Photosynthesis in Tall Fescue : V. Analysis of High PSI Activity in a Decaploid Genotype

Previous work in our laboratory (Krueger, Miles 1981 Plant Physiol 68: 1110-1114) indicated that a decaploid genotype (I-16-2) of tall fescue (Festuca arundinacea Schreb.) which exhibits unusually high net photosynthesis rates also had high potential rates of photosynthetic electron transport through photosystem I (PSI) compared to the typical hexaploid genotype (V6-802). Analysis of electron transport activity revealed that the oxidizing side of PSI as the major site of difference. Examination of the whole thylakoids and subchloroplast particle protein components of the common hexaploid and the decaploid genotypes had major polypeptide differences at 30, 21, and 12.5 kilodaltons. These differences could not be assigned to a specific physiological function in PSI. The decaploid had increased P₇₀₀ and plastocyanin content on a chlorophyll basis. Antibodies raised against fescue plastocyanin were used to quantitate plastocyanin in crude (Triton X-100) solubilized extracts of plant material. Results showed that the decaploid had 16% and 40% more plastocyanin on a weight and area basis, respectively. The antibodies did not inhibit electron transport (diaminodiurene to methyl viologen) in isolated thylakoids strengthening the hypothesis of plastocyanin as an internal mobile electron shuttle. The trend of inhibition of plastocyanin by KCN was similar in the two genotypes but the decaploid had 15 to 20% higher rates of electron flow under nearly all inhibiting conditions.     

Ribulose bisphosphate carboxylase: altered genetic expression in tall fescue

A decaploid tall fescue (Festuca arundinacea Schreb) genotype has been found which exhibits net photosynthetic rates of 32 to 41 mg CO₂/dm²·hour as opposed to a mean of 22 mg CO₂/dm²·hour for 10 hexaploid genotypes. The decaploid genotype exhibited a ribulose 1,5-bisphosphate (RuBP) carboxylase specific activity 1.3- to 2-fold higher than typical tall fescue genotypes. Specific activities of photorespiratory enzymes and nitrate reduction enzymes were lower in the decaploid than the hexaploid genotypes. Results suggest that genetic expression of RuBP carboxylase activity may have been altered to increase the net photosynthesis rate in the decaploid genotype.     

Translocation of photoassimilate from leaves of two popyploid genotypes of tall fescue differing in photosynthetic rates

The photosynthetic rate of a decaploid genotype (1-16-2) of tall fescue ( Festuca arundinacea Schreb.) is about twice that of a common hexaploid genotype (V6-802) (Plant Physiol. 72: 16–21, 1983). Translocation of photosynthate out of the leaves is a possible means of regulating carbon assimilation. To evaluate this possibility, we have examined a) translocation velocity, b) time course of translocation from leaves, c) photoassimilate partitioning pattern into whole plants in pulse and chase experiments, and d) interveinal distances between two ploidy genotypes. Most of the ¹⁴C accumulated in sucrose, and the labelled carbon moved down the leaf blades at similar velocities (6 to 10 cm h⁻¹) in both genotypes. Recent ¹⁴C assimilate was rapidly translocated from the fed area of the leaf blade. For example, the decaploid and the common hexaploid had translocated 40 and 26% of the ¹⁴C, respectively, at 6 h, and 79 and 49% of the ¹⁴C, respectively, at 24 h. Partitioning of ¹⁴C among plant organs was considerably different between the genotypes after a 24 h chase. For example, out of the total ¹⁴C recovered from the whole plant, the decaploid had retained 40% in the labelled leaf with 10, 33 and 29% in other leaves, stem bases and roots, respectively; whereas the hexaploid had retained 91% in the labelled leaf with 4, 3 and 2% in other leaves, stem bases and roots, respectively. However, the higher rate of translocation was correlated with greater interveinal distances in the decaploid genotype. These results suggested that the higher translocation percentage in the decaploid than the hexaploid genotype was due to greater sink activity.     

