokinin application on pod set in field crops of lupin. However, selection or genetic engineering of plants with higher levels of endogenous cytokinins in flowers or flower parts at anthesis may provide a means by which to assess the importance of this factor in determining yield stability.

Predominance of flowers and newly formed pods in reproductive abscission of Lupinus angustifolius L

The stage of development at which reproductive organs abscised on the main inflorescence of Lupinus angustifolius L. plants was studied in field plot trials which incorporated site, sowing date, density and cultivar treatments. The average number of pods on the main inflorescence at final harvest ranged over 1.8 to 8.4, the Reproductive Index (number of seeds per gram total above-ground dry weight, excluding seeds) varied from 3.1 to 7.6, and the Harvest Index from 0.17 to 0.50. Four stages at which abscission occurred were identified (flower [ovaries 7-8 mm long], pl [pods 9-10 mm], p2 [pods 11-13 mm], p3 [pods 14-27 mm long]) and used to assess reproductive development on a week by week basis. In all treatments, losses of flowers and pl pods accounted for 92% or more of total reproductive abscission and, of this, the majority was due to flower abscission. The number of abscised pl pods was relatively stable across treatments, but the number of p2 and p3 pods which abscised varied markedly with sowing date, possibly indicating a relationship with changes in environmental conditions. Examination of ovaries following cryosectioning and fluorescence microscopy showed that pollination of ovules occurred with a similarly high frequency in flowers destined to abscise (77%) compared with those destined to form pods (88%). The difference was due to a 50% probability of pollination of the fifth ovule in ovaries of flowers forming pods and only a 25% probability in those which abscised. The data are discussed in relation to the likely physiological basis for the regulation of reproductive abscission in the species.

Adaptation of Nodulated Soybean (Glycine max L. Merr.) to Growth in Rhizospheres Containing Nonambient pO(2)

Nodulated soybean (Glycine max L. Merr. cv White Eye inoculated with Bradyrhizobium japonicum strain CB 1809) plants were cultured in the absence of combined N from 8 to 28 days with their root systems maintained continuously in 1, 2.5, 5, 10, 20, 40, 60, or 80% O(2) (volume/volume) in N(2). Plant dry matter yield was unaffected by partial pressure of oxygen (pO(2)) and N(2) fixation showed a broad plateau of maximum activity from 2.5 to 40 or 60% O(2). Slight inhibition of nitrogenase activity occurred at 1% O(2) and as much as 50% inhibition occurred at 80% O(2). Low pO(2) (less than 10%) decreased nodule mass on plants, but this was compensated for by those nodules having higher specific nitrogenase activities. Synthesis and export of ureides in xylem was maintained at a high level (70-95% of total soluble N in exudate) over the range of pO(2) used. Measurements of nitrogenase (EC 1.7.99.2) activity by acetylene reduction indicated that adaptation of nodules to low pO(2) was largely due to changes in ventilation characteristics and involved increased permeability to gases in those grown in subambient pO(2) and decreased permeability in those from plants cultured with their roots in pO(2) greater than ambient. A range of structural alterations in nodules resulting from low pO(2) were identified. These included increased frequency of lenticels, decreased nodule size, increased volume of cortex relative to the infected central tissue of the nodule, as well as changes in the size and frequency of extracellular voids in all tissues. In nodules grown in air, the inner cortex differentiated a layer of four or five cells which formed a band, 40 to 50 micrometers thick, lacking extracellular voids. This was reduced in nodules grown in low pO(2) comprising one or two cell layers and being 10 to 20 micrometers thick in those from 1% O(2). Long-term adaptation to different external pO(2) involved changes which modify diffusive resistance and are additional to adjustments in the variable diffusion barrier.

Effect of pO(2) on the Formation and Status of Leghemoglobin in Nodules of Cowpea and Soybean

Nodulated cowpea (Vigna unguiculata [L.] Walp. cv Vita 3: Bradyrhizobium strain CB756) and soybean (Glycine max [L.] Merr. cv White Eye: Bradyrhizobium strain CB1809) were grown with their root systems maintained in a flowing gas stream containing a range of pO(2) (1-80%, v/v) in N(2) for up to 28 days after planting. At the extremes of sub- and supra-ambient pO(2), the levels of leghemoglobin (Lb) in nodules were reduced. However, neither the proportional composition of Lb component proteins (eight in soybean, three in cowpea) nor their oxidation state was affected by pO(2). Short-term changes in pO(2) (transferring plants grown with sub- or supra-ambient pO(2) in the rhizosphere to air or vice versa) caused a significant decline in Lb content and, in cowpea but not soybean, where pO(2) was increased, a higher percentage of oxidation of Lb. Combining data on changes in Lb level of cowpea nodules grown in sub-ambient pO(2) with those for their structural adaptation to an under supply of O(2) indicated that, despite the nodules having a lower level of Lb, the amount per infected cell was increased by up to twofold and per bacteroid up to fivefold (in those from 1% O(2)) compared to those grown in air. Progressive decline in pO(2) resulted in a progressive increase on this basis, indicating a close relationship between Lb content and the adaptation of nodule functioning to external O(2) level.

N(2)-Fixation by Freshly Isolated Nostoc from Coralloid Roots of the Cycad Macrozamia riedlei (Fisch. ex Gaud.) Gardn

Nitrogenase (EC 1.7.99.2) activity (acetylene reduction) and nitrogen fixation ((15)N(2) fixation) were measured in cyanobacteria freshly isolated from the coralloid roots of Macrozamia riedlei (Fisch. ex Gaud.) Gardn. Light and gas phase oxygen concentration had marked interactive effects on activity, with higher (up to 100-fold) rates of acetylene reduction and (15)N(2) fixation in light. The relationship between ethylene formation and N(2)-fixation varied in the freshly isolated cyanobacteria from 4 to 7 nanomoles of C(2)H(4) per nanomole (15)N(2). Intact coralloid roots, incubated in darkness and ambient air, showed a value of 4.3. Maximum rates of nitrogenase activity occurred at about 0.6% O(2) in light, while in darkness there was a broad optimum around 5 to 8% O(2). Inhibition of nitrogenase, in light, by pO(2) above 0.6% was irreversible. Measurements of light-dependent O(2) evolution and (14)CO(2) fixation indicated negligible photosynthetic electron transport involving photosystem II and, on the basis of inhibitor studies, the stimulatory effect of light was attributed to cyclic photophos-phorylation. Nitrogenase activity of free-living culture of an isolate from Macrozamia (Nostoc PCC 73102) was only slightly inhibited by O(2) levels above 6% O(2) and the inhibition was reversible. These cells showed rates of light-dependent O(2) evolution and (14)CO(2) fixation which were 100- to 200-fold higher than those by the freshly isolated symbiont. Furthermore, nitrogenase activity was dependent on both photosynthetic electron transport and photophosphorylation. These data indicate that cyanobacteria within cycad coralloid roots are differentiated specifically for symbiotic functioning in a microaerobic environment. Specializations include a high heterocyst frequency, enhanced permeability to O(2), and a direct dependence on the cycad for substrates to support nitrogenase activity.

Cytokinins in the Phloem Sap of White Lupin (Lupinus albus L.)

Cytokinin-like activity in samples of xylem and phloem sap collected from field-grown plants of white lupin (Lupinus albus L.) over a period of 9 to 24 weeks after sowing was measured using the soybean hypocotyl callus bioassay following paper chromatographic separation. The phloem sap was collected from shallow incisions made at the base of the stem, the base of the inflorescence (e.g. stem top), the petioles, and the base and tip of the fruit. Xylem sap was collected as root exudate from the stump of plants severed a few centimeters above ground level. Concentration of cytokinin-like substances was highest in phloem sap collected from the base of the inflorescence and showed an increase over the entire sampling period (from week 10 [61 nanogram zeatin equivalents] to week 24 [407 nanogram zeatin equivalents]). Concentrations in the xylem sap and in the other phloem saps were generally lower. Relatively high concentrations of cytokinin-like substances in petiole phloem sap (70 to 130 nanogram zeatin equivalents per milliliter) coincided in time with high concentrations in sap from the base of the inflorescence (see above). Concentrations in sap (phloem or xylem) from the base of the stem were very much lower. This finding is consistent with movement of cytokinins from leaves into the developing inflorescence and fruit, rather than direct input to the fruit from xylem sap. However, an earlier movement of cytokinins from roots into leaves via the xylem cannot be ruled out. Sap collected at an 18-week harvest was additionally separated by sequential C(18) reversed-phase high performance liquid chromatography --> NH(2) normal phase high performance liquid chromatography, bioassayed, and then analyzed by electron impact gas chromatography-mass spectrometry. Identification of zeatin riboside and dihydrozeatin as two of the major cytokinins in combined sap samples was accomplished by gas chromatography-mass spectrometry-selected ion monitoring.

