Ultrastructural localization of heavy metals in the extraradical mycelium and spores of the arbuscular mycorrhizal fungus Glomus intraradices

Arbuscular mycorrhizal fungi, obligate symbionts of most plant species, are able to accumulate heavy metals, thereby, protecting plants from metal toxicity. In this study, the ultrastructural localization of Zn, Cu, and Cd in the extraradical mycelium and spores of the arbuscular mycorrhizal fungus Glomus intraradices grown in monoxenic cultures was investigated. Zinc, Cu, or Cd was applied to the extraradical mycelium to final concentrations of 7.5, 5.0, or 0.45 mmol/L, respectively. Samples were collected at time 0, 8 h, and 7 days after metal application and were prepared for rapid freezing and freeze substitution. Metal content in different subcellular locations (wall, cytoplasm, and vacuoles), both in hyphae and spores, was determined by energy-dispersive X-ray spectroscopy. In all treatments and fungal structures analysed, heavy metals accumulated mainly in the fungal cell wall and in the vacuoles, while minor changes in metal concentrations were detected in the cytoplasm. Incorporation of Zn into the fungus occurred during the first 8 h after metal addition with no subsequent accumulation. On the other hand, Cu steadily accumulated in the spore vacuoles over time, whereas Cd steadily accumulated in the hyphal vacuoles. These results suggest that binding of metals to the cell walls and compartmentalization in vacuoles may be essential mechanisms for metal detoxification.

Light and electron microscopic and energy dispersive X-ray microanalysis studies of globoids in protein bodies of embryo tissues and the aleurone layer of rice (Otyza sativa L.) grains

To understand the differences in mineral nutrient storage within tissues and organs of rice (Oryza sativa L.) grains, the distribution of globoids in rice embryo and endosperm tissues was examined using light and transmission electron microscopy and energy dispersive X-ray microanalysis was used to study globoid composition. Globoids were found in most embryo tissues, including provascular cells, and their location and size in sections of protein bodies is described. While P, Mg, and K were commonly detected in all globoids, other elements such as Ca, Mn, Fe, and Zn were sometimes detected in globoids of specific tissues and (or) regions. High peak-to-background ratios for P were obtained in globoids of scutellar and aleurone cells, and moderately high values were detected in ground meristem regions of the mesocotyl and coleoptile. Relatively high K levels were found in globoids in parenchyma cells of the scutellum and coleorhiza; in provascular cells of the radicle; and in ground meristem cells from the mesocotyl, coleoptile, and plumule. Calcium was mainly detected in globoids of the aleurone layer. Iron was mostly found in radicle tissue globoids. Zinc was commonly found in globoids of the scutellar epithelium and in provascular tissues of the mesocotyl, coleoptile, and radicle. Manganese was distributed throughout most of the tissues examined, but the highest levels of Mn were detected in globoids from the coleoptile tip regions and the plumule. A novel finding was that, in the provascular tissues of the coleoptile tip, distinctive differences were found in Mn, Fe, and Zn storage between globoids in the future xylem and the future phloem. Key words: EDX analysis, embryos, globoids, mineral storage, phytate, Oryza sativa, rice.

Large globoid particles in the cotyledons of Cucurbita maxima seedlings

Light microscopy and transmission electron microscopy revealed the location of previously identified large protein body globoids within the cotyledons of Cucurbita maxima seedlings. Large globoids formed in the three cell layers of the spongy mesophyll above the lower epidermis. The large globoids share morphological similarities to typical globoids by being spherical and naturally electron dense, but they were approximately four times the size of regular globoids, ranging from 4 to 8 μm in diameter. The formation of the large globoid particles occurred around 3 days of seedling growth and likely resulted from fusion of a number of typically sized globoids. This method of formation was supported by the appearance of clusters of regular globoids prior to the formation of the unusually large globoids. The volume of a large globoid was less than the combined volume of all the typical-sized globoids found within a mesophyll cell earlier in seedling growth. Keywords: large globoids, cotyledon, Cucurbita maxima, mesophyll, seedling, protein bodies.

