Differences in steady-state net ammonium and nitrate influx by cold- and warm-adapted barley varieties

Temperature effects on nitrogen form uptake by seedling roots of three contrasting conifers

Plant species may show a preference for uptake of particular nitrogen (N) forms, but little is known about how N form preference is influenced by soil temperature. Potential future changes in soil N form availability and plant N form preference in warmer soils might shift competitive interactions among forest tree species. We compared the N uptake and growth of three conifer species from contrasting environments grown at rhizosphere temperatures of 10, 16 or 20 °C and supplied with ammonium (NH4 (+)) or nitrate (NO3 (-)) or a mix of arginine and alanine. Short-term N uptake was assessed using ion-selective microelectrodes and application of (15)N, and long-term uptake was assessed by plant N status. Species exhibited preferences for particular N forms, and these preferences related to the N form most available in native soils. Specifically, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) showed a preference for nitrate (a N form commonly found in warmer areas), Sitka spruce (Picea sitchensis (Bong.) Carr.) preferred ammonium (a N form abundant in cold soils) and Engelmann spruce (Picea engelmannii Parry ex Engelm.) showed a preference for ammonium and organic N (organic N is often abundant in cold soils). Relative N form preference, as indicated by plant growth, changed with temperature in some species, indicating that these species could acclimate to changing rhizosphere temperatures. Understanding how conifers utilize available soil nutrients at different temperatures can help to predict species' future performance as soil temperatures rise.

Preferential expression of an ammonium transporter and of two putative nitrate transporters in root hairs of tomato

Root hairs as specialized epidermal cells represent part of the outermost interface between a plant and its soil environment. They make up to 70% of the root surface and, therefore, are likely to contribute significantly to nutrient uptake. To study uptake systems for mineral nitrogen, three genes homologous to Arabidopsis nitrate and ammonium transporters (AtNrt1 and AtAmt1) were isolated from a root hair-specific tomato cDNA library. Accumulation of LeNrt1-1, LeNrt1-2, and LeAmt1 transcripts was root-specific, with no detectable transcripts in stems or leaves. Expression was root cell type-specific and regulated by nitrogen availability. LeNrt1-2 mRNA accumulation was restricted to root hairs that had been exposed to nitrate. In contrast, LeNrt1-1 transcripts were detected in root hairs as well as other root tissues under all nitrogen treatments applied. Analogous to LeNrt1-1, the gene LeAmt1 was expressed under all nitrogen conditions tested, and root hair-specific mRNA accumulation was highest following exposure to ammonium. Expression of LeAMT1 in an ammonium uptake-deficient yeast strain restored growth on low ammonium medium, confirming its involvement in ammonium transport. Root hair specificity and characteristics of substrate regulation suggest an important role of the three genes in uptake of mineral nitrogen.

Kinetics of NH4+ Influx in Spruce

Influxes of 13NH4+ across the root plasmalemma were measured in intact seedlings of Picea glauca (Moench) Voss. Two kinetically distinct uptake systems for NH4+ were identified. In N-deprived plants, a Michaelis-Menten-type high-affinity transport system (HATS) operated in a 2.5 to 350 [mu]M range of external NH4+ concentration ([NH4 +]o). The Vmax of this HATS was 1.9 to 2.4 [mu]mol g-1 h-1, and the Km was 20 to40 [mu]M. At [NH4+]o from 500 [mu]M to 50 mM, a linear low-affinity system (LATS) was apparent. Both HATS and LATS were constitutive. A time-dependence study of NH4+ influx in previously N-deprived seedlings revealed a small transient increase of NH4+ influx after 24 h of exposure to 100 [mu]M [NH4+]o. This was followed by a decline of influx to a steady-state value after 4 d. In seedlings exposed to 100 [mu]M external NO3- concentration for 3 d, the Vmax for NH4+ uptake by HATS was increased approximately 30% compared to that found in N-deprived seedlings, whereas LATS was down-regulated. The present study defines the much higher uptake capacity for NH4+ than for N03- in seedlings of this species.

Methylammonium as a Transport Analog for Ammonium in Tomato (Lycopersicon esculentum L.)