Morphological and structural adaptation of nodules of cowpea to functioning under sub- and supra-ambient oxygen pressure

Nodules of cowpea (Vigna unguiculata (L.) Walp. cv. Vita 3:Bradyrhizobium CB 756) from 28-d-old plants cultured for 23 d with their root systems maintained in O2 levels from 1 to 80% (v/v, in N2) in the external gas phase showed a range of structural changes which have been interpreted in relation to an over- or under-supply of O2. A response to the partial pressure of O2 in the gas phase (pO2) was noted with respect to nodule size, lenticel development, the relative distributions of cortical and infected central tissue, the differentiation of cortex, especially the inner cortex, the frequency and size of infected and uninfected interstitial cells, the volume of extracellular spaces both in cortex and infected tissue, and in the frequency of bacteroids. As a consequence of these changes the surface area of inner cortex relative to the nitrogenase-containing units of fixing tissue (infected cells or bacteroids) was increased by as much as 20-fold. Effectiveness of bacteroid functioning increased from 0.10 ± 0.02 · 10(-9) μmol acetylene reduced per bacteroid in air-grown nodules to 0.9 ± 0.16 · 10(-9) (same units) per bacteroid in those cultured in 1% O2.

Effect of oxygen pressure on synthesis and export of nitrogenous solutes by nodules of cowpea

Nodules of cowpea plants (Vigna unguiculata (L.) Walp. cv. Vita 3 :Bradyrhizobium CB756) cultured for periods of 23 d with their root systems maintained in atmospheres containing a range of partial pressures of O2 (pO2; 1-80%, v/v, in N2) formed and exported ureides (allantoin and allantoic acid) as the major products of fixation at all pO2 tested. In sub-ambient pO2 (1 and 2.5%) nodules contained specific activities of uricase (urate: O2 oxidoreductase; EC 1.7.3.3) and allantoinase (allantoin hydrolyase; EC 3.5.2.5) as much as sevenfold higher than in those from air. On a cell basis, uninfected cells in nodules from 1% O2 contained around five times the level of uricase. Except for NAD: glutamate synthase (EC 1.4.1.14), which was reduced in sub-ambient O2, the activities of other enzymes of ureide synthesis were relatively unaffected by pO2. Short-term effects of pO2 on assimilation of fixed nitrogen were measured in nodules of air-grown plants exposed to subambient pO2 (1, 2.5 or 5%, v/v in N2) and(15)N2. Despite a fall in total(15)N2 fixation, ureide synthesis and export was maintained at a high level except in 1% O2 where formation was halved. The data indicate that in addition to the structural and diffusional adaptations of cowpea nodules which allow the balance between O2 supply and demand to be maintained over a wide range of pO2, nodules also show evidence of biochemical adaptations which maintain and enhance normal pathways for the assimilation of fixed nitrogen.

Effect of pO(2) on Growth and Nodule Functioning of Symbiotic Cowpea (Vigna unguiculata L. Walp.)

Nodulated cowpea (Vigna unguiculata L. Walp. cv Vita 3:Bradyrhizobium CB 756) plants were cultured with their whole root system or crown root nodulation zone maintained for periods from 5 to 69 days after planting in atmospheres containing a range of pO(2) (1-80%, v/v) while the rest of the plant grew in normal air. Growth (dry matter yield) and N(2) fixation were largely unaffected by pO(2) from 10 to 40%. Decrease in fixation at pO(2) below 5% was due to lower nodulation and nodule mass and, at pO(2) above 60%, to a fall in specific N(2)-fixing activity of nodules. Root:shoot ratios were significantly lower at pO(2) below 2.5%. The effect of pO(2) on nitrogenase activity (acetylene reduction), both of whole nodulated root systems and crown root nodulation zones, varied with plant age but was generally lower at supra- and subambient extremes of O(2). H(2) evolution showed a sharp optimum at 20% O(2) but was at most 4% of total nitrogenase activity. The ratio of CO(2) evolved to substrate (C(2)H(2)+H(+)) reduced by crown root nodulation zones was constant (6 moles CO(2) per mole substrate reduced) from 2.5 to 60% O(2) but at levels below 2.5 and above 80% O(2) reached values between 20 and 30 moles CO(2) per mole substrate reduced. Effects of long-term growth with nonambient pO(2) on adaptation and efficiency of functioning of nodules are discussed.

Effect of pO(2) during Growth on the Gaseous Diffusional Properties of Nodules of Cowpea (Vigna unguiculata L. Walp.)

Adaptations of nodules of cowpea (Vigna unguiculata L. Walp. cv Vita 3: Bradyrhizobium CB 756) to growth in pO(2) ranging from 1 to 80% O(2) (volume/volume) involved both readily reversible mechanisms of adjustment and more stable alterations which together resulted in nodules with widely ranging resistance to diffusion of gases. Those grown in subambient pO(2) (1-5% O(2) were altered such that rapid diffusional adjustment was unable to prevent irreversible loss of nitrogenase on their transfer to higher levels of O(2). Those cultured in 80% had adapted to over-supply of O(2) such that their transfer to lower levels of O(2) limited both nitrogenase and respiratory CO(2) release. There was also some evidence for ;protective respiration.' Measurement of diffusional properties based on gas exchange kinetics indicated that gaseous permeability values for nodules from 5 to 40% O(2) were relatively constant around 20 x 10(-3) millimeters per second, while those for nodules from 1% O(2) were as high as 67.7 x 10(-3) millimeter per second and from 80% as low as 6.8 x 10(-3) millimeters per second. Estimates of the thickness of the diffusion barrier ranged from 7.5 micrometers for nodules from 1% O(2) to 71.9 micrometers in those from 80% O(2).

Nitrogen Nutrition of Nodules in Relation to ;N-Hunger' in Cowpea (Vigna unguiculata L. Walp)

Early growth, nodule development, and nitrogen fixation by two cultivars of cowpea (Vigna unguiculata L. Walp), one large-seeded (Vita 3; 146.0 +/- 0.9 milligrams seed dry weight, 4.1 +/- 0.2 milligrams seed N), the other small-seeded (Caloona; 57.5 +/- 2.5 milligrams seed dry weight, 1.8 +/- 0.1 milligrams seed N), were compared under conditions of sand culture with nutrient solution free of combined N. The seed stocks used had been obtained from plants uniformly labeled with (15)N, thus enabling changes with time in distribution of cotyledon and fixed N among plant parts to be measured by isotope dilution. Caloona, but not Vita 3, showed physiological symptoms of ;N hunger,' i.e. transient loss of chlorophyll (visible yellowing) and N from the first-formed unifoliolate leaves at or around the onset of symbiotic functioning and N(2) fixation. The smaller-seeded Caloona showed higher early nitrogenase activity than the larger-seeded Vita 3 and by 28 days had fixed 6.6 milligrams of N per milligram of seed N [mg N . (mg seed N)(-1)] versus only 3.5 mg N . (mg seed N)(-1) in Vita 3. Both cultivars lost around 30% of their initial seed N at germination, mostly as fallen cotyledons. Abscised cotyledons of Caloona contained 1.21 +/- 0.17% N; those of Vita 3 contained 2.61 +/- 0.37% N. When compared on the basis of cotyledon N available for seedling growth, Caloona was shown to have fixed 10.6 mg N . (mg seed N)(-1) and Vita 3 only 5.3 mg N . (mg seed N)(-1). Most of the cotyledon N withdrawn from the unifoliolate leaf pair of Caloona during ;N-hunger' was committed to early nodule growth and, in total, 20 to 25% of the cotyledon N resource of this cultivar was ultimately invested in establishment of symbiosis compared with only 7% in Vita 3.