Photosynthesis in Polyploid Tall Fescue : II. PHOTOSYNTHESIS AND RIBULOSE-1, 5-BISPHOSPHATE CARBOXYLASE OF POLYPLOID TALL FESCUE

Net photosynthesis on a leaf area and leaf weight basis increased significantly with ploidy in a 4X, 6X, 8X and 10X allopolyploid series of tail fescue (Festuca arundinacea Schreb.). Total protein did not increase significantly with ploidy. Rocket immunoelectrophoresis was used to quantitate ribulose-1, 5-bisphosphate carboxylase (RuBPCase) protein. RuBPCase content, expressed on both a concentration basis and as a percentage of total protein increased significantly with ploidy in both field and greenhouse experiments. The range of RuBPCase content was 16 to 73% of total protein and 2.8 and 6.5 mg/ml of extract. Specific activity of RuBPCase did not increase significantly with ploidy. Chlorophyll concentration increased as a quadratic function of ploidy, with the mean for 8X genotypes representing maximal chlorophyll content. Evidence is presented that increasing concentrations of RuBPCase are associated with higher net photosynthesis rates in tall fescue. This suggests that RuBPCase may represent a marker for increased net photosynthesis. RuBPCase was extracted in a partially active state or inhibited state and must be fully activated by Mg²⁺ and HCO₃⁻ to measure maximal activities. Polyploidization appeared to increase selectively the allocation of total protein for synthesis of RuBPCase; however, there was also a range for carboxylase content among the genotypes within a given ploidy level.     

Photosynthesis in Fescue : III. RATES OF ELECTRON TRANSPORT IN A POLYPLOID SERIES OF TALL FESCUE PLANTS

Photosystem I electron transport activity has been found to be considerably higher in a decaploid tall fescue (Festuca arundinacea Schreb.) genotype as compared to a common hexaploid genotype. The decaploid genotype also displayed a higher photosystem whole chain (Photosystem II plus Photosystem I uncoupled) activity, suggesting a connection between polyploidy and increased electron transport activity. However, when a polyploidy series of tall fescue, ranging from diploid to decaploid with several different genetic isolates at each ploidy level, was examined in natural growth conditions, no effect of increasing genome content on electron transport and photophosphorylation was found. These results suggest that a gene component of one of the genomes involved may be responsible for the increased activity rather than simply the total chromosome content.     

Evidence for Mitochondrial Regulation of Photosynthesis by a Starchless Mutant of Nicotiana sylvestris

Mutant NS458 of Nicotiana sylvestris (Speg. et Comes) contains a defective plastid phosphoglucomutase and accumulates only trace amounts of starch. Determinations of carbon partitioning using tracer d-[3-¹⁴C]glyceric acid showed that the maximal CO₂ assimilation by mature leaves of the mutant at saturating [CO₂] and light and low [O₂] was close to the flux for sucrose formation in the wild type. The mutant is characterized by exceptionally slow oscillations in maximal CO₂ assimilation. The postulate that these slow oscillations follow changes in the cytosolic rate of sucrose phosphate synthesis has been investigated. Studies with wild-type and mutant leaf discs subjected to various treatments failed to indicate that any significant activation-inactivation cycle in sucrose-P synthase activity can occur. The rate of sucrose phosphate synthesis, however, might be altered by variations in the supply of uridine UDP-glucose which is controlled by the rate of ATP regeneration (via UTP regeneration). Treating mutant leaf protoplasts and young leaves with oligomycin, an inhibitor of mitochondrial ATP regeneration, reduced photosynthesis by as much as 25 and 40%, respectively. The wild type failed to show inhibition by oligomycin, i.e. its effect is masked when starch and sucrose synthesis can interact. It is concluded that maximal CO₂ assimilation in the mutant is fine tuned by mitochondrial metabolism such that interactions between sucrose synthesis and mitochondrial processes may generate the observed oscillations.     