Changes in the element composition of globoids from Cucurbita maxima and Cucurbita andreana cotyledons during early seedling growth

Energy dispersive X-ray analysis was used to study the element composition of globoids from the cotyledons of Cucurbita maxima and Cucurbita andreana seeds and seedlings at various stages of growth. The influence of light and mineral nutrient conditions on changes in globoid composition was also investigated. The element composition of globoids changed markedly during early seedling growth. In both species, regardless of light and mineral nutrient conditions, K decreased markedly, Mg, Ca, Fe, Mn, and Zn generally increased, and P remained relatively constant. There were differences in globoid composition changes between the two species that could be attributed, at least in part, to differences in the Ca content of the mature, dry embryos. In C. andreana, which had a higher initial Ca content, there was a large increase in the Ca content of the globoids during seedling growth and no significant increase in Mn. In C. maxima globoids, there was only a slight increase in Ca, but there was a much larger increase in Fe, Zn, and Mn than occurred in C. andreana. Light conditions alone did not have a significant influence on the changes in globoid composition. Light, in combination with mineral nutrients, resulted in a more rapid degradation of globoids in the later stages of growth. Mineral nutrient conditions affected mostly elements that were initially present in large amounts. There were very large, globoid-like particles present in some later stage samples of both species, from each of the different growth conditions. These particles had element compositions that were consistent with their being composed of phytate. Keywords: Cucurbita, energy dispersive X-ray analysis, globoids, mineral nutrient reserves, large particles.

The influence of experimentally induced changes in the (Mg + Ca):K balance on protein bodies formed in developing Cucurbita seeds

Phytate, the main mineral nutrient storage compound in seeds, is stored inside protein bodies. Phytate is usually concentrated in dense bodies called globoid crystals. In 1985, Lott and co-workers proposed that the balance of (Mg + Ca):K may be important in controlling globoid crystal formation and provided some experimental evidence to support this proposal. To test this hypothesis further, developing Cucurbita fruits were injected with sterile K salt solutions. Squash cotyledons generally have large globoid crystals and a relatively high (Mg + Ca):K ratio. We hypothesized that experimental reduction of the ratio by the addition of K would result in the alteration of the size and number of globoid crystals. Developing seeds were remarkably resistant to attempts to alter the elements taken up for storage. The elemental content of embryo tissues was measured quantitatively with neutron activation analysis. Controls plus those few samples showing a distinct shift in the ratio owing to elevated K content were analyzed further. Energy dispersive X-ray analysis of cryogenically prepared samples was used to study the elemental content of globoid crystals and transmission electron microscopy was used to study the ultrastructure of the protein bodies. The results provide additional support for the hypothesis being tested. Key words: phytate, globoid crystals, Cucurbita, seeds, mineral nutrients, protein bodies.

Studies of Capsicum annuum seeds: structure, storage reserves, and mineral nutrients

Storage reserves of Capsicum annuum (sweet pepper) seeds were studied using light microscopy, histochemical techniques, and electron microscopy. The seed had mainly protein and lipid as storage reserves. Starch was not detected. Thickened endosperm cell walls likely are a store of nonstarch polysaccharide. Protein bodies had protein crystalloids and globoid crystals as inclusions in the proteinaceous matrix. Protein bodies in embryo tissues varied greatly in size both inter- and intra-cellularly, while endosperm protein bodies were similar in size. Energy dispersive X-ray analysis and neutron activation analysis revealed the element content of C. annuum seeds. The presence of P, K, and Mg in all globoid crystals is consistent with their being phytin-rich. The average ratios of Mg:K, based on peak-to-background values for globoid crystals, were lowest in provascular tissues, where most globoid crystals were very small, and were highest in ground meristem tissues, where globoid crystals were large. Fe, Zn, Mn, Ca, S, and Cl were detected in some globoid crystals. In globoid crystals of both the cotyledon and hypocotyl – radicle axis, Fe was always detected in protoderm cells, was occasionally detected in ground meristem cells, and was never detected in provascular cells. Neutron activation analysis results showed that Mg, K, S, and Cu were higher in the embryo than in the endosperm, while Cl, Na, and Ca were higher in the endosperm. Mn had similar concentrations in both tissues. Key words: Capsicum annuum, storage reserves, protein body, globoid crystal, phosphorus, seed.