Pathways of assimilation and transfer of fixed nitrogen in coralloid roots of cycad-Nostoc symbioses

Freshly detached coralloid roots of several cycad species were found to bleed spontaneously from xylem, permitting identification of products of nitrogen transfer from symbiotic organ to host. Structural features relevant to the export of fixed N were described for Macrozamia riedlei (Fisch. ex Gaud.) Gardn. the principal species studied. Citrulline (Cit), glutamine (Gln) and glutamic acid (Glu), the latter usually in a lesser amount, were the principal translocated solutes in Macrozamia (5 spp.), Encephalartos (4 spp.) and Lepidozamia (1 sp.), while Gln and a smaller amount of Glu, but no Cit were present in xylem sap of Bowenia (1 sp.),and Cycas (2 spp.). Time-course studies of (15)N enrichment of the different tissue zones and the xylem sap of (15)N2-pulse-fed coralloid roots of M. riedlei showed earlier (15)N incorporation into Gln than into Cit, and a subsequent net decline in the (15)N of Gln of the coralloid-root tissues, whereas Cit labeling continued to increase in inner cortex and stele and in the xylem sap. Hydrolysis of the (15)N-labeled Cit and Gln consistently demonstrated much more intense labeling of the respective carbamyl and amide groups than of the other N-atoms. Coralloid roots of M. riedlei pulse-fed (14)CO2 in darkness showed (14)C labeling of aspartic acid (Asp) and Cit in all tissue zones and of Cit of xylem bleeding sap. Lateral roots and uninfected apogeotropic roots of M. riedlei and M. moorei also incorporated (14)CO2 into Cit. The (14)C of Cit was restricted to the carbamyl-C. Comparable (15)N2 and CO2-feeding studies on corallid roots of Cycas revoluta showed Gln to be the dominant product of N2 fixation, with Asp and alanine as other major (14)C-labeled amino compounds, but a total absence of Cit in labeled or unlabeled form.

Inhibition of nodule functioning in cowpea by a xanthine oxidoreductase inhibitor, allopurinol

Allopurinol (1H-pyrazolo-[3,4-d]pyrimidine-4-ol), an inhibitor of xanthine oxidation in ureide-producing nodulated legumes, was taken up from the rooting medium, translocated in xylem, and transferred to nodules of both the ureide-forming cowpea (Vigna unguiculata L. Walp.) and the amide-forming white lupin (Lupinus albus L.). Cowpea suffered severe nitrogen deficiency, extreme chlorosis, and reduced growth, whereas lupin was unaffected by the inhibitor. Similar results were obtained with oxypurinol (1H-pyrazolo-[3,4-d]pyrimidine-4,6-diol). Xylem composition of symbiotic cowpea was markedly changed by allopurinol. Ureides fell to a very low level, but xanthine and, to a lesser extent, hypoxanthine increased markedly. Xylem glutamine was also reduced, but there was little change in other amino acids. Nitrogenase (EC 1.7.99.2) activity of intact nodulated plants or nodulated root segments of plants treated with allopurinol or oxypurinol for 24 hours or more was severely inhibited in cowpea but unaffected in lupin for periods of exposure up to 9 days. Nitrogenase activity of slices of nodules prepared from allopurinol-treated cowpea showed inhibition comparable to that of intact plants. Breis prepared from nodules of treated plants showed no reduction in nitrogenase, nor was there reduction in activity of breis following addition of allopurinol, xanthine, or a range of purine pathway intermediates. Increasing the O(2) concentration in assays above 20% (volume/volume) reversed inhibition of nitrogenase by allopurinol in intact nodulated roots. It was concluded for cowpea that allopurinol not only inhibited ureide synthesis but also caused inhibition of nitrogenase activity, thereby leading to progressive dysfunction and eventual senescence of nodules. The mechanistic relationships between inhibition of ureide biosynthesis, changes in gaseous diffusion resistance, and reduced nitrogenase activity remain obscure.

Pathways of Nitrogen Assimilation in Cowpea Nodules Studied using N(2) and Allopurinol

In the presence of 0.5 millimolar allopurinol (4-hydroxypyrazolo [3,4-d]pyrimidine), an inhibitor of NAD:xanthine oxidoreductase (EC 1.2.3.2), intact attached nodules of cowpea (Vigna unguiculata L. Walp. cv Vita 3) formed [(15)N]xanthine from (15)N(2) at rates equivalent to those of ureide synthesis, confirming the direct assimilation of fixed nitrogen into purines. Xanthine accumulated in nodules and was exported in increasing amounts in xylem of allopurinol-treated plants. Other intermediates of purine oxidation, de novo purine synthesis, and ammonia assimilation did not increase and, over the time course of experiments (4 hours), allopurinol had no effect on nitrogenase (EC 1.7.99.2) activity. Negligible (15)N-labeling of asparagine from (15)N(2) was observed, suggesting that the significant pool (up to 14 micromoles per gram of nodule fresh weight) of this amide in cowpea nodules was not formed directly from fixation but may have accumulated as a consequence of phloem delivery.

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with (15)N(2) gas showed asparagine to be the principal nitrogen product exported from N(2)-fixing nodules. Maintaining root systems in an N(2)-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N(2) fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of (15)N-nitrate, and a (15)N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N(2) or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N(2). Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N(2) and nitrate are being used in peanuts.

The extrafloral nectaries of cowpea (Vigna unguiculata (L.) Walp.) II. Nectar composition, origin of nectar solutes, and nectary functioning

Nectar was collected from the extrafloral nectaries of leaf stipels and inflorescence stalks, and phloem sap from cryopunctured fruits of cowpea plants. Daily sugar losses as nectar were equivalent to only 0.1-2% of the plant's current net photosynthate, and were maximal in the fourth week after anthesis. Sucrose:glucose:fructose weight ratios of nectar varied from 1.5:1:1 to 0.5:1:1, whereas over 95% of phloem-sap sugar was sucrose. [(14)C]Sucrose fed to leaves was translocated as such to nectaries, where it was partly inverted to [(14)C]glucose and [(14)C]fructose prior to or during nectar secretion. Invertase (EC 3.2.1.26) activity was demonstrated for inflorescence-stalk nectar but not stipel nectar. The nectar invertase was largely associated with secretory cells that are extruded into the nectar during nectary functioning, and was active only after osmotic disruption of these cells upon dilution of the nectar. The nectar invertase functioned optimally (phloem-sap sucrose as substrate) at pH 5.5, with a starting sucrose concentration of 15% (w/v). Stipel nectar was much lower in amino compounds relative to sugars (0.08-0.17 mg g(-1) total sugar) than inflorescence nectar (22-30 mg g(-1)) or phloem sap (81-162 mg g(-1)). The two classes of nectar and phloem sap also differed noticeably in their complements of organic acids. Xylem feeding to leaves of a range of (14)C-labelled nitrogenous solutes resulted in these substrates and their metabolic products appearing in fruit-phloem sap and adjacent inflorescence-stalk nectar. (14)C-labelled asparagine, valine and histidine transferred freely into phloem and appeared still largely as such in nectar. (14)C-labelled glycine, serine, arginine and aspartic acid showed limited direct access to phloem and nectar, although labelled metabolic products were transferred and secreted. The ureide allantoin was present in phloem, but absent from both types of nectar. Models of nectary functioning are proposed.

Nitrogen nutrition and metabolic interconversions of nitrogenous solutes in developing cowpea fruits

Budgets for import and utilization of ureide, amides, and a range of amino acids were constructed for the developing first-formed fruit of symbiotically dependent cowpea (Vigna unguiculata [L.] Walp. cv Vita 3). Data on fruit total N economy, and analyses of the xylem and phloem streams serving the fruit, were used to predict the input of various solutes while the compositions of the soluble and protein pools of pod, seed coat, and embryo were used to estimate the net consumption of compounds. Ureides and amides provided virtually all of the fruit's N requirements for net synthesis of amino compounds supplied inadequately from the parent plant. Xylem was the principal source of ureide to the pod, while phloem was the major source of amides to pod and seed. All fruit parts showed in vitro activity of urease (EC 3.5.1.5), allantoinase (EC 3.5.2.5), asparaginase (EC 3.5.11), ammonia-assimilating enzymes and aspartate and alanine aminotransferases (EC 2.61.1 and EC 2.6.1.1.2). Asparagine:pyruvate aminotransferase (EC 2.6.1.14) was recovered only from the pod. The pod was initially the major site for processing and incorporating N; later seed coats and finally embryos became predominant. Ureides were broken down mainly in the pod and seed coat. Amide metabolism occurred in all fruit organs, but principally in the embryo during much of seed growth. Seed coats released N to embryos mainly as histidine, arginine, glutamine, and asparagine, hardly at all as ureide. Amino compounds delivered in noticeably deficient amounts to the fruit were arginine, histidine, glycine, glutamate, and aspartate, while seeds received insufficient arginine, histidine, serine, glycine, and alanine. Quantitatively based schemes are proposed depicting the principal metabolic transformation accompanying N-flow between seed compartments during development.