Methylammonium (CH3NH3+) has been widely used as an analog of ammonium (NH4+) for examining transport in bacteria and fungi. We compared the kinetics of root CH3NH3+ and NH4+ uptake from solution culture in intact tomato (Lycopersicon esculentum cv T5) plants. Efflux of NH4+ and CH3NH3+ was negligible. The apparent maximum rate of absorption (apparent Vmax) was similar for NH4+ and CH3NH3+, but the apparent affinity (apparent Km) was about 10-fold greater for NH4+ than for CH3NH3+. In characterizing the interaction between NH4+ and CH3NH3+ transport, we used [15N]NH4+ and [14C]CH3NH3+ as well as improved methods for analysis of nonisotopic CH3NH3+ and NH4+. CH3NH3+ acted as an inhibitor of NH4+ influx. Relatively low concentrations of NH4+ strongly inhibited CH3NH3+ influx. Treatments with 1 mM methionine sulfoximine that blocked NH4+ assimilation had little influence on NH4+ inhibition of CH3NH3+ influx. These results suggest that the two ions share a common transport system in tomato, but because this transport system has a much greater affinity for NH4+, CH3NH3+ may be used as a transport analog only when ambient concentrations of NH4+ are very low.

Ammonium Uptake by Rice Roots (I. Fluxes and Subcellular Distribution of 13NH4+)

The time course of 13NH4+ uptake and the distribution of 13NH4+ among plant parts and subcellular compartments was determined for 3-week-old rice (Oryza sativa L. cv M202) plants grown hydroponically in modified Johnson's nutrient solution containing 2,100, or 1000 [mu]M NH4+ (referred to hereafter as G2, G100, or G1000 plants, respectively). At steady state, the influx of 13NH4+ was determined to be 1.31, 5.78, and 10.11 [mu]mol g-1 fresh weight h-1, respectively, for G2, G100, and G1000 plants; efflux was 11, 20, and 29%, respectively, of influx. The NH4+ flux to the vacuole was calculated to be between 1 and 1.4 [mu]mol g-1 fresh weight h-1. By means of 13NH4+ efflux analysis, three kinetically distinct phases (superficial, cell wall, and cytoplasm) were identified, with t1/2 for 13NH4+ exchange of approximately 3 s and 1 and 8 min, respectively. Cytoplasmic [NH4+] was estimated to be 3.72, 20.55, and 38.08 mM for G2, G100, and G1000 plants, respectively. These concentrations were higher than vacuolar [NH4+], yet 72 to 92% of total root NH4+ was located in the vacuole. Distributions of newly absorbed 13NH4+ between plant parts and among the compartments were also examined. During a 30-min period G100 plants metabolized 19% of the influxed 13NH4+. The remainder (81%) was partitioned among the vacuole (20%), cytoplasm (41%), and efflux (20%). Of the metabolized 13N, roughly one-half was translocated to the shoots.

Effects of Exposure to Ammonium and Transplant Shock upon the Induction of Nitrate Absorption

In barley (Hordeum vulgare L. cv Steptoe) seedlings, the time course for induction of root nitrate absorption varied significantly with pretreatment. Net nitrate uptake of nitrogen-deprived plants more than doubled during the 12 hours after first exposure to nitrate. For these plants, gentle physical disturbance of the roots inhibited net nitrate absorption for more than 6 hours and potassium absorption for 2 hours. Pretreatment with ammonium appeared sufficient to induce nitrate absorption; plants either grown for 2 weeks on or exposed for only 10 hours to a medium containing ammonium as a sole nitrogen source showed high rates of net nitrate uptake when first shifted to a medium containing nitrate. Gentle physical manipulation of these plants inhibited nitrate absorption for 2 hours and potassium absorption for more than 12 hours. These results indicate (a) that experimental protocols should avoid physical manipulation of the roots when-ever possible and (b) that ammonium or a product of ammonium assimilation can induce nitrate absorption.

Root excision decreases nutrient absorption and gas fluxes

The roots of barley plants (Hordeum vulgare L. cv Steptoe) were monitored before and after excision for net uptake of carbon dioxide, oxygen, ammonium, potassium, nitrate, and chloride and for their content of sucrose, glucose, fructose, and malic acid. All fluxes began to attenuate within 2 hours after excision. Net potassium uptake returned to control levels 6 hours after excision, but carbon dioxide, oxygen, ammonium, and nitrate fluxes continued to diminish for the remainder of the observation period. The addition of 0.1 molar glucose or 0.1 molar sucrose to excision medium had no significant effect on these changes in ion and gas fluxes. Net chloride uptake was negligible for all treatments. Sugar and malic acid content of the root declined after excision. Sucrose and glucose levels remained depressed for the entire observation period, whereas fructose and malic acid returned to control levels after 9 hours. These results indicate that excision has profound, adverse effects on root respiration and the absorption of mineral nitrogen.