Mineral analyses of storage reserves of Cucurbita maxima and Cucurbita andreana pollen

Cucurbita maxima and Cucurbita andreana are so closely related that hybridization is possible. These two species also have been shown to have very different levels of calcium storage in their seeds. Our neutron activation analysis studies have shown that the total amount of P, Ca, K, and Mg in pollen of the two species was similar. Energy dispersive X-ray analysis studies also showed that the composition of electron-dense particles in the tube cells of the two species was similar. Thus the differences in Ca levels in phytate reserves in the seeds of these two Cucurbita species do not appear to be paralleled by differences in mineral reserves in the pollen of the two species. Specimen preparation studies demonstrated that even though elements such as P, K, Mg, and Ca are mostly retained by an anhydrous fixation and embedding protocol, thin-sectioning of such blocks on a water-filled microtome boat resulted in major loss of elements. Key words: pollen, mineral nutrients, energy dispersive X-ray analysis, neutron activation analysis, phytate, Cucurbita.

Protein bodies in Datura stramonium seeds: structure and mineral nutrient composition

The structure of protein bodies in the endosperm and embryo of Datura stramonium was studied with a variety of light-and electron-microscopic techniques. Protein bodies had one to several globoid crystals and one or two protein crystalloids in the proteinaceous matrix. Although the embryo protein bodies rarely had more than two globoid crystals, the endosperm protein bodies had varying sizes and numbers of globoid crystals, even within the same cell. Energy-dispersive X-ray analysis of globoid crystals revealed the presence of P, K, and Mg in all cases. Traces of Fe, Mn, and Zn were also found in globoid crystals of protein bodies from certain cell types. The distribution patterns of these three elements were quite specific; for example, Mn traces were found only in the globoid crystals of the protoderm. Neutron-activation analysis of endosperm and embryo tissues was used to quantitatively measure the concentration of Ca, Cl, Cu, I, K, Mg, Mn, Na, and S. The results from structural studies and the element analysis studies are discussed in the context of solanaceous seeds in particular but also with relation to seeds in general. Key words: protein bodies, Datura stramonium, seed, globoid crystals, energy-dispersive X-ray analysis, Solanaceae.

Effects of silica level on some properties of Oryza sativa straw and hull

Reduction of silica content of two rice varieties obtained by growth in hydroponic solutions (0–400 ppm added SiO2) increased leaf openness and decreased both stem bending hardness and hull weight. The number of silica bodies in the surface of leaf blades was similarly decreased. Mapping by energy dispersive X-ray analysis techniques showed high concentrations of silicon in silica bodies inside silica cells on leaf blades and uniform distribution on lemma and palea surfaces. Vascular tissues and bulliform cells were also rich in silica. Decrease in silica content to less than 10% dry weight increased the in vivo dry matter digestibility of straw in both varieties and increased the in vitro organic matter digestibility of IR36 straw.

Protein bodies in umbelliferous seeds. III. Characterization of calcium-rich crystals

The chemical composition of the calcium-rich crystal inclusions present in the seed protein bodies of carrot (Daucus carota L. cv. Imperator 408), wild carrot (Daucus carota L.), caraway (Carum carvi L.), anise (Pimpinella anisum L.), dill (Anethum graveolens L.), celery (Apium graveolens L. cv. Tall Utah), fennel (Foeniculum vulgare Mill.), parsnip (Pastinaca sativa L. cv. Hollow Crown), parsley (Petroselinum sativum L. cv. Moss Curled), and chervil (Anthriscus cerefolium L. cv. Curled) was determined. Using a variety of methods including X-ray diffraction, infrared spectroscopy, microincineration, energy dispersive X-ray analysis, solubility studies, and staining, the chemical composition of the calcium-rich crystal inclusions was identified as calcium oxalate.