Mechanism of Photosynthesis Decrease by Verticillium dahliae in Potato

Young, visually symptomless leaves from potato (Solanum tuberosum) plants infected with Verticillium dahliae exhibited reduced carbon assimilation rate, stomatal conductance, and intercellular CO₂, but no increase in dark respiration, no change in the relationship between carbon assimilation rate versus intercellular CO₂, and no change in light use efficiency when intercellular CO₂ was held constant. Therefore, the initial decrease in photosynthesis caused by V. dahliae was caused by stomatal closure. Errors in the intercellular CO₂ calculation caused by uneven distribution of carbon assimilation rate across the leaf were tested by ¹⁴CO₂ autoradiography. Patchiness was found at a low frequency. Low stomatal conductance was correlated with low leaf water potentials. Infection did not affect leaf osmotic potentials.     

Effects of the Fungal Endophyte Acremonium coenophialum on Nitrogen Accumulation and Metabolism in Tall Fescue

Infection by the fungal endophyte Acremonium coenophialum affected the accumulation of inorganic and organic N in leaf blades and leaf sheaths of KY 31 tall fescue (Festuca arundinacea Schreb.) grown under greenhouse conditions. Total soluble amino acid concentrations were increased in either the blade or sheath of the leaf from infected plants. A number of amino acids were significantly increased in the sheath, but only asparagine increased in the blade. Infection resulted in higher sheath NH₄⁺ concentrations, whereas NO₃⁻ concentrations decreased in both leaf parts. The effects on amino acid, NO₃⁻, and NH₄⁺ concentrations were dependent upon the level of N fertilization and were usually apparent only at the high rate (10 millimolar) of application. Administration of ¹⁴CO₂ to the leaf blades increased the accumulation of ¹⁴C in their amino acid fraction but not in the sheaths of infected plants. This may indicate that infection increased amino acid synthesis in the blade but that translocation to the sheath, which is the site of fungal colonization, was not affected. Glutamine synthetase activity was greater in leaf blades of infected plants at high and low N rates of fertilization, but nitrate reductase activity was not affected in either part of the leaf. Increased activities of glutamine synthetase together with the other observed changes in N accumulation and metabolism in endophyte-infected tall fescue suggest that NH₄⁺ reassimilation could also be affected in the leaf blade.     

Photosynthesis, photorespiration and productivity of wheat and soybean genotypes

The results of the numerous measurements obtained during the last 40 years on gas exchange rate, photosynthetic carbon metabolism by exposition in ¹⁴CO₂ and activities of primary carbon fixation enzyme, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBPC/O), in various wheat and soybean genotypes grown over a wide area in the field and contrasting in photosynthetic traits and productivity are presented in this article. It was established that high productive wheat genotypes (7–9 t ha^?1) with the optimal architectonics possess higher rate of CO₂ assimilation during the leaf ontogenesis. Along with the high rate of photosynthesis, high values of photorespiration are characteristic for the high productive genotypes. Genotypes with moderate (4–5 t ha^?1) and low (3 t ha^?1) grain yield are characterized by relatively low rates of both CO₂ assimilation and photorespiration. A value of photorespiration constitutes 28–35% of photosynthetic rate in contrasting genotypes. The activities of RuBPC and RuBPO were changing in a similar way in the course of the flag leaf and ear elements development. High productive genotypes are also characterized by a higher rate of biosynthesis and total value of glycine–serine and a higher photosynthetic rate. Therefore, contrary to conception arisen during many years on the wastefulness of photorespiration, taking into account the versatile investigations on different aspects of photorespiration, it was proved that photorespiration is one of the evolutionarily developed vital metabolic processes in plants and the attempts to reduce this process with the purpose of increasing the crop productivity are inconsistent.     

