Effect of Photoperiod on Photosynthate Partitioning and Diurnal Rhythms in Sucrose Phosphate Synthase Activity in Leaves of Soybean (Glycine max L. [Merr.]) and Tobacco (Nicotiana tabacum L.)

Studies were conducted to identify the existence of diurnal rhythms in sucrose phosphate synthase (SPS) activity in leaves of three soybean (Glycine max L. [Merr.]) and two tobacco (Nicotiana tabacum L.) cultivars and the effect of photoperiod (15 versus 7 hours) on carbohydrate partitioning and the rhythm in enzyme activity. Acclimation of all the genotypes tested to a short day (7 hours) photoperiod resulted in increased rates of starch accumulation, whereas rates of translocation, foliar sucrose concentrations, and activities of SPS were decreased relative to plants acclimated to long days (15 hours). Under the long day photoperiod, two of the three soybean cultivars (`Ransom' and `Jupiter') and one of the two tobacco cultivars (`22NF') studied exhibited a significant diurnal rhythm in SPS activity. With the soybean cultivars, acclimation to short days reduced the activity of SPS (leaf fresh weight basis) and tended to dampen the amplitude of the rhythm. With the tobacco cultivars, photoperiod affected the shape of the SPS-activity rhythm. The mean values for SPS activity (calculated from observations made during the light period) were correlated positively with translocation rates and were correlated negatively with starch accumulation rates. Overall, the results support the postulate that SPS activity is closely associated with starch/sucrose levels in leaves, and that acclimation to changes in photoperiod may be associated with changes in the activity of SPS.     

DNA,RNA, AND PROTEIN COMPARISONS BETWEEN NODULATED AND NON-NODULATED MALE-STERILE AND MALE-FERTILE GENOTYPES OF SOYBEAN (GLYCINE MAX L.)

Four soybean (Glycine max L. Merr.) lines isogenic except for loci controlling male sterility (ms₁) and nodulation (rj₁) were developed to study the effects of reproductive development and nitrogen source on the nucleic acid and protein levels within the leaves. Changes in DNA, RNA, protein, and cellular viability were measured from flowering (77 days after emergence) until maturity (147 days after emergence) in leaves of nodulated and non-nodulated male-sterile and fertile soybean genotypes. Leaf nuclei from the sterile genotypes yielded DNA amounts that were significantly higher than those from the fertile lines. The average DNA values for the nodulated sterile and nodulated fertile lines at 147 days after emergence were 7.01 and 2.45 picograms, respectively. The average 2C DNA amount as determined from dividing root-tip nuclei was 2.83 picograms, which indicated occurrence of endopolyploid mechanisms in the sterile lines and age-related loss of DNA in fertile lines. Similar to DNA findings, the RNA and protein values in the sterile lines were significantly higher than those values observed in the fertile lines, suggesting an increased capacity to synthesize protein. The soybean leaf nuclear DNA declined, especially in the fertile lines in terms of the percent endopolyploid nuclei as well as the average DNA content during maturation. The DNA decline in leaves of fertile genotypes suggests that the leaves may be exporting nucleosides and phosphates to the seeds during embryo formation. In the sterile lines, due to the reduced pod-set, these ready reserves of nucleosides and phosphates tended to accumulate in the chromatin of the leaf nucleus as manifested by the DNA specific Feulgen stain. By the end of the study (147 days after emergence), the nodulated fertile genotypes had experienced a dramatic loss in DNA, RNA, and protein. The nodulated sterile genotypes, however, indicated 65% more DNA, 59% more RNA, and 53% more protein as compared to the nodulated fertile genotypes at 147 days after emergence. The sterile lines also indicated the slowest increase in the death of cells, while the fertile lines indicated the fastest increase in nonviable cells, as shown by trypan blue staining. The fertile lines displayed normal monocarpic senescence throughout the study. The reproductive structures of fertile plants utilized the molecules in seed production, whereas in the sterile lines, these accumulated in leaf cells.     