Purification and properties of inosine monophosphate oxidoreductase from nitrogen-fixing nodules of cowpea (Vigna unguiculata L. Walp)

Using ammonium sulfate precipitation, gel filtration, and affinity chromatography, inosine monophosphate (IMP) oxidoreductase (EC 1.2.1.14) was isolated from the soluble proteins of the plant cell fraction of nitrogen-fixing nodules of cowpea (Vigna unguiculata L. Walp). The enzyme, purified more than 140-fold with a yield of 11%, was stabilized with glycerol and required a sulfydryl-reducing agent for maximum activity. Gel filtration indicated a molecular weight of 200,000, and sodium dodecyl sulfate-gel electrophoresis a single subunit of 50,000 Da. The final specific activity ranged from 1.1 to 1.5 mumol min-1 mg protein-1. The enzyme had an alkaline pH optimum and showed a high affinity for IMP (Km = 9.1 X 10(-6) M at pH 8.8 and NAD levels above 0.25 mM) and NAD (Km = 18-35 X 10(-6) M at pH 8.8). NAD was the preferred coenzyme, with NADP reduction less than 10% of that with NAD, while molecular oxygen did not serve as an electron acceptor. Intermediates of ureide metabolism (allantoin, allantoic acid, uric acid, inosine, xanthosine, and XMP) did not affect the enzyme, while AMP, GMP, and NADH were inhibitors. GMP inhibition was competitive with a Ki = 60 X 10(-6) M. The purified enzyme was activated by K+ (Km = 1.6 X 10(-3) M) but not by NH+4. The K+ activation was competitively inhibited by Mg2+. The significance of the properties of IMP oxidoreductase for regulation of ureide biosynthesis in legume root nodules is discussed.

Diurnal water balance of the cowpea fruit

The vascular network of the cowpea (Vigna unguiculata [L.] Walp.) fruit exhibits the anatomical potential for reversible xylem flow between seeds, pod, and parent plant. Feeding of cut shoots with the apoplast marker acid fuchsin showed that fruits imported regularly via xylem at night, less frequently in early morning, and only rarely in the afternoon. The dye never entered seeds or inner dorsal pod strands connecting directly to seeds. Root feeding (early morning) of intact plants with (32)PO(4) or (3)H(2)O rapidly (20 min) labeled pod walls but not seeds, consistent with uptake through xylem. Weak subsequent (4 hours) labeling of seeds suggested slow secondary exchange of label with the phloem stream to the fruit. Vein flap feeding of subtending leaves with [(14)C]sucrose, (3)H(2)O, and (32)PO(4) labeled pod and seed intensely, indicating mass flow in phloem to the fruit. Over 90% of the (14)C and (3)H of fruit cryopuncture phloem sap was as sucrose and water, respectively. Specific (3)H activities of transpired water collected from fruits and peduncles were assayed over 4 days after feeding (3)H(2)O to roots, via leaf flaps, or directly to fruits. The data indicated that fruits transpired relatively less xylem-derived (apoplastic) water than did peduncles, that fruit and peduncle relied more heavily on phloem-derived (symplastic) water for transpiration in the day than at night, and that water diffusing back from the fruit was utilized in peduncle transpiration, especially during the day. The data collectively support the hypothesis of a diurnally reversing xylem flow between developing fruit and plant.

Economy of water, carbon, and nitrogen in the developing cowpea fruit

The nutritional economy of the fruit of cowpea (Vigna unguiculata (L.) Walp cv Vita 3) was assessed quantitatively from intake and utilization of carbon, nitrogen, and water. Fruits failed to make net gains of CO(2) from the atmosphere during daytime, although pod photosynthesis did play a role in the fruit's carbon economy by refixing a proportion of the fruit's respired CO(2). Of every 100 units by weight of carbon entering the fruit, 70.4 were finally incorporated into seeds, 10.3 remained as nonmobilizable material in pod walls, and the remaining 19.3 were lost in fruit respiration. Phloem supplied 97% of the fruit's carbon and 72% of its nitrogen. The xylem contribution of nitrogen occurred mainly in early growth. Ninety-six% of the fruit's nitrogen was incorporated into seeds, approximately 10% of this mobilized from the senescing pod. The mean transpiration ratio of the fruit was very low-8 milliliters water transpired per gram dry matter accumulated. Models of carbon, nitrogen, and water flow were constructed for the two consecutive 11 day periods of fruit development, and indicated a considerably greater entry of water through xylem and phloem than could be accounted for in changes in fruit tissue water and transpiration loss. This discrepancy was greater in the second half of fruit growth and was interpreted as evidence that a significant fraction of the water entering the fruit through phloem cycled back to the parent plant via the xylem.

Effects of short-term n(2) deficiency on N metabolism in legume nodules

The study aimed to test the hypothesis that ammonia production by Rhizobium bacteroids provides not only a source of nitrogen for growth but has a central regulatory role in maintaining the metabolic activity and functional integrity of the legume nodule. Production of ammonia in intact, attached nodules was interrupted by short-term (up to 3 days) exposure of the nodulated root systems of cowpea (Vigna unguiculata L. Walp cv Vita 3: Rhizobium CB 756) and lupin (Lupinus albus L. cv Ultra: Rhizobium WU 425) to atmospheres of argon:oxygen (80:20; v/v). Treatment did not affect nodule growth, levels of plant cell and bacteroid protein, leghaemoglobin content, or nitrogenase (EC 1.7.99.2) activity (acetylene reduction) but severely reduced (by 90%) synthesis and export of the major nitrogenous solutes produced by the two symbioses (ureides in cowpea, amides in lupin). Glutamine synthetase (EC 6.3.1.2) and NAD:glutamate oxidoreductase (EC I.4.1.2) were more or less stable to Ar:O(2) treatment, but activities of the glutamine-utilizing enzymes, glutamate synthase (EC 2.6.1.53), asparagine synthetase (EC 6.3.5.4) (lupin only), and de novo purine synthesis (cowpea only), were all markedly reduced. Production and export of nitrogenous solutes by both symbioses resumed within 2 hours after transferring Ar:O(2)-treated plants back to air. In each case the major exported product of fixation after transfer was initially glutamine, reflecting the relative stability of glutamine synthetase activity. Subsequently, glutamine declined and products of its assimilation became predominant consistent with resurgence of enzymes for the synthesis of asparagine in lupin and ureides in cowpea. Enzymes not directly involved with either ammonia or glutamine assimilation (purine synthesis, purine oxidation, and carbon metabolism of both bacteroids and plant cells) also showed transient changes in activity following interruption of N(2) supply. These data have been interpreted to indicate a far-reaching effect of the production of ammonia by bacteroids on a wide range of enzymes, possibly through control of protein turnover, rather than a highly specific effect of ammonia, or some product of its assimilation, on a few enzyme species.

Nitrogen nutrition and the development of biochemical functions associated with nitrogen fixation and ammonia assimilation of nodules on cowpea seedlings

During early development (up to 18 d after sowing) of nodules of an "effective" cowpea symbiosis (Vigna unguiculata (L.) Walp cv. Vita 3: Rhizobium strain CB756), rapidly increasing nitrogenase (EC 1.7.99.2) activity and leghaemoglobin content were accompanied by rapid increases in activities of glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 2.6.1.53), enzymes of denovo purine synthesis (forming inosine monophosphate) xanthine oxidoreductase (EC 1.2.3.2), urate oxidase (EC 1.7.3.3), phosphoenolpyruvate carboxylase (EC 4.1.1.31) and led to increased export of ureides (allantoin and allantoic acid) to the shoot of the host plant in the xylem. Culturing plants with the nodulated root systems maintained in the absence of N2 (in 80 Ar: 20 O2, v/v) had little effect on the rates of induction and increase in nitrogenase activity and leghaemoglobin content but, in the absence of N2 fixation and consequent ammonia production by bacteroids, there was no stimulation of activity of enzymes of ammonia assimilation or of the synthesis of purines or ureides. Addition of NO 3 (-) (0.1-0.2 mM) relieved host-plant nitrogen deficiency caused by the Ar: O2 treatment but failed to increase levels of enzymes of N metabolism in either the bacteroid or the plant-cell fractions of the nodule. Premature senescence in Ar: O2-grown nodules occurred at 18-20 d after sowing, and resulted in reduced levels of nitrogenase activity and leghaemoglobin but increased the activity of hydroxybutyrate oxidoreductase (EC 1.1.1.30).