Photosynthetic CO(2) Fixation Products and Activities of Enzymes Related to Photosynthesis in Bermudagrass and Other Plants

After a 5-second exposure of illuminated bermudagrass (Cynodon dactylon L. var. `Coastal') leaves to ¹⁴CO₂, 84% of the incorporated ¹⁴C was recovered as aspartate and malate. After transfer from ¹⁴CO₂-air to ¹²CO₂-air under continuous illumination, total radioactivity decreased in aspartate, increased in 3-phosphoglyceric acid and alanine, and remained relatively constant in malate. Carbon atom 1 of alanine was labeled predominantly, which was interpreted to indicate that alanine was derived from 3-phosphoglyceric acid. The activity of phosphoenolpyruvate carboxylase, alkaline pyrophosphatase, adenylate kinase, pyruvate-phosphate dikinase, and malic enzyme in bermudagrass leaf extracts was distinctly higher than those in fescue (Festuca arundinacea Schreb.), a reductive pentose phosphate cycle plant. Assays of malic enzyme activity indicated that the decarboxylation of malate was favored. Both malic enzyme and NADP⁺-specific malic dehydrogenase activity were low in bermudagrass compared to sugarcane (Saccharum officinarum L.). The activities of NAD⁺-specific malic dehydrogenase and acidic pyrophosphatase in leaf extracts were similar among the plant species examined, irrespective of the predominant cycle of photosynthesis. Ribulose-1, 5-diphosphate carboxylase in C₄-dicarboxylic acid cycle plant leaf extracts was about 60%, on a chlorophyll basis, of that in reductive pentose phosphate cycle plants.     

Effect of Ploidy on Quality of Tall Fescue, Italian Ryegrass x Tall Fescue and Tall Fescue x Giant Fescue Hybrids

Selected quality parameters were measured for forage leaf tissuefrom a spaced-plant nursery. The genotypes used were Ky 31 tallfescue and hybrids of Italian ryegrass (Lolium multiflorum Lam.)x tall fescue (Festuca arundinacea Schreb.) and tall fescuex giant fescue [Fescue gigantea (L ) Vill.]. Hybrid ploidy rangedfrom 2n = 28 to 84 chromosomes. Forage quality was characterizedby neutral detergent fibre (NDF), acid detergent fibre (ADF),total soluble carbohydrates (TSC) nutritive value index (NVI),hemicellulose, and in vitro dry matter disappearance (DMD). Quality of tall fescue, as measured by increased DMD, was improvedby hybridization with giant fescue. Improved DMD and NVI correlatedwith lower NDF and ADF in the hybrids. A few hybrids of Italianryegrass x tall fescue (2n = 28) were higher in some qualityparameters than Ky 31. Tall fescue x giant fescue hybrids (2n= 80 to 84), as a group, had significant quality improvementover Ky 31 in higher DMD and NVI and lower NDF and ADF. Whilesome individual hybrids within each group were significantlyhigher in quality, only the 2n = 80 to 84 chromosome group wasconsistently higher than Ky 31. Prediction equations for DMD,NDF, and ADF were established based on solvent extraction withnear-infrared reflectance spectroscopy (NIRS). Linear correlationcoefficients between chemical measurement and NIRS for eachquality parameter were 0–95 or higher. Acid detergent fibre, neutral detergent fibre, dry matter disappearance, hemicellulose, nutritive value index, Festuca arundinacea, Festuca gigantea, Lolium multiflorum     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: IV. ANALYSIS OF REDUCED OXYGEN RESPONSE IN PANICUM MILIOIDES AND PANICUM SCHENCKII