Biochemical Basis for Partitioning of Photosynthetically Fixed Carbon between Starch and Sucrose in Soybean (Glycine max Merr.) Leaves

The control of photosynthetic starch/sucrose formation in leaves of soybean (Glycine max L. Merr.) cultivars was studied in relation to stage of plant development, photosynthetic photoperiod, and nitrogen source. At each sampling, leaf tissue was analyzed for starch content, activities of sucrose-metabolizing enzymes, and labeling of starch and sucrose (by ¹⁴CO₂ assimilation) in isolated cells. In three of the four varieties tested, nodulated plants had lower leaf starch levels and higher activities of sucrose phosphate synthetase (SPS), and isolated mesophyll cells incorporated more carbon (percentage of total ¹⁴CO₂ fixed) into sucrose and less into starch as compared to nonnodulated (nitrate-dependent) plants. The variation among cultivars and nitrogen treatments observed in the activity of SPS in leaf extracts was positively correlated with labeling of sucrose in isolated cells (r = 0.81) and negatively correlated with whole leaf starch content (r = −0.66). The results suggested that increased demand for assimilates by nodulated roots may be accommodated by greater partitioning of carbon into sucrose in the mesophyll cells. We have also confirmed the earlier report (Chatterton, Silvius 1979 Plant Physiol 64: 749-753) that photoperiod affects partitioning of fixed carbon into starch. Within two days of transfer of nodulated soybean Ransom plants from a 14-hour to a 7-hour photoperiod, leaf starch accumulation rates doubled, and this effect was associated with increased labeling of starch and decreased labeling of sucrose in isolated cells. Concurrently, activities of SPS, sucrose synthase, and uridine diphosphatase in leaves were decreased.     

Studies on genetic male-sterile soybeans : v. Effects of male-sterility on the function and glycerolipid composition of chloroplast thylakoids

Soybean (Glycine max L. Merr.) germplasm, isogenic except for loci controlling male sterility (ms₁), was utilized to study the effects of reproductive development on certain aspects of photosynthesis. Plants were sampled at various times between flowering (77 days after transplanting) and maturity (147 days after transplanting). During that period photosynthetic rates declined more rapidly in the male-sterile genotypes than male-fertile genotypes; and after 105 days, the sterile genotypes maintained low but relatively constant carbon exchange rates. The decline of leaf photosynthesis in the male-sterile genotype occurred concomitantly with an inhibition of the photosynthetic electron transport chain associated with photosystem II. Changes in photosystem I activities, cytochrome f levels, and chlorophyll a/b ratios per se were not responsible for the decline in whole leaf photosynthesis. These conditions were independent of the source of nitrogen nutrition. Lipid analyses of the thylakoids revealed that a loss of phosphatidylglycerol was highly correlated with the inhibition of photosystem II activity. These results suggested a relation between the decline in leaf carbon exchange and the decline in photosynthetic electron transport activity.     

Studies on Genetic Male-Sterile Soybeans : IV. Effect of Male Sterility and Source of Nitrogen Nutrition on Accumulation, Partitioning, and Transport of Nitrogen

Soybean (Glycine max [L.] Merr.) germplasm, isogenic except for loci controlling male sterility (ms₁) and nodulation (rj₁), was used to investigate the effects of reproductive tissue development and source of nitrogen nutrition on accumulation, transport, and partitioning of nitrogen in a greenhouse experiment. Nodulated plants were supplied nitrogen-free nutrient solution, and nonnodulated plants were supplied nutrient solution containing 20 millimolar KNO₃. Plants were sampled from flowering until maturity (77 to 147 days after transplanting).

Accumulation rates of nitrogen in whole plants during reproductive growth were not significantly different among the four plant types. Nitrogen accumulation in the sterile, nonnodulated plants, however, ceased 2 weeks earlier than in fertile, nonnodulated or fertile and sterile, nodulated plants. This early cessation in nitrogen accumulation resulted in sterile, nonnodulated plants accumulating significantly less whole plant nitrogen by 133 days after transplanting (DAT) than fertile, nonnodulated plants. Thus, changing the site of nitrogen assimilation from nodules (N₂-fixing plants) to roots and leaves (NO₃-fed plants) resulted in similar whole-plant nitrogen accumulation rates in fertile and sterile plants, despite the absence of seed in the latter.

Leaflet and stem plus petiole tissues of both types of sterile plants had significantly higher nitrogen concentrations after 119 DAT than both types of fertile plants. Significantly higher concentrations and exudation rates of nonureide, reduced-nitrogen in xylem sap of sterile than of fertile plants after 105 DAT were observed. These latter results indicated possible cycling of nonureide, reduced-nitrogen from the downward phloem translocation stream to the upward xylem translocation stream in roots of sterile plants. Collectively, these results suggest a lack of sinks for nitrogen utilization in the shoots of sterile plants. Hence, comparison of nitrogen accumulation rates for sterile and fertile plants does not provide a definitive test of the hypothesis that reproductive tissue development limits photosynthate availability for support of N₂ fixation and nitrate assimilation in determinate soybeans.