Nitrogen nutrition and the development and senescence of nodules on cowpea seedlings

Cowpea (Vigna unguiculata (L.) Walp cv. Vita 3) seedlings inoculated with Rhizobium strain CB756 were cultured with their root systems maintained in air or in Ar: O2 (80:20, v/v) during early nodule development (up to 24 d after sowing). Compared with those in air, seedlings in Ar:O2 showed progressive N deficiency with inhibited shoot growth, reduced ribulose-1,5-bisphosphate carboxylase and total protein levels and loss of chlorophyll in the leaves. Nodule initiation, differentiation of infected and uninfected nodule tissues and the ultrastructure of bacteriod-containing cells were similar in the air and Ar: O2 treatments up to 16 d after sowing. Thereafter the Ar: O2 treatment caused cessation of growth and development of nodules, reduced protein levels in bacteroids and nodule plant cells, and progressive degeneration of nodule ultrastructure leading to premature senescence of these organs. Provision of NO 3 (-) (0.1-0.2 mM) to Ar: O2-grown seedlings overcame the abovementioned consequences of N2 deficiency on nodule and plant growth, but merely delayed the degenerative effects of Ar: O2 treatment on nodule structure and senescence. Treatment of Ar: O2-grown seedlings with NO 3 (-) greatly increased the protein level of nodules but the increase was largely restricted to the plant cell fraction as opposed to the bacteroids. By contrast, NO 3 (-) treatment of air-grown seedlings increased protein of bacteroid and host nodule fractions to the same relative extents when compared with air-grown plants not supplemented with NO 3 (-) . These findings, taken together with studies of the distribution of N in nodules of symbiotically effective plants grown from (15)N-labeled seed, indicate that direct incorporation of fixation products by bacteroids may be a critical feature in the establishment and continued growth of an effective symbiosis in the cowpea seedling.

Effects of n(2) deficiency on transport and partitioning of C and N in a nodulated legume

Nodulated root systems of white lupin (Lupinus albus L. cv Ultra: Rhizobium strain WU425) were exposed to Ar:O(2) (80:20, v/v) or Ar:N(2):O(2) (70:10:20, v/v/v) and C and N partitioning were examined over a 9- or 10-day period in comparison with control plants with nodulated roots retained in air. Accumulation of N ceased in plants exposed to Ar:O(2) or was much reduced in plants exposed to Ar:N(2):O(2), but net C assimilation rates and profiles of C utilization remained similar to those of control N(2)-fixing plants. There was, however, a proportional reduction in CO(2) evolution from nodulated roots of the Ar:O(2) treatment. Xylem N levels fell rapidly after application of Ar:O(2). C:N ratios of phloem sap of petioles and of stem base rose during the first day of Ar:O(2) treatment and then fell progressively back to levels close to that of control plants as leaf reserves of N became available for loading of phloem. Stem top phloem sap increased progressively in C:N ratio throughout Ar:O(2) treatment, presumably due to increasing shortage of xylem derived N for xylem to phloem exchange. Reexposure of Ar:O(2)-treated nodulated root systems to air prompted a rapid recovery of N(2) fixation and restoration of plant N status. Rates of N(2) fixation in plants whose roots were exposed to a range of N(2) concentrations indicated an apparent K(m) of 10% N(2) for the attached intact white lupin nodule.

Spontaneous Phloem bleeding from cryopunctured fruits of a ureide-producing legume

The vasculature of the dorsal suture of cowpea (Vigna unguiculata [L.] Walp) fruits bled a sugar-rich exudate when punctured with a fine needle previously cooled in liquid N(2). Bleeding continued for many days at rates equivalent to 10% of the estimated current sugar intake of the fruit. A phloem origin for the exudate was suggested from its high levels (0.4-0.8 millimoles per milliliter) of sugar (98% of this as sucrose) and its high K(+) content and high ratio of Mg(2+) to Ca(2+). Fruit cryopuncture sap became labeled with (14)C following feeding of [(14)C]urea to leaves or adjacent walls of the fruit, of (14)CO(2) to the pod gas space, and of [(14)C] asparagine or [(14)C]allantoin to leaflets or cut shoots through the xylem. Rates of translocation of (14)C-assimilates from a fed leaf to the puncture site on a subtended fruit were 21 to 38 centimeters per hour. Analysis of (14)C distribution in phloem sap suggested that [(14)C]allantoin was metabolized to a greater extent in its passage to the fruit than was [(14)C] asparagine. Amino acid:ureide:nitrate ratios (nitrogen weight basis) of NO(3)-fed, non-nodulated plants were 20:2:78 in root bleeding xylem sap versus 90:10:0.1 for fruit phloem sap, suggesting that the shoot utilized NO(3)-nitrogen to synthesize amino acids prior to phloem transfer of nitrogen to the fruit. Feeding of (15)NO(3) to roots substantiated this conclusion. The amino acid:ureide ratio (nitrogen weight basis) of root xylem sap of symbiotic plants was 23:77 versus 89:11 for corresponding fruit phloem sap indicating intense metabolic transfer of ureide-nitrogen to amino acids by vegetative parts of the plant.

Effect of temperature on nitrogenase functioning in cowpea nodules

Nitrogenase (EC 1.7.99.2) activity of a cowpea (Vigna unguiculata (L.) Walp cv Caloona) symbiosis formed with a Rhizobium strain (176A27) lacking uptake hydrogenase and maintained under conditions of a 12-hour day at an air temperature of 30 degrees C (800-1000 microeinsteins per square meter per second) and a 12-hour night at an air temperature of 20 degrees C showed a marked diurnal variation in ratio of nitrogen fixed to hydrogen evolved. As little as 0.3 micromole nitrogen was fixed per micromole hydrogen evolved in the photoperiod versus up to 0.6 in the dark period. In plants maintained under the same diurnal illumination regime but at constant (day and night) air temperature (30 degrees C), this difference was abolished and a relatively constant ratio of nitrogen fixed to hydrogen evolved (around 0.3 micromole per micromole) was observed day and night. Exposure of nodulated roots to a range of temperatures maintained for 2 hours in a single photoperiod indicated that, whereas hydrogen evolution increased with increasing temperature from 15 degrees C to a maximum around 35 degrees C, nitrogen fixation was largely unaffected over this temperature range. Both functions of the enzyme declined sharply at temperatures above 38 degrees C. A similar general response of nitrogen fixation to root temperature was observed in glasshouse-grown, sand-cultured plants maintained under a range of temperatures (from 15 to 35 degrees C) for a 14-day period in mid vegetative growth. The effect of temperature on the proportion of electrons allocated to proton reduction compared with nitrogen reduction showed a linearly increasing relationship (correlation coefficient = 0.96) between 15 degrees C and 47 degrees C.

Role of Inosine Monophosphate Oxidoreductase in the Formation of Ureides in Nitrogen-Fixing Nodules of Cowpea (Vigna unguiculata L. Walp.)

Cell-free extracts from nodules of cowpea (Vigna unguiculata L. (Walp.) cv Caloona:Rhizobium strain CB756) prepared in the presence of 15% (v/v) glycerol showed high rates (30 to 60 nanomoles NAD reduced per minute per gram fresh weight nodule) of inosine monophosphate oxidoreductase (EC 1.2.1.14) activity. The enzyme was labile (half-life of activity less than 3 hours) but could be stabilized for up to 18 hours by inclusion of the substrates NAD and inosine monophosphate in the breaking media. Activity showed a broad pH optimum between 8.5 and 9.5, had an apparent K(m) (inosine monophosphate) of 4 and 12 micromolar at pH 7.5 and 9.0, respectively, and was largely (96%) associated with the plant cell cytosol fraction of the nodule.Metabolism of [8-(14)C]inosine monophosphate and [1-(14)C]glycine by the cell-free system showed two pathways for purine base production from inosine monophosphate, one via xanthosine monophosphate, xanthosine, and xanthine, the other via inosine and hypoxanthine. The proportion of inosine monophosphate utilized by inosine monophosphate oxidoreductase and the xanthine-based pathway was increased from 30% at 0.5 millimolar to 80% at 0.01 millimolar inosine monophosphate. The data are interpreted to indicate that in vivo inosine monophosphate oxidation rather than dephosphorylation is the predominant metabolic route leading to ureide synthesis and that inosine monophosphate provides the link between de novo purine nucleotide synthesis in the plastid and ureide production in the plant cell cytosol.