Elemental composition of globoid crystals in protein bodies of wheat grain grown on soil treated with sewage sludge

Wheat grain grown on soil treated with a heavy application rate of sewage sludge exhibited higher iron, zinc, copper, and cadmium contents than did wheat grain grown on an unfertilized control treatment. The sludge supplied more than the maximum amounts of zinc, copper, cadmium, and lead recommended for addition to Ontario agricultural lands.Globoid crystals, which are inclusions in protein bodies, are the major mineral storage component of wheat grain. The elemental composition of globoid crystals in wheat grain grown with and without sludge treatment was determined using energy dispersive X-ray (EDX) analysis. Globoid crystals from sludge-treated and control grains exhibited similar elemental compositions with the following exceptions. When comparing sludge-treated grains versus the controls: more globoid crystals in the aleurone layer farthest from the peduncle contained calcium; more coleoptile mesophyll globoid crystals contained calcium, manganese, and (or) iron; some globoid crystals in the young foliage leaves contained manganese; and some globoid crystals in the stele of the radicle contained calcium. The globoid crystals exhibited no tendency to accumulate toxic metals supplied by the sludge indicating that the mineral storage system in wheat is remarkably specific despite varying soil conditions.

Thin-section, freeze-fracture, and energy dispersive x-ray analysis studies of the protein bodies of tomato seeds

Protein bodies of dry seeds of tomato (Lycopersicon esculentum) from radicle, hypocotyl, cotyledon, and endosperm tissue were extensively studied using thin-sectioning, freeze-fracturing and energy dispersive x-ray (EDX) analysis. Protein bodies varied in size, were oval to circular in section, and generally consisted of a proteinaceous matrix, globoid crystal, and protein crystalloid components. Size, shape, and arrangements of globoid crystals and protein crystalloids varied even within the same cell. Globoid crystals were generally oval to circular in section. They were always surrounded by a proteinaceous matrix. In a given protein body the number present ranged from a few to numerous. A protein body generally contained only one protein crystalloid. In section, protein crystalloids were irregular or angular in shape. They were composed of substructural particles which formed lattice planes. EDX analysis of tomato seed globoid crystals revealed the presence of P, K, and Mg in all cases, a fact that is consistent with globoid crystals being phytin-rich. Rarely, small amounts of calcium were found along with P, K, and Mg in globoid crystals of each of the tissue regions considered. The distribution pattern of cells with Ca containing globoid crystals was random. Small amounts of Fe and Mn were also found in the globoid crystals of protein bodies from certain cell types. These two elements, unlike calcium, were specific in terms of their distribution. Globoid crystals from the protodermal cells often contained Mn and Fe. The globoid crystals from provascular tissue of radicle, hypocotyl, and cotyledon regions often contained Fe while globoid crystals in the first layer of large cells surrounding these provascular areas always contained Fe. Results from EDX analysis of the proteinaceous material from the protein bodies are presented and discussed as are variations in elemental content due to different fixations.

An energy dispersive x-ray analysis study of protein bodies in the seeds of Astragalus bisulcatus and Oxytropis Lambertii

Elemental composition of protein body regions in radicle, stem, and cotyledon areas of the leguminous plants Astragalus bisulcatus and Oxytropis Lambertii were determined using energy dispersive x-ray (EDX) analysis. Globoid crystals from all tissue regions of both species routinely contained P, K, Mg, and Ca. Possible relationships between seed size and the presence of calcium in globoid crystals of plants in the legume subfamily Papilionateae are discussed. Although the two species studied are selenium accumulators, no Se was ever detected in either globoid crystal or proteinaceous regions. The unusual mineral uptake properties of these plants seemingly does not influence the types of mineral reserves deposited in the protein bodies of the seed.

Location of reserves of mineral elements in seed protein bodies: macadamia nut, walnut, and hazel nut

This paper presents the results of thin-section, freeze-fracture, energy dispersive x-ray (EDX) analysis, and chemical analysis studies of cotyledon tissue from dry seeds of macadamia nut (Macadamia integrifolia), walnut (Juglans regia), and hazel nut (Corylus avellana). Studies concentrated on mineral elements in the storage protein bodies. Protein bodies of macadamia nut contained no protein crystalloids but did contain some globoid crystals. Aqueous fixation of macadamia tissue appeared to result in a rapid solubilization of proteinaceous matrix material. EDX analysis of macadamia nut globoid crystals revealed the presence of P, K, and Mg in most cases but the occasional crystal was very high in calcium. Walnut protein bodies often contained many large protein crystalloids. Globoid crystals were present between the protein crystalloids and in the proteinaceous matrix. EDX analysis indicated the presence of P, K, and Mg as the main elements in walnut globoid crystals. Hazel nut protein bodies contained one or more small protein crystalloids, globoid crystals of various sizes, and proteinaceous matrix. Some protein bodies contained druse crystals. EDX analysis showed that the druse crystals were rich in calcium and thus are presumably calcium oxalate. EDX analysis of globoid crystals revealed the presence of elements consistent with the phytin-rich nature of globoid crystals. Variations in globoid crystal composition are discussed.