Reduced photorespiration has been reported in Panicum milioides on the basis of lower CO₂ compensation concentrations than in C₃ species, lower CO₂ evolution in the light, and less response of apparent photosynthesis to O₂ concentration. The lower response to O₂ in P. milioides could be due to reduced O₂ competition with CO₂ for reaction with ribulose 1,5-bisphosphate, to a reduced loss of CO₂, or to an initial fixation of CO₂ by phosphoenolpyruvate carboxylase. Experiments were carried out with Panicum maximum Jacq., a C₄ species having no apparent photorespiration; tall fescue (Festuca arundinacea Schreb.), a C₃ species; P. milioides Nees ex Trin.; and Panicum schenckii Hack. The latter two species are closely related and have low photorespiration rates. CO₂ exchange was measured at five CO₂ concentrations ranging from 0 to 260 microliters per liter at both 2 and 21% O₂. Mesophyll conductance or carboxylation efficiency was estimated by plotting substomatal CO₂ concentrations against apparent photosynthesis. In the C₄ species P. maximum, mesophyll conductance was 0.96 centimeters per second and was unaffected by O₂ concentration. At 21% O₂ mesophyll conductance of tall fescue was decreased 32% below the value at 2% O₂. Decreases in mesophyll conductance at 21% O₂ for P. milioides and P. schenckii were similar to that for tall fescue. On the other hand, loss of CO₂ in CO₂-free air, estimated by extrapolating the CO₂ response curve to zero CO₂, was increased from 1.8 to 6.5 milligrams per square decimeter per hour in tall fescue as O₂ was raised from 2-21%. Loss of CO₂ was less than 1 milligram per square decimeter per hour for P. milioides and P. schenckii and was unaffected by O₂. The results suggest that the reduced O₂ response in P. milioides and P. schenckii is due to a lower loss of CO₂ in the light rather than less inhibition of carboxylation by O₂, since the decrease in carboxylation efficiency at 21% O₂ was similar for P. milioides, P. schenckii, and tall fescue. The inhibition of apparent photosynthesis by 21% O₂ in these three species at low light intensities was similar at 31 to 36% which also indicates similar O₂ effects on carboxylation. Apparent photosynthesis at high light intensity was inhibited less by 21% O₂ in P. milioides (16.8%) and P. schenckii (23.8%) than in tall fescue (28.4%). This lower inhibition in the Panicum species may have been due to a higher degree of recycling of photorespired CO₂ in these species than in tall fescue.     

Photorespiration and Oxygen Inhibition of Photosynthesis in Chlorella pyrenoidosa

The inhibition of photosynthesis by O₂ in air-grown Chlorella pyrenoidosa was investigated using three experimental techniques (artificial leaf, aqueous method, and O₂ electrode) to measure carbon assimilation. CO₂ response curves were determined under different O₂, pH, and temperature conditions. Regardless of the experimental technique and condition, O₂ inhibition was not evident until a concentration of 50% was reached; V_max values were reduced whereas K_m (CO₂) values were unaffected by the increasing O₂ concentration. The response of photosynthesis to O₂ was independent of CO₂ and HCO₃⁻ concentrations as well as temperature. Relative rates of photosynthesis showed a 4 to 5% stimulation in 2% O₂, a 12% inhibition in 50% O₂, and a 24% inhibition in 100% O₂. The inhibition by 50% O₂ was still reversible after 20 minutes exposure whereas 100% O₂ caused irreversible inhibition after only 4 minutes.     

Utilization of Exogenous Inorganic Carbon Species in Photosynthesis by Chlorella pyrenoidosa

The nature of the inorganic carbon utilized during photosynthesis by Chlorella pyrenoidosa was investigated using three experimental techniques (open gas analysis system with “artificial leaf” or “aqueous” chambers and O₂ electrode system) to measure carbon assimilation. Photosynthesis was studied as a function of pH and CO₂ concentration. The CO₂ concentration was inadequate to meet the requirements of photosynthesis only when HCO₃⁻ was added at high pH. Under all other conditions, the low and constant K_m (CO₂), in contrast to the highly variable K_m (HCO₃⁻), suggested that CO₂ was the major species utilized.     

Oxalate metabolism by tobacco leaf discs

The turnover rate of oxalate in leaf discs of Nicotiana tabacum, var Havana Seed, during photosynthesis was estimated to be 1 to 2 micromoles per gram fresh weight per hour. Radioactivity from the enzymic oxidation of [¹⁴C]oxalate rapidly appeared in neutral sugars (mainly sucrose), organic acids (mainly malate), and amino acids. Only 5% of the radioactivity was released to the atmosphere as ¹⁴CO₂, and no formate or formaldehyde could be detected. The metabolism of oxalate was not increased by raising the O₂ concentration from 1% to 21% to 60%, nor was the formation of [¹⁴C]oxalate from [2-¹⁴C]glyoxylate changed under the same conditions as was previously observed in vitro (Havir 1983 Plant Physiol 71: 874-878). While oxalate is not an inert end product of the glycolate pathway, it contributes little to the formation of photorespiratory CO₂.     