Cellular and subcellular organization of pathways of ammonia assimilation and ureide synthesis in nodules of cowpea (Vigna unguiculata L. Walp.)

Fractionation of cell organelles of nitrogen-fixing nodules of cowpea (Vigna unguiculata L. Walp) by discontinuous and continuous sucrose density centrifugation indicated that starch-containing plastids possessed the complete pathway for purine nucleotide synthesis together with significant activities of some other enzymes associated with the provision of substrates in purine synthesis; triosephosphate isomerase (EC 5.3.1.1), NADH-glutamate synthase (EC 2.6.1.53), aspartate aminotransferase (EC 2.6.1.1), phosphoglycerate oxidoreductase (EC 1.1.1.95), and methylene tetrahydrofolate oxidoreductase (EC 1.5.1.5). Enzymes of purine oxidation, xanthine oxidoreductase (EC 1.2.3.2), and urate oxidase (EC 1.7.3.3) were recovered in the soluble fraction; glutamine synthetase (EC 6.3.1.2) occurred in bacteroids and in the cytosol. Intact, infected (bacteroid-containing) and uninfected cells were prepared by enzymatic maceration of the central zone of the nodule and partially separated by centrifugation on discontinuous sucrose gradients. Glutamine synthetase was largely restricted to infected cells whereas plastid enzymes, de novo purine synthesis, and urate oxidase were present in both cell types. Although the levels of all enzymes assayed were higher in infected cells, both cell types possessed the necessary enzyme complement for ureide formation. A model for the cellular and subcellular organization of nitrogen metabolism and the transport of nitrogenous solutes in cowpea nodules is proposed.

Diurnal variation in the functioning of cowpea nodules

Nitrogenase (EC 1.7.99.2) activity of nodules of cowpea (Vigna unguiculata [L.] Walp), maintained under conditions of a 12-hour day at 30 degrees C and 800 to 1,000 microeinsteins per square meter per second (photosynthetically active radiation) and a 12-hour night at 20 degrees C, showed a marked diurnal variation with the total electron flux through the enzyme at night being 60% of that in the photoperiod. This diurnal pattern was, however, due to changes in hydrogen evolution. The rate of nitrogen fixation, measured by short-term (15)N(2) assimilation or estimated from the difference in hydrogen evolution in air or Ar:O(2) (80:20; v/v), showed no diurnal variation. Carbon dioxide released from nodules showed a diurnal variation synchronized with that of nitrogenase functioning and, as a consequence, the apparent ;respiratory cost' of nitrogen fixation in the photoperiod was almost double that at night (9.74 +/- 0.38 versus 5.70 +/- 0.90 moles CO(2) evolved per mole N(2) fixed). Separate carbon and nitrogen balances constructed for nodules during the photoperiod and dark period showed that, at night, nodule functioning required up to 40% less carbohydrate to achieve the same level of nitrogen fixation as during the photoperiod (2.4 versus 1.4 moles hexose per mole N(2) fixed).Stored reserves of nonstructural carbohydrate of the nodule only partly satisfied the requirement for carbon at night, and fixation was dependent on continued import of translocated assimilates at all times. Measurements of the soluble nitrogen pools of the nodule together with (15)N studies indicated that, both during the day and night, nitrogenous products of fixation were effectively translocated to all organs of the host plant despite low rates of transpiration at night. Reduced fluxes of water through the plant at night were apparently counteracted by increased concentration of nitrogen, especially as ureides, in the xylem stream.

Xylem and Phloem transport and the functional economy of carbon and nitrogen of a legume leaf

Exchanges of CO(2) and changes in content of C and N were studied over the life of a leaf of Lupinus albus L. These data were combined with measurements of C:N weight ratios of xylem (upper stem tracheal) and phloem (petiole) sap to determine net fluxes of C and N between leaf and plant. Phase 1 of leaf development (first 11 days, leaf to one-third area) showed increasing net import of C and N, with phloem contributing 61% of the imported C and 18% of the N. (14)C feeding studies suggested the potential for simultaneous import and export through phloem over the period 9 to 12 days. Phase 2 (11-20 days, leaf attaining maximum area and net photosynthesis rate) exhibited net import through xylem and increasing export through phloem. Eighty-two% of xylem-delivered N was consumed in leaf growth, the remainder exported in phloem. Phase 3 (20-38 days) showed high but declining rates of photosynthesis, translocation, and net export of N. Phase 4 (38-66 days) exhibited substantial losses of N and declining photosynthesis and translocation of C. C:N ratio of xylem sap remained constant (2.3-2.6) during leaf life; petiole phloem sap C:N ratio varied from 25 to 135 over leaf development. The relationships between net photosynthesis and N import in xylem were: phase 1, 4.8 milligrams C per milligram N; phase 2, 24.7 milligrams C per milligram N; phase 3, 91.9 milligrams C per milligram N; and phase 4, 47.7 milligrams C per milligram N.

Amino Acid transport and metabolism in relation to the nitrogen economy of a legume leaf

Net balances of amino acids were constructed for stages of development of a leaf of white lupin (Lupinus albus L.) using data on the N economy of the leaf, its exchanges of amino acids through xylem and phloem, and net changes in its soluble and protein-bound amino acids. Asparagine, aspartate, and gamma-aminobutyrate were delivered to the leaf in excess of amounts consumed in growth and/or phloem export. Glutamine was supplied in excess until full leaf expansion (20 days) but was later synthesized in large amounts in association with mobilization of N from the leaf. Net requirements for glutamate, threonine, serine, proline, glycine, alanine, valine, isoleucine, leucine, tyrosine, phenylalanine, histidine, lysine, and arginine were met mainly or entirely by synthesis within the leaf. Amides furnished the bulk of the N for amino acid synthesis, asparagine providing from 24 to 68%. In vitro activity of asparaginase (EC 3.5.1.1) exceeded that of asparagine:pyruvate aminotransferase (EC 2.6.1.14) during early leaf expansion, when in vivo estimates of asparagine metabolism were highest. Thereafter, aminotransferase activity greatly exceeded that of asparaginase. Rates of activity of one or both asparagine-utilizing enzymes exceeded estimated rates of asparagine catabolism throughout leaf development. In vitro activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1) were consistently much higher than that of glutamate dehydrogenase (EC 1.4.1.3), and activities of the former two enzymes more than accounted for estimated rates of ammonia release in photorespiration and deamidation of asparagine.

Significance of hydrogen evolution in the carbon and nitrogen economy of nodulated cowpea

The carbon and nitrogen economies of a single cultivar of cowpea (Vigna unguiculata (L.) Walp.cv Caloona) nodulated with either a high H(2)-evolving strain (176A27) or a low H(2)-evolving strain (CB756) of Rhizobium were compared. The two symbioses did not differ in total dry matter production, seed yield, nitrogen fixed, the spectrum of nitrogenous solutes produced by nodules for export, or the partitioning of net photosynthate within the plant throughout the growth cycle. Detailed examination of the carbon and nitrogen economy of the nodules, however, showed a significant difference between the symbioses. Nodules formed with CB756 lost less CO(2) in respiration compared to the higher H(2)-evolving symbioses and this could have been largely responsible for a 36% better economy of carbon use in CB756 nodules during the period of maximum H(2) evolution (48-76 days) and over the whole growth period (20-90 days), a 16% economy. In terms of overall net photosynthate generated by the plant, these economies were equivalent to 5% and 2% of the carbon utilized in the two periods, respectively. From the differences in H(2) evolution and CO(2) production by nodules of the two symbioses, the cost of H(2) evolution was found to be 3.83+/-0.6 millimoles CO(2)/millimoles H(2) for plants grown in sand culture and 1.69 +/- 0.48 millimoles CO(2)/millimoles H(2) for those in water culture. In both symbioses, the ratio of H(2) evolution to N(2) fixed varied markedly during ontogeny, indicating a significant variation in the relative efficiency and thus metabolic cost of N(2) fixation at different stages during development.