Inclusions in seed protein bodies in members of the Compositae and Anacardiaceae: comparison with other dicotyledonous families

Thin sections, freeze-fracture replicas, energy dispersive x-ray (EDX) analyses, and chemical analyses of cotyledon tissue from seeds of sunflower (Helianthus annuus), golden everlasting daisy (Helichrysum bracteatum), cashew (Anacardium occidentale), and pistachio nut (Pistacia vera) showed that in all four species the protein bodies consisted of a proteinaceous matrix surrounding numerous globoid crystals. Some variation in globoid crystal size did occur, but they were generally small in relation to the size of the protein body. No protein crystalloids were found in the cotyledon protein bodies of these four species. EDX analysis results were consistent with the globoid crystals being phytin rich. Results of this study, combined with an evaluation of protein body structure in other dicot embryos that have been studied, support the theory that protein body structure is related to taxonomic grouping.

Thin sectioning, freeze fracturing, energy dispersive x-ray analysis, and chemical analysis in the study of inclusions in seed protein bodies: almond, Brazil nut, and quandong

Protein bodies from almond (Prunus dulcis), Brazil nut (Bertholletia excelsa), and quandong (Santalum acuminatum) have been studied in thin sections of fixed and embedded tissue, in freeze-fracture replicas of unfixed tissue, by chemical analysis of tissue for P, K, Mg, and Ca, and by energy dispersive x-ray (EDX) analysis of both sections of glutaraldehyde-fixed tissue and freeze-dried tissue powders. The protein bodies in all three species contained globoid crystals, protein crystalloids, and proteinaceous matrix regions. Results of EDX analyses were consistent with globoid crystals being rich in phytin. Variation in both the structure and the elemental composition of globoids was common. In almond some globoids were lobed rather than spherical, and large globoid crystals often contained considerable calcium whereas small globoid crystals contained little if any calcium. The globoid crystals of Brazil nut often contained barium in addition to P, K, Ca, and Mg. Protein crystalloids of Brazil nut were compound crystals. Protein bodies of quandong seed, which is largely endosperm rather than embryo, were unexceptional.

Calcium oxalate druse crystals and other inclusions in seed protein bodies: Eucalyptus and jojoba

Druse crystals of calcium oxalate are known to occur in some protein bodies of some plant seeds. This paper reports observations on crystals, and other contents, of protein bodies of Eucalyptus erythrocorys and Simmondsia chinensis (jojoba). Results are presented from thin section studies of glutaraldehyde–OsO4 fixed, dehydrated, and embedded tissue; freeze-fracture studies; energy dispersive x-ray (EDX) analysis studies of freeze-dried tissue powders (Eucalyptus only); and chemical analysis studies of P, Mg, K, and Ca content (Eucalyptus only). Many Eucalyptus protein bodies contained large druse crystals rich in Ca but devoid of P, K, and Mg, and an occasional protein body from jojoba contained some dispersed crystals. Eucalyptus seeds were exceptionally high in Ca content. EDX analysis results provide evidence favouring the phytin-rich nature of globoid crystals in the two species. Structural variation in the globoid crystals was great, especially in jojoba. Eucalyptus, whose protein bodies contained very large globoid crystals rich in P, Mg, and K, was found to have higher levels of these elements than most species investigated to date.