Nitrogen assimilation during reproductive growth met a larger proportion of the reproductive-tissue nitrogen requirement of nitrate-dependent plants (73%) than of N₂-fixing plants (63%). Hence, vegetative-tissue nitrogen mobilization to reproductive tissue was a more prominent process in N₂-fixing than in nitrate-dependent plants. N₂-fixing plants partitioned nitrogen to reproductive tissue more efficiently than nitrate-dependent plants as the reproductive tissues of the former and latter contained 65 and 55%, respectively, of the whole-plant nitrogen at the time that nitrogen accumulation in reproductive parts had ceased (133 DAT).

     

Up-regulation of sucrose phosphate synthase in rice grown under elevated CO2 and temperature

Rice (Oryza sativa L. cv. IR-30) was grown season-long in outdoor, controlled-environment chambers at 33 Pa CO₂ with day/night/paddy-water temperatures of 28/21/25 °C, and at 66 Pa CO₂ with five different day/night/paddy-water temperature regimes (25/18/21, 28/21/25, 31/24/28, 34/27/31 and 37/30/34 °C). Sucrose phosphate synthase (SPS) activities in leaf extracts at 21, 48 and 81 days after planting (DAP) were assayed under saturating and selective (limiting) conditions. Diel SPS activity data indicated that rice SPS was light regulated; with up to 2.2-fold higher rates during the day. Throughout the growth season, leaf SPS activities were up-regulated in the CO₂-enriched plants, averaging 20 and 12% higher than in ambient-CO₂ grown plants in selective and saturating assays, respectively. Similarly, SPS activities increased 2.4% for each 1 °C rise in growth temperature from 25 to 34 °C, but de creased 11.5% at 37 °C. Leaf sucrose content was higher, and mirrored SPS activity better, than starch, although starch was more responsive to CO₂ treatment. Leaf sucrose and starch contents were significantly higher throughout the season in plants at elevated CO₂, but the N content averaged 6.5% lower. Increasing growth temperatures from 25 to 37 °C caused a linear decrease (62%) in leaf starch content, but not in sucrose. Consequently, the starch:sucrose ratio declined with growth temperature. The data are consistent with the hypothesis that the up-regulation of leaf SPS may be an acclimation response of rice to optimize the utilization and export of organic-C with the increased rates of inorganic-C fixation in elevated CO₂ or temperature growth regimes.     

Characterization of diurnal changes in activities of enzymes involved in sucrose biosynthesis

Experiments were conducted with vegetative soybean plants (Glycine max [L.] Merr., `Ransom') to determine whether the activities in leaf extracts of key enzymes in sucrose metabolism changed during the daily light/dark cycle. The activity of sucrose-phosphate synthase (SPS) exhibited a distinct diurnal rhythm, whereas the activities of UDP-glucose pyrophosphorylase, cytoplasmic fructose-1,6-bisphosphatase, and sucrose synthase did not. The changes in extractable SPS activity were not related directly to photosynthetic rates or light/dark changes. Hence, it was postulated that the oscillations were under the control of an endogenous clock. During the light period, the activity of SPS was similar to the estimated rate of sucrose formation. In the dark, however, SPS activity declined sharply and then increased even though degradation of starch was linear. The activity of SPS always exceeded the estimated maximum rate of sucrose formation in the dark. Transfer of plants into light during the normal dark period (when SPS activity was low) resulted in increased partitioning of photosynthate into starch compared to partitioning observed during the normal light period. These results were consistent with the hypothesis that SPS activity in situ was a factor regulating the rate of sucrose synthesis and partitioning of fixed carbon between starch and sucrose in the light.     

Up-regulation of sucrose metabolizing enzymes in Oncidium goldiana grown under elevated carbon dioxide