Metabolism and translocation of allantoin in ureide-producing grain legumes

Transfer of the nitrogen and carbon of allantoin to amino acids and protein of leaflets, stems and petioles, apices, peduncles, pods, and seeds of detached shoots of nodulated cowpea (Vigna unguiculata L. Walp. cv. Caloona) plants was demonstrated following supply of [2-(14)C], [1,3-(15)N]allantoin in the transpiration stream. Throughout vegetative and reproductive growth all plant organs showed significant ureolytic activity and readily metabolized [2-(14)C]allantoin to (14)CO(2). A metabolic pathway for ureide nitrogen utilization via allantoic acid, urea, and ammonia was indicated. Levels of ureolytic activity in extracts from leaves and roots of nodulated cowpea were consistently maintained at higher levels than in non-nodulated, NO(3) (-) grown plants.[(14)C]Ureides were recovered in extracts of aphids (Aphis craccivora and Macrosiphum euphorbieae) feeding at different sites on cowpea plants supplied with [2-(14)C]allantoin through the transpiration stream or to the upper surface of single leaflets. The data indicated that the ureides were effectively transferred from xylem or leaf mesophyll to phloem, and then translocated in phloem to fruits, apices, and roots.

De Novo Purine Synthesis in Nitrogen-Fixing Nodules of Cowpea (Vigna unguiculata [L.] Walp.) and Soybean (Glycine max [L.] Merr.)

Partially purified, cell-free extracts from nodules of cowpea (Vigna unguiculata L. Walp. cv. Caloona) and soybean (Glycine max L. Merr. cv. Bragg) showed high rates of de novo purine nucleotide and purine base synthesis. Activity increased with rates of nitrogen fixation and ureide export during development of cowpea plants; maximum rates (equivalent to 1.2 micromoles N(2) per hour per gram fresh nodule) being similar to those of maximum nitrogen fixation (1-2 micromoles N(2) per hour per gram fresh nodule). Extracts from actively fixing nodules of a symbiosis not producing ureides, Lupinus albus L. cv. Ultra, showed rates of de novo purine synthesis 0.1% to 0.5% those of cowpea and soybean. Most (70-90%) of the activity was associated with the particulate components of the nodule, but up to 50% was released from this fraction by osmotic shock. The accumulated end products with particulate fractions were inosine monophosphate and aminoimidazole carboxamide ribonucleotide. Further metabolism to purine bases and ureides was restricted to the soluble fraction of the nodule extract. High rates of inosine monophosphate synthesis were supported by glutamine as amide donor, lower rates (10-20%) by ammonia, and negligible rates with asparagine as substrate.

Purification and some properties of urate oxidase from nitrogen-fixing nodules of cowpea

Urate oxidase (urate: oxygen oxidoreductase, EC 1.7.3.3) was purified 166-fold from nitrogen-fixing root nodules of cowpea Vigna unguiculata [L.] Walp. The purified enzyme showed a specific activity of 5.7 mumol urate oxidised/min per mg protein, a molecular mass of 100 kdaltons, pH optimum between 9 and 10, isoelectric point at PH 6.8, Km(urate) = 18 muM and Km(oxygen) = 29 muM. A number of metal complexing and chelating reagents were inhibitory, as were divalent cations, including Cu2+. Iron stimulated the enzyme. Low concentrations of ammonia, glutamine and xanthine were also inhibitory. The regulation of urate oxidase in relation to the assimilation of fixed nitrogen in legume nodules is discussed.

Partitioning of carbon and nitrogen and the nutrition of root and shoot apex in a nodulated legume

Empirically based models depicting exchanges of C, N, and H(2)O in phloem and xylem among organs of nodulated white lupin (Lupinus albus cv Ultra) were constructed for the interval 51 to 58 days after sowing. Information was incorporated on the economy of C, N, and H(2)O in plant parts, the solute composition of transport fluids collected at selected sites on the plant, and the photosynthetic inputs, transpirational losses, and translocatory activities of different age groups of leaflets and stem + petiole segments of the shoot. Partitioning of C and N showed preferential transfer of N to the shoot apex, which imported 13 milligrams C per milligram N, compared with 54 milligrams C per milligram N for the nodulated root. Leaves translocated assimilates at a C:N weight ratio of 43 to 59, and older leaves serving the roots produced the translocate most rich in N relative to C. The shoot apex was enriched with N, additional to its intake from leaves, by direct uptake of xylem fluid (C:N ratio, 2.4) and receipt of nitrogenous solutes transferred from xylem to upward-moving phloem streams in upper regions of the stem. The models for flow of N and H(2)O indicated that xylem streams passing to leaves were substantially less rich in N than the adjacent stream moving through the body of the stem and that a progressive increase in concentration of N occurred within stem xylem elements from base to top of the shoot. This apparently resulted from an abstraction of N from xylem of departing leaf traces, possibly by xylem transfer cells, and a subsequent feedback of this N to xylem streams passing on up the shoot. Upper leaves and shoot apex, therefore, acquired more N from xylem per unit of H(2)O transpired than lower parts of the shoot.

Synthesis, Storage, and Utilization of Amino Compounds in White Lupin (Lupinus albus L.)

Changes in total N and in free amino compounds were followed during growth of nodulated white lupin. Leaflets contained the greatest fraction of plant N but had lower proportions (1 to 4%) of their N in soluble amino form than stem + petioles (10 to 27%) and reproductive parts (15 to 33%). Mobilization of free amino compounds from plant parts to fruits contributed at most only 7% of the total N intake of fruits, compared with 50% in mobilization of other forms of N and 43% from fixation during fruiting. Asparagine was usually the most abundant free amino compound in plant parts, followed by glutamine and alanine. Valine, glycine, isoleucine, aspartic acid and gamma-aminobutyric acid comprised the bulk of the remaining soluble amino N. Composition of tissue pools of amino-N closely resembled that of xylem and phloem exudates. Data on N flow and utilization were combined with information on composition of transport fluids to quantify syntheses, exchanges, and consumptions of asparagine, glutamine, aspartic acid, and valine by organs of the 51- to 58-day plant. These amino compounds carried 56, 29, 5, and 2%, respectively, of the N exported from nodules and contributed in roughly commensurate proportions to transport exchanges and N increments of plant parts. There were, however, more than expected involvements of glutamine and valine in mobilization of N from lower leaves, of asparagine in xylem to phloem transfer, and of aspartic acid in cycling of N through the root, and there was a less than expected participation of aspartic acid in xylem to phloem transfer and in phloem translocation to the shoot apex. The significance of these differences is discussed.

Economy of Carbon and Nitrogen in Nodulated and Nonnodulated (NO(3)-grown) Cowpea [Vigna unguiculata (L.) Walp.]

The response of non-nodulated cowpea (Vigna unguiculata (L.) Walp. cv Caloona) to a wide range of NO(3) levels in the rooting medium was studied 40 days after sowing by in vitro assays of plant organs for NO(3) reductase (EC 1.6.6.1) and analyses of root bleeding (xylem) sap for nitrogenous solutes. Plants fed 1, 5, 10, 20, and 40 millimolar NO(3) showed, respectively, 64, 92, 94, and 91% of their total reductase activity in shoots and 34, 30, 66, 62, and 58% of the total N of their xylem sap as NO(3). These data, and the absence in the plants of significant pools of stored NO(3), indicated that shoots were major organs of NO(3) assimilation, especially at levels of NO(3) (10 to 40 millimolar) that maintained plant growth at near maximum rates. Partitioning and utilization of C and N were studied in nodulated, minus NO(3) plants and non-nodulated plants fed 10 or 20 millimolar NO(3), the levels of NO(3) which gave rates of growth and N assimilation closest to those of the symbiotic plants. The conversion of the C of net photosynthate to dry matter was similar in nodulated plants (67%) and NO(3)-grown plants (64%), but greater proportions of photosynthate were translocated to below ground parts of nodulated plants (37%) than of NO(3)-fed plants (23 to 26%). Greater photosynthate consumption by nodulated roots was associated with proportionately greater root growth and respiration and 2-fold greater export of C in xylem than in the NO(3)-fed plants. Theoretical considerations suggest that the elevated CO(2) output of nodulated roots was due not only to CO(2) loss associated with nodule function, but also to a much greater nonassimilatory component of respiration in the supporting root of the nodulated plant compared to roots of the NO(3)-fed plants. Data are compared with previously published information from other legumes.