The effects of chlordane on the green algae Scenedesmus quadricauda and Chlamydomonas sp

Chlordane is an organochlorine soil insecticide whose effects upon aquatic ecosystems, and particularly on algae, are little known. Experimental results indicate that chlordane concentrations ranging from 0.1 to 100 μg/ℓ significantly stimulate cell division of Scenedesmus quadricauda, a common planktonic green alga. Chlordane in concentrations from 0.1 to 50 μg/ℓ is also stimulatory to the growth of Chlamydomonas sp., isolated from soil. However, 100 μg/ℓ was inhibitory to cell division in this species.Significant stimulatory effects on the respiration rates of these two species were observed at all concentrations within 3 to 4 h of chlordane addition. Stimulation of respiration increased with increasing chlordane concentration. Similar stimulatory effects on respiration were produced using 2,4-dinitrophenol (DNP), a known uncoupling agent of oxidative phosphorylation. Long-term inhibition of photosynthetic response of Scenedesmus quadricauda treated with the higher chlordane concentrations was observed over the course of the experiment even though a significant increase in cell numbers over the control was noted. A highly significant chlordane–light interaction was observed on photosynthesis of both S. quadricauda and Chlamydomonas sp.

Cell walls in Cucurbita maxima cotyledons in relation to imbibition

The scanning electron microscope and the freeze-etch technique were used to demonstrate the presence of corrugated cell walls in cotyledon mesophyll cells from dry squash seeds. These wavy cell walls straightened out during imbibition. In the scanning electron micrographs no wavy cell walls were observed in the smaller epidermal cells. Tissue imbibed for 2 h and then air-dried contained corrugated cell walls, whereas similar imbibed tissue dehydrated in ethanol and propylene oxide contained smooth cell walls. Thus the method of seed dehydration is important in determining the appearance of wavy cell walls. Glutaraldehyde fixation during the 2-h imbibition process did not alter the conditions under which wavy cell walls reappear.

Light intensity and photosynthetic rates in phytoplankton

The physiological behavior of various phytoplankton species has been examined under increasing and decreasing light regimes. A marked asymmetry has been found, with photosynthesis rates under rising light intensities exceeding those under dimming lights. Photoinhibition has been observed at moderate light intensities (0.4–1.0 ly/min). Variations in the asymmetry and in photoinhibition appear to be correlated with the light regimes experienced by the population. Under constant illumination by full sunlight a steady decrease in the photosynthesis rate was observed. As a result of photosynthesis measurements in fluctuating light regimes there is now evidence for photorespiration as the controlling mechanism. Data are presented which show that prolonged exposure to high light has an adverse effect on photosynthesis and indicate that 14C moored bottle experiments may show reduced surface rates because of this.

The structure of protein bodies in Cucurbita maxima cotyledons

The results of this freeze-etching study of imbibed seeds confirm the findings of an earlier study on the structure of protein bodies in the cotyledons of dormant squash seeds. The cotyledon mesophyll cell protein bodies were found to have at least four components, which are the proteinaceous matrix, the protein crystalloid, the soft globoid, and the globoid crystal.

Frequency and distribution of nuclear pores in Cucurbita maxima cotyledons as revealed by freeze-etching

The frequency and distribution of nuclear pores in the cotyledon mesophyll cells of Cucurbita maxima were studied during germination. During the early stages of seed germination the nuclear envelope of the mesophyll cells was found to contain a distinctive pattern of nuclear pores. The nuclear pores were in short rows that divided the nuclear membrane into areas in which pores were absent. Mesophyll cells from a later stage of germination contained nuclear envelopes with significantly more nuclear pores per unit area. At the later stage of germination the distinctive pattern of nuclear pores was less readily distinguishable by visual observation of electron micrographs, but pattern analysis revealed that the pattern was still present.

Protein bodies from the cotyledons of Cucurbita maxima

The structure of protein bodies from cotyledon tissue of dormant squash seeds was studied using several different preparative techniques. Four major components, the proteinaceous matrix, the protein crystalloid, the soft globoid, and the globoid crystal were found and verified to be present in protein bodies from the large mesophyll cells. Some of these four components were found to be absent in protein bodies of epidermal cells or in protein bodies near the nucleus of mesophyll cells. The globoid crystal, which was hard and electron dense after osmium fixation, is probably the site of phytin storage. The globoid crystal was usually surrounded by the soft globoid, a material of undetermined composition which tended to smear during freeze-etching. Some protein bodies contained several of these globoid regions although most protein bodies seemed to have only one globoid region. One or several protein crystalloids were usually found in protein bodies. Both the protein crystalloids and the globoid regions were surrounded by a proteinaceous matrix substance.Glutaraldehyde–permanganate fixation of tissue clearly showed the presence of the proteinaceous ground substance and the protein crystalloid but the globoid structures were absent. In tissue chemically fixed with glutaraldehyde–osmium all four components were sometimes found but interpretation, on the basis of this fixation alone, was difficult because of the presence of fixation artifacts. Isolation of protein bodies from cotyledon tissue made faster and better quality osmium or glutaraldehyde–osmium fixations possible. In such protein body pellets all four components were seen. Freeze-etching of tissue verified the presence of the globoid crystal and the soft globoid in most protein bodies from cotyledon mesophyll cells. In dormant seed tissue the two proteinaceous components usually were not distinguishable by the freeze-etching technique however. Thus only through the use of several different preparative techniques was it possible to locate and verify the presence of the various structural components in protein bodies from squash cotyledons.