Experiments were conducted in controlled growth chambers to evaluate how increase in CO₂ concentration affected sucrose metabolizing enzymes, especially sucrose phosphate synthase (SPS; EC 2.4.1.14) and sucrose synthase (SS; EC 2.4.1.13), as well as carbon metabolism and partitioning in a tropical epiphytic orchid species (Oncidium goldiana). Response of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) to elevated CO₂ was determined along with dry mass production, photosynthesis rate, chlorophyll content, total nitrogen and total soluble protein content. After 60 days of growth, there was a 80% and 150% increase in dry mass production in plants grown at 750 and 1 100 μl l^?1 CO₂, respectively, compared with those grown at ambient CO₂ (about 370 μl l^?1). A similar increase in photosynthesis rate was detected throughout the growth period when measured under growth CO₂ conditions. Concomitantly, there was a decline in leaf Rubisco activity in plants in elevated CO₂ after 10 days of growth. Over the growth period, leaf SPS and SS activities were up‐regulated by an average of 20% and 40% for plants grown at 750 and 1100 μl l^?1 CO₂, respectively. Leaf sucrose content and starch content were significantly higher throughout the growth period in plants grown at elevated CO₂ than those at ambient CO₂. The partitioning of photosynthetically fixed carbon between sucrose and starch appeared to be unaffected by the 750 μl l^?1 CO₂ treatment, but it was favored into starch under the 1 100 μl l^?1 CO₂ condition. The activities of SPS and SS in leaf extracts were closely associated with photosynthetic rates and with partitioning of carbon between starch and sucrose in leaves. The data are consistent with the hypothesis that the up‐regulation of leaf SPS and SS might be an acclimation response to optimize the utilization and export of organic carbon with the increased rate of inorganic‐carbon fixation in elevated CO₂ conditions.     

Sucrose Phosphate Synthase and Acid Invertase as Determinants of Sucrose Concentration in Developing Muskmelon (Cucumis melo L.) Fruits

Fruits of orange-fleshed and green-fleshed muskmelon (Cucumis melo L.) were harvested at different times throughout development to evaluate changes in metabolism which lead to sucrose accumulation, and to determine the basis of differences in fruit sucrose accumulation among genotypes. Concentrations of sucrose, raffinose saccharides, hexoses and starch, as well as activities of the sucrose metabolizing enzymes sucrose phosphate synthase (SPS) (EC 2.4.1.14), sucrose synthase (EC 2.4.1.13), and acid and neutral invertases (EC 3.2.1.26) were measured. Sucrose synthase and neutral invertase activities were relatively low (1.7 ± 0.3 micromole per hour per gram fresh weight and 2.2 ± 0.2, respectively) and changed little throughout fruit development. Acid invertase activity decreased during fruit development, (from as high as 40 micromoles per hour per gram fresh weight) in unripe fruit, to undetectable activity in mature, ripened fruits, while SPS activity in the fruit increased (from 7 micromoles per hour per gram fresh weight) to as high as 32 micromoles per hour per gram fresh weight. Genotypes which accumulated different amounts of sucrose had similar acid invertase activity but differed in SPS activity. Our results indicate that both acid invertase and SPS are determinants of sucrose accumulation in melon fruit. However, the decline in acid invertase appears to be a normal function of fruit maturation, and is not the primary factor which determines sucrose accumulation. Rather, the capacity for sucrose synthesis, reflected in the activity of SPS, appears to determine sucrose accumulation, which is an important component of fruit quality.     

Effects of gibberellic acid on sucrose accumulation and sucrose biosynthesizing enzymes activity during banana ripening

During banana ripening there is a massive conversion into sugars, mainly sucrose, which can account for more than 10% of the fresh weight of the fruit. An ethylene burst is the trigger of the banana ripening process but there is evidence that other compounds can act as modulators of some biochemical pathways. As previously demonstrated, gibberellic acid (GA₃) can impair the onset of starch degradation and affect some degradative enzymes, but effects on the sucrose biosynthetic apparatus have not yet been elucidated. Here, the activity and amount of sucrose synthase (SuSy; E.C. 2.4.1.13) and sucrose–phosphate synthase (SPS; E.C. 2.4.1.14), respiration rates, ethylene production, and carbohydrate levels, were evaluated in GA₃-infiltrated and non-infiltrated banana slices. The exogenous supply of gibberellin did not alter the respiration or the ethylene profile but delayed sucrose accumulation by at least 2 days. While SuSy activity was similar in control and treated slices, SPS increase and sucrose accumulation was related in treated slices. Western blotting with specific antiserum showed no apparent effects of GA₃ on the amount of SuSy protein, but impaired the increase in SPS protein during ripening. The overall results indicate that although GA₃ did not block carbohydrate mobilisation in a irreversibly way, it clearly affected the triggering of starch breakdown and sucrose synthesis. Also, the delayed sucrose accumulation in GA₃-infiltrated slices could be explained by the disturbance of SPS activity. In conclusion, gibberellins can play an important role during banana ripening and our results also reinforce the idea of multiple regulatory components in the ripening pathway, as evidenced by the GA₃ effects.     