Biosynthesis of Ureides from Purines in a Cell-free System from Nodule Extracts of Cowpea [Vigna unguiculata (L) Walp.]

The synthesis of (14)C-labeled xanthine/hypoxanthine, uric acid, allantoin, allantoic acid, and urea from [8-(14)C]guanine or [8-(14)C]hypoxanthine, but not from [8-(14)C]adenine, was demonstrated in a cell-free extract from N(2)-fixing nodules of cowpea (Walp.). The (14)C recovered in the acid/neutral fraction was present predominantly in uric acid and allantoin (88-97%), with less than 10% of the (14)C in allantoic acid and urea. Time courses of labeling in the cell-free system suggested the sequence of synthesis from guanine to be uric acid, allantoin, and allantoic acid. Ureide synthesis was confined to soluble extracts from the bacteroid-containing tissue, was stimulated by pyridine nucleotides and intermediates of the pathways of aerobic oxidation of ureides, but was completely inhibited by allopurinol, a potent inhibitor of xanthine dehydrogenase (EC 1.2.1.37). The data indicated a purine-based pathway for ureide synthesis by cowpea nodules, and this suggestion is discussed.

Nitrogen Nutrition and Xylem Transport of Nitrogen in Ureide-producing Grain Legumes

Xylem sap composition was examined in nodulated and nonnodulated cowpea (Vigna unguiculata [L.] Walp.) plants receiving a range of levels of NO(3) and in eight other ureide-forming legumes utilizing NO(3) or N(2) as sole source of nitrogen. A (15)N dilution technique determined the proportions of plant nitrogen derived from N(2) in the nodulated cowpeas fed NO(3). Xylem sap composition of NO(3)-fed, nodulated cowpea varied predictably with the relative extents to which N(2) and NO(3) were being utilized. The ratios of asparagine to glutamine (N/N) and of NO(3) to ureide (N/N) in xylem sap increased with increasing dependence on NO(3) whereas per cent of xylem nitrogen as ureide and the ratio of ureide plus glutamine to asparagine plus NO(3) (N/N) in xylem sap increased with increasing dependence on N(2) fixation. The amounts of NO(3) and ureides stored in leaflets, stems plus petioles, and roots of cowpea varied in a complex manner with level of NO(3) and the presence or absence of N(2) fixation. All species showed higher proportions of organic nitrogen as ureide and several-fold lower ratios of asparagine to glutamine in their xylem sap when relying on N(2) than when utilizing NO(3). In nodulated (minus nitrate) cowpea and mung bean (Vigna radiata [L.] Wilczek) the percentage of xylem nitrogen as ureide remained constant during growth but the ratio of asparagine to glutamine varied considerably. The biochemical significance of the above differences in xylem sap composition was discussed.

Economy of Carbon and Nitrogen in a Nodulated and Nonnodulated (NO(3)-grown) Legume

Partitioning and utilization of assimilated C and N were compared in nonnodulated, NO(3)-fed and nodulated, N(2)-fed plants of white lupin (Lupinus albus L.). The NO(3) regime used (5 millimolar NO(3)) promoted closely similar rates of growth and N assimilation as in the symbiotic plants. Over 90% of the N absorbed by the NO(3)-fed plants was judged to be reduced in roots. Empirically based models of C and N flow demonstrated that patterns of incorporation of C and N into dry matter and exchange of C and N among plant parts were essentially similar in the two forms of nutrition. NO(3)-fed and N(2)-fed plants transported similar types and proportions of organic solutes in xylem and phloem. Withdrawal of NO(3) supply from NO(3)-fed plants led to substantial changes in assimilate partitioning, particularly in increased translocation of N from shoot to root. Nodulated plants showed a lower (57%) conversion of C or net photosynthate to dry matter than did NO(3)-fed plants (69%), and their stems were only half as effective as those of NO(3)-fed plants in xylem to phloem transfer of N supplied from the root. Below-ground parts of symbiotic plants consumed a larger share (58%) of the plants' net photosynthate than did NO(3)-fed roots (50%), thus reflecting a higher CO(2) loss per unit of N assimilated (10.2 milligrams C/milligram N) by the nodulated root than by the root of the NO(3)-fed plant (8.1 milligrams C/milligram N). Theoretical considerations indicated that the greater CO(2) output of the nodulated root involved a slightly greater expenditure for N(2) than for NO(3) assimilation, a small extra cost due to growth and maintenance of nodule tissue, and a considerably greater nonassimilatory component of respiration in root tissue of the symbiotic plant than in the root of the NO(3)-fed plant.

Assimilation and Transport of Nitrogen in Nonnodulated (NO(3)-grown) Lupinus albus L

The response of nonnodulated white lupin (Lupinus albus L. cv. Ultra) plants to a range of NO(3) levels in the rooting medium was studied by in vitro assays of extracts of plant parts for NO(3) reductase (EC 1.6.6.1) activity, measurements of NO(3)-N in plant organs, and solute analyses of root bleeding (xylem) sap and phloem sap from stems and petioles. Plants were grown for 65 days with 5 millimolar NO(3) followed by 10 days with 1, 5, 15, or 30 millimolar NO(3). NO(3) reductase was substrate-induced in all tissues. Roots contained 76, 68, 62 and 31% of the total NO(3) reductase activity of plants fed with 1, 5, 15, and 30 millimolar NO(3), respectively. Stem, petioles, and leaflets contained virtually all of the NO(3) reductase activity of a shoot, the activity in extracts of fruits amounting to less than 0.3% of the total enzyme recovered from the plant. Xylem sap from NO(3)-grown nonnodulated plants contained the same organic solutes as from nodulated plants grown in the absence of combined N. Asparagine accounted for 50 to 70% and glutamine 10 to 20% of the xylem-borne N. The level of NO(3) in xylem sap amounted to 4, 13, 12, and 17% of the total xylem N at 1, 5, 15, and 30 millimolar NO(3), respectively. Xylem to phloem transfer of N appeared to be quantitatively important in supplying fruits and vegetative apices with reduced N, especially at low levels of applied NO(3). NO(3) failed to transfer in any quantity from xylem to phloem, representing less than 0.3% of the phloem-borne N at all levels of applied NO(3). Shoot organs were ineffective in storing NO(3). Even when NO(3) was supplied in great excess (30 millimolar level) it accounted for only 8% of the total N of stem and petioles, and only 2 and 1% of the N of leaflets and fruits, respectively.

Economy of Photosynthate Use in Nitrogen-fixing Legume Nodules: Observations on Two Contrasting Symbioses

The economy of C use by root nodules was examined in two symbioses, Vigna unguiculata (L.) Walp. (cv. Caloona):Rhizobium CB756 and Lupinus albus L. (cv. Ultra):Rhizobium WU425 over a 2-week period in early vegetative growth. Plants were grown in minus N water culture with cuvettes attached to the nodulated zone of their primary roots for collection of evolved CO(2) and H(2). Increments in total plant N and in C and N of nodules, and C:N weight ratios of xylem and phloem exudates were studied by periodic sampling from the plant populations. Itemized budgets were constructed for the partitioning and utilization of C in the two species. For each milligram N fixed and assimilated by the cowpea association, 1.54 +/- 0.26 (standard error) milligrams C as CO(2) and negligible H(2) were evolved and 3.11 milligrams of translocated C utilized by the nodules. Comparable values for nodules of the lupin association were 3.64 +/- 0.28 milligrams C as CO(2), 0.22 +/- 0.05 milligrams H(2), and 6.58 milligrams C. More efficient use of C by cowpea nodules was due to a lesser requirement of C for synthesis of exported N compounds, a smaller allocation of C to nodule dry matter, and a lower evolution of CO(2). The activity of phosphoenolpyruvate carboxylase in nodule extracts and the rate of (14)CO(2) fixation by detached nodules were greater for the cowpea symbiosis (0.56 +/- 0.06 and 0.22 milligrams C as CO(2) fixed per gram fresh weight per hour, respectively) than for the lupin 0.06 +/- 0.02 and 0.01 milligrams C as CO(2) fixed per gram fresh weight per hour. The significance of the data was discussed in relation to current information on theoretical costs of nitrogenase functioning and associated nodule processes.