Ureide Catabolism in Soybeans : III. Ureidoglycolate Amidohydrolase and Allantoate Amidohydrolase Are Activities of an Allantoate Degrading Enzyme Complex

We demonstrate that allantoate is catabolized in soybean seedcoat extracts by an enzyme complex that has allantoate amidohydrolase and ureidoglycolate amidohydrolase activities. Soybean seedcoat extracts released ¹⁴CO₂ from [ureido-¹⁴C]ureidoglycolate under conditions in which urease is not detectable. CO₂ and glyoxylate are enzymically released in a one to one ratio indicating that ureidoglycolate amidohydrolase is the responsible activity. Ureidoglycolate amidohydrolase has a K_m of 85 micromolar for ureidoglycolate. Glyoxylate and CO₂ are enzymically released from allantoate at linear rates in a one to 2.3 ratio from 5 to 30 min. This ratio is consistent with the degradation of allantoate to two CO₂ and one glyoxylate with approximately 23% of the allantoate degraded reacting with 2-mercaptoethanol to yield 2-hydroxyethylthio, 2′-ureido, acetate (RG Winkler, JC Polacco, DG Blevins, DD Randall 1985 Plant Physiol 79: 787-793). That [¹⁴C]urea production from [2,7-¹⁴C]allantoate is not detectable indicates that allantoate-dependent glyoxylate production is enzymic and not a result of nonenzymic hydrolysis of a ureido intermediate (nonenzymic hydrolysis releases urea). These results and those from intact tissue studies (RG Winkler DG Blevins, JC Polacco, DD Randall 1987 Plant Physiol 83: 585-591) suggest that soybeans have a second amidohydrolase reaction (ureidoglycolate amidohydrolase) that follows allantoate amidohydrolase in allantoate catabolism. The rate of ¹⁴CO₂ release from [2,7-¹⁴C]allantoate is not reduced when the volume of the reaction mixture is increased, suggesting that the release of ¹⁴CO₂ is not dependent on the accumulation of free intermediates. That [2,7-¹⁴C]allantoate dependent ¹⁴CO₂ release is not proportionally diluted by unlabeled ureidoglycolate indicates that the reaction is carried out by an enzyme complex. This is the first report of ureidoglycolate amidohydrolase activity in any organism and the first in vitro demonstration in plants that the ureido-carbons of allantoate can be completely degraded to CO₂ without a urea intermediate.     

Formation of C-Labeled Alanine from Pyruvate during Short Term Photosynthesis in a C(4) Plant

Large amounts of alanine are produced in the first few seconds of photosynthesis in Portulaca oleracea L. The normal precursor-product relationship (phosphoglyceric acid → pyruvate → alanine) does not appear to operate in this species since labeling in pyruvate precedes that in phosphoglyceric acid. Pulse-chase experiments show that the alanine is rapidly metabolized. After a 6-second pulse of ¹⁴CO₂, the percentage of ¹¹C in alanine drops more than 30% in the first 10 seconds of a ¹²CO₂ chase period. The percentage of ¹⁴C in the other early-labeled photosynthetic products, aspartate and malate, also decreases during the ¹²CO₂ chase. The decrease of label in these compounds is concomitant with an increase in the labeling of sucrose and alanine, which in this case is formed via phosphoglyceric acid. Randomization of label within alanine increases gradually throughout the 2-minute chase.     