Relationships between Carbon Assimilation, Partitioning, and Export in Leaves of Two Soybean Cultivars

To evaluate leaf carbon balance during rapid pod-fill in soybean (Glycine max [L.] Merrill), measurements were made of CO₂ assimilation at mid-day and changes in specific leaf weight, starch, and sucrose concentrations over a 9-hour interval. Assimilate export was estimated from CO₂ assimilation and leaf dry matter accumulation. Chamber-grown `Amsoy 71' and `Wells' plants were subjected on the day of the measurements to one of six photosynthetic photon flux densities in order to vary CO₂ assimilation rates.

Rate of accumulation of leaf dry matter and rate of export both increased as CO₂ assimilation rate increased in each cultivar.

Starch concentrations were greater in Amsoy 71 than in Wells at all CO₂ assimilation rates. At low CO₂ assimilation rates, export rates in Amsoy 71 were maintained in excess of 1.0 milligram CH₂O per square decimeter leaf area per hour at the expense of leaf reserves. In Wells, however, export rate continued to decline with decreasing CO₂ assimilation rate. The low leaf starch concentration in Wells at low CO₂ assimilation rates may have limited export by limiting carbon from starch remobilization.

Both cultivars exhibited positive correlations between CO₂ assimilation rate and sucrose concentration, and between sucrose concentration and export rate. Carbon fixation and carbon partitioning both influenced export rate via effects on sucrose concentration.

     

Decrease in leaf sucrose synthesis leads to increased leaf starch turnover and decreased RuBP regeneration-limited photosynthesis but not Rubisco-limited photosynthesis in Arabidopsis null mutants of SPSA1

We investigated the individual effect of null mutations of each of the four sucrose‐phosphate synthase (SPS) genes in Arabidopsis (SPSA1, SPSA2, SPSB and SPSC) on photosynthesis and carbon partitioning. Null mutants spsa1 and spsc led to decreases in maximum SPS activity in leaves by 80 and 13%, respectively, whereas null mutants spsa2 and spsb had no significant effect. Consistently, isoform‐specific antibodies detected only the SPSA1 and SPSC proteins in leaf extracts. Leaf photosynthesis at ambient [CO₂] was not different among the genotypes but was 20% lower in spsa1 mutants when measured under saturating [CO₂] levels. Carbon partitioning at ambient [CO₂] was altered only in the spsa1 null mutant. Cold treatment of plants (4 °C for 96 h) increased leaf soluble sugars and starch and increased the leaf content of SPSA1 and SPSC proteins twofold to threefold, and of the four null mutants, only spsa1 reduced leaf non‐structural carbohydrate accumulation in response to cold treatment. It is concluded that SPSA1 plays a major role in photosynthetic sucrose synthesis in Arabidopsis leaves, and decreases in leaf SPS activity lead to increased starch synthesis and starch turnover and decreased Ribulose 1,5‐bisphosphate regeneration‐limited photosynthesis but not ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco)‐limited photosynthesis, indicating a limitation of triose‐phosphate utilization (TPU).     

Effect of N-source on soybean leaf sucrose phosphate synthase, starch formation, and whole plant growth

Soybeans (Glycine max L. Merr. cv Tracy and Ransom) were grown under N₂-dependent or NO₃⁻-supplied conditions, and the partitioning of photosynthate and dry matter was characterized. Although no treatment effects on photosynthetic rates were observed, NO₃⁻-supplied plants in both cultivars had lower starch accumulation rates than N₂-dependent plants. Leaf extracts of NO₃⁻-supplied plants had higher activities of sucrose phosphate synthase (SPS) and cytoplasmic fructose-1,6-bisphosphatase (FBPase) than N₂-dependent plants. The variation in starch accumulation was correlated negatively with the activity of SPS, but not the activity of FBPase, UDP-glucose pyrophosphorylase, or ADP-glucose pyrophosphorylase. These results suggested that starch accumulation is biochemically controlled, in part, by the activity of SPS. Leaf starch content at the beginning of the photoperiod was lower in NO₃⁻-supplied plants than N₂-dependent plants in both cultivars which suggested that net starch utilization as well as accumulation was affected by N source.

Total dry matter accumulation and dry matter distribution was affected by N source in both cultivars, but the cultivars differed in how dry matter was partitioned between the shoot and root as well as within the shoot. The activity of SPS was correlated positively with total dry matter accumulation which suggested that SPS activity is related to plant growth rate. The results suggested that photosynthate partitioning is an important but not an exclusive factor which determines whole plant dry matter distribution.