Effects of NaCl and CaCl(2) on Ion Activities in Complex Nutrient Solutions and Root Growth of Cotton

Sodium displaces Ca(2+) from membranes (GR Cramer, A Läuchli, VS Polito Plant Physiol 1985 79: 207-211) and this can be related to the (Ca(2+))/(Na(+))(2) activity ratio in the external solution (GR Cramer, A Läuchli 1986 J Exp Bot 37: 321-330). Supplemental Ca(2+) is known to mitigate the adverse effects of salinity on plant growth. In this report we investigated the effects of NaCl (0-250 millimolar) and Ca(2+) (0.4 and 10 millimolar) on the ion activities in solution and on root growth of cotton (Gossypium hirsutum L.). Ion activities were analyzed using the computer program, GEOCHEM. Most ion activities in a 0.1 modified Hoagland solution were significantly reduced by both NaCl and supplemental Ca(2+). Ion-pair formation and precipitation were significant for some ions, especially phosphate. Root growth of 6-day-old seedlings was stimulated by low NaCl concentrations (25 millimolar). At higher NaCl concentrations, root growth was inhibited; the concentration at which this occurred depended on the Ca(2+) concentration and the growth index used. Supplemental Ca(2+) mitigated the inhibition of root growth caused by NaCl. There was a curvilinear relationship between root growth and the (Ca(2+))/(Na(+))(2) ratio in the nutrient solution. The mechanisms by which Na(+) and Ca(2+) may affect root growth are discussed.

The influence of ammonium and chloride on potassium and nitrate absorption by barley roots depends on time of exposure and cultivar

Net uptakes of K(+) and NO(3) (-) were monitored simultaneously and continuously for two barley (Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K(+) and NO(3) (-) uptake in the absence of NH(4) (+) or Cl(-). Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH(4) (+), K(+), Cl(-), or NO(3) (-) affected the cultivars very differently: (a) the presence of NH(4) (+) as NH(4)Cl stimulated net NO(3) (-) uptake in Prato barley but inhibited net NO(3) (-) uptake in Olli barley; (b) Cl(-) inhibited net NO(3) (-) uptake in Prato but had little effect in Olli; and (c) NH(4) (+) as (NH(4))(2)SO(4) inhibited net K(+) uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl(-) initially inhibited net K(+) uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH(4) (+) and Cl(-) uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption.

Relationship between Mineral Nitrogen Influx and Transpiration in Radish and Tomato

Net ammonium and nitrate influx were independent of transpiration rate for intact seedlings of both a wild species of radish (Raphanus raphanistrum) and a wilty tomato mutant (Lycopersicon esculentum Mill. cv RR flacca).

Regulation of k influx in barley : effects of low temperature

Influx and accumulation of K(+) in barley (Hordeum vulgare L. cv Fergus) roots were measured at two temperatures (10 degrees C and 20 degrees C) in plants which had been grown with roots and shoots at 20 degrees C (HT plants), with roots and shoots at 10 degrees C (LT plants), and with roots at 10 degrees C and shoots at 20 degrees C (DT plants). Under conditions where K(+) was in limited supply during the prior growth period, K(+) influx and accumulation were consistently higher in roots of DT and LT plants than in those of HT plants. Thus, it would appear that this low temperature response is not limited specifically to conditions in which temperature differentials are maintained between roots and shoots. Nevertheless, it was generally the case that increases of influx were larger in DT and LT plants so that the temperature differentials may intensify the low temperature response. When K(+) influx was examined over a wide range of root [K(+)], it was seen that the characteristic reduction of influx associated with increased internal [K(+)] was substantially greater in HT than DT or LT plants. Transfer of plants grown under HT conditions to DT or LT regimes led to both short-term and long-term adjustments of influx. The former became apparent within 6 hours of exposure to the new conditions and decayed within minutes of transfer back to 20 degrees C. The long-term adjustments were only apparent after prolonged exposure (days) to the lower root temperature and these did not decay as rapidly. Regardless of shoot temperature, the transfer of roots from 20 degrees C to 10 degrees C caused a gradual increase of root [K(+)] so that 4 days later LT and DT roots contained, respectively, 53.3 and 49.83 micromoles per gram compared to 17.82 micromoles per gram for roots maintained at 20 degrees C.