Photosynthesis by protoplasm extruded from Chara and Nitella

(a) Photosynthesis with protoplasm isolated from Chara or Nitella as measured by C¹⁴ fixation has been obtained at a rate 12 to 15 per cent of that of the whole cells. (b) Photosynthesis by cut cells of Chara or Nitella with the vacuolar sap removed was at a rate comparable to that of the whole cells. (c) Both the protoplasm and the cut cells reduced CO₂ in the light to sucrose and hexose phosphates. Other products formed were also detected by paper chromatography. In contrast, dark controls fixed the C¹⁴ into products associated with plant respiration. (d) An important difference in the products from the extruded protoplasm was the absence of C¹⁴-labelled pentoses or sedoheptulose which were formed, however, by the whole or cut cells. This suggests that the most sensitive site affected by disruption of the cells may be the steps involved in the regeneration of the "C-2 acceptor" for CO₂ fixation in photosynthesis.     

Mitochondrial Respiration Can Support NO(3) and NO(2) Reduction during Photosynthesis : Interactions between Photosynthesis, Respiration, and N Assimilation in the N-Limited Green Alga Selenastrum minutum

Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH₄⁺, NO₂⁻, NO₃⁻) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO₂ release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH₄⁺ assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O₂ consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO₃⁻ and NO₂⁻ assimilation resulted in a larger stimulation of TCA cycle CO₂ release than did NH₄⁺, but a much smaller stimulation of mitochondrial O₂ consumption. NH₄⁺ assimilation was the same in the light and dark and insensitive to DCMU, but was 82% inhibited by anaerobiosis in both the light and dark. NO₃⁻ and NO₂⁻ assimilation rates were maximal in the light, but assimilation could proceed at substantial rates in the light in the presence of DCMU and in the dark. Unlike NH₄⁺, NO₃⁻ and NO₂⁻ assimilation were relatively insensitive to anaerobiosis. These results indicated that operation of the mitochondrial electron transport chain was not required to maintain TCA cycle activity during NO₃⁻ and NO₂⁻ assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O₂ consumption during NO₃⁻ and NO₂⁻ assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO₃⁻ and NO₂⁻ reduction.     

Photosynthesis, reserve mobilization and enzymes of sucrose metabolism in soybean (Glycine max) cotyledons

Soybean (Glycine max L. [Merr] cv. Ransom II) seedlings were grown under a light/ dark regime or in continuous darkness. Cotyledons were harvested daily for measurements of reserve mobilization, net carbon exchange rate, chlorophyll content and activities of certain enzymes involved in sucrose metabolism. Seedlings lost dry weight for the first 3 to 4 days after planting, then maintained a constant dry weight in the etiolated seedlings, and gained dry weight (via net fixation of CO₂) in the light-grown seedlings. In general, the patterns of reserve mobilization were as expected based on the collective work of other investigators. Soluble sugars were mobilized first, followed by protein and lipid. Galactinol, previously uncharacterized in soybean cotyledons, was present at low concentrations and was rapidly depleted within 2 days after planting. Mobilization of reserves was most important during the first 8 days after planting, whereas net cotyledonary photosynthesis began at 6 days after planting and was the primary source of assimilates after 8 days. Maximum rates of cotyledon photosynthesis were higher [up to 18 mg CO₂ (g dry weight)^?1 h^?1] than previously reported and accounted for about 75% of the assimilates transported from the cotyledons to the growing seedling during the functional life of the cotyledon. Enzyme activities in light-grown cotyledons peaked 7 to 10 days after planting and then declined. Sucrose phosphate synthase (EC 2.4.1.14) and sucrose synthase (EC 2.4.1.13) activities were similar in etiolated and light-grown seedlings, whereas uridine-5′-di-phosphatase (EC 3.6.1.6) activity was substantially higher in light-grown seedlings. During the period of reserve mobilization, the maximum sucrose phosphate synthase activity in cotyledonary extracts was in excess of the calculated rate of sucrose formation. However, when the cotyledons had highest net photosynthetic rates (14 days after planting), sucrose phosphate synthase activity was similar to the rate of carbon assimilation. It appears that soybean cotyledons are adapted for high rates of sucrose formation (from reserve mobilization and/or photosynthesis) for export to the rapidly growing tissues of the seedling.