     

Changes in Starch Formation and Activities of Sucrose Phosphate Synthase and Cytoplasmic Fructose-1,6-bisphosphatase in Response to Source-Sink Alterations

Short term experiments were conducted with vegetative soybean plants (Glycine max L. Merr. `Ransom' or `Arksoy') to determine whether sourcesink manipulations, which rapidly changed the `demand' for sucrose and partitioning of photosynthetically fixed carbon into starch, were associated with alterations in activities of sucrose-P synthase and/or cytoplasmic fructose-1,6-bisphosphatase in leaf extracts. When demand for sucrose from a particular source leaf was increased by defoliation of other source leaves, starch accumulation was restricted and activities of both enzymes were markedly enhanced. When demand for sucrose from source leaves was limited by excision, starch accumulation in the detached leaves was increased while activity of sucrose-P synthase declined sharply. The consistent responsiveness of sucrose-P synthase activity to changes in demand for sucrose supports the contention that regulation of sucrose-P synthase is an integral component of the system which controls sucrose biosynthesis and partitioning of carbon between starch and sucrose biosynthesis in the light.     

Relations between carbohydrate accumulation in leaves, sucrose phosphate synthase activity and photoassimilate transport in chilling treated maize seedlings

Sucrose and starch concentration, sucrose phosphate synthase (SPS) activity in leaves, and long distance transport were studied in maize seedlings treated with moderate chilling (14 °C/12 °C - day/night). Two inbred lines were tested: chilling-tolerant KW1074 and chilling-sensitive CM109. Seedlings were grown in phytotrone on water nutrient until the 4-th leaf appearance. The estimations were done on fully developed 2-nd leaf. Six days after the temperature was lowered, leaves of line KW 1074 plants contained 5-fold more sucrose and starch than the control ones. The same treatment of CM 109 seedlings resulted in accumulation of sucrose and starch by 2-fold and 8.5-fold, respectively. As the result of chilling-treatment, ¹⁴C assimilation rate (P_a), transport speed in the leaf blade (TS₁) and along the plant (TS_m) decreased by about 50 % in both lines. On the other hand, time necessary for radiolabel movement into the phloem loading region (AT) increased strongly, especially in chilling-sensitive line CM 109. It was also noted, that the radioactivity exported from leaves (R₁) and imported by roots (R_m) decreased in line CM 109, and increased slightly in line KW 1074. The activity of SPS extracted from leaves of both lines decreased by about 3.3 when temperature was lowered form 30°C to 10°C. There was no effect of 6 day treatment of chilling on SPS activity. Changes in sucrose and starch concentration, SPS activity as well as differences in transport parameters observed in KW1074 and CM109 seedlings at moderate low temperatures are discussed in terms of mechanism of maize chilling-sensitivity.     

Transgenic cotton over-producing spinach sucrose phosphate synthase showed enhanced leaf sucrose synthesis and improved fiber quality under controlled environmental conditions

Prior data indicated that enhanced availability of sucrose, a major product of photosynthesis in source leaves and the carbon source for secondary wall cellulose synthesis in fiber sinks, might improve fiber quality under abiotic stress conditions. To test this hypothesis, a family of transgenic cotton plants (Gossypium hirsutum cv. Coker 312 elite) was produced that over-expressed spinach sucrose-phosphate synthase (SPS) because of its role in regulation of sucrose synthesis in photosynthetic and heterotrophic tissues. A family of 12 independent transgenic lines was characterized in terms of foreign gene insertion, expression of spinach SPS, production of spinach SPS protein, and development of enhanced extractable V _max SPS activity in leaf and fiber. Lines with the highest V _max SPS activity were further characterized in terms of carbon partitioning and fiber quality compared to wild-type and transgenic null controls. Leaves of transgenic SPS over-expressing lines showed higher sucrose:starch ratio and partitioning of ¹⁴C to sucrose in preference to starch. In two growth chamber experiments with cool nights, ambient CO₂ concentration, and limited light below the canopy, the transgenic line with the highest SPS activity in leaf and fiber had higher fiber micronaire and maturity ratio associated with greater thickness of the cellulosic secondary wall.     

Carbon partitioning and export from mature cotton leaves

The partitioning of carbon in intact, mature cotton (Gossypium hirsutum L.) leaves was examined by steady-state ¹⁴CO₂ labeling. Plants were exposed to dark periods of varying lengths, followed by similar illuminated labeling periods. These treatments produced leaves with a range of starch and soluble sugar contents, carbon exchange, and