Changes in the cotyledons of Cucurbita maxima during germination. II. Development of mitochondrial function

Mitochondrial respiration in squash cotyledons was followed during germination. Cytochrome oxidase and succinoxidase activities were determined in isolated mitochondrial preparations; tissue localizations of cytochrome oxidase and succinic dehydrogenase were determined histochemically in fresh tissue slices. The respiratory activities differed in light- and dark-germinated plants. In light-grown plants, the respiratory activity reached a peak at 3 days, when the root and the stem were actively growing, and then declined markedly as the tissue became photosynthetic. During the peak, the respiratory activity was present in all the cotyledon tissues; in the foliaceous cotyledons the respiratory activity was localized in the veins. In dark-grown plants the peak of respiratory activity extended over several days and the respiratory rate remained high throughout the cotyledon's life. This broad peak of activity may be related to the mobilization of storage materials for the growth of the embryo axis. In dark-germinated plants, the respiratory activity was widespread throughout the cotyledonary tissues during the peak of activity; the activity became vein localized by 8 days. Electron microscopic studies of cotyledon tissues showed the presence of many mitochondrial profiles in the veinlet regions.

Changes in the cotyledons of Cucurbita maxima during germination. I. General characteristics

Various structural and physiological changes in squash cotyledons were followed as a function of germination time. Temperature, to which squash plants are very sensitive, was kept constant at 31 °C but light conditions were varied.In light-germinated plants (16 h light/day) the cotyledons changed from storage organs to greatly enlarged photosynthetic leaves, within 5 days. Incorporation of major amounts of thymidine-2-14C throughout the cotyledons occurred only during the enlargement phase.Dark-germinated plants, in which the cotyledons enlarged very little, produced a long, stout hypocotyl. No major incorporation of thymidine-2-14C into the cotyledons occurred, suggesting that little cell division occurs in the cotyledons of dark-germinated squash plants.Fresh- and dry-weight changes in the cotyledons were consistent with the changes in morphology of light- and dark-germinated plants. The cotyledons of plants which were dark-germinated for 6 days and then placed in 16 h light per day did not reach the fresh weight per cotyledon or dry weight per cotyledon level attained by cotyledons from plants germinated, from the beginning in 16 h light per day.

Changes in the cotyledons of Cucurbita maxima during germination. III. Plastids and chlorophylls

The chronological age of a seedling does not ensure that the tissue is similar in cell structure. At 4 days of age the positioning of the 8-h night cycle with relation to the planting time had profound influence over plastid structure. Mesophyll tissue, which had a long light period before fixation at 4 days, contained chloroplasts with well-developed grana and fret systems. Tissue which had an 8-h dark period before fixation at 4 days contained plastids with large prolamellar bodies and no grana. Apparently during a short time period the night period can result in the formation of crystalline prolamellar bodies in chlorophyll containing tissue (about 0.5 mg chlorophyll per gram fresh weight). Thus in greening squash cotyledons the prolamellar body is a part of normal plastid development.Mesophyll tissue, which received a long light period before fixation at 5 days, contained normal chloroplasts with abundant starch whereas tissue receiving 8 h darkness before fixation at 5 days contained normal chloroplasts which were devoid of starch.The chlorophyll a to chlorophyll b ratio changed during the greening of the cotyledons. Prolonged dark germination greatly reduced the ability of the cotyledons to synthesize chlorophyll when the plants were subsequently placed in 16 h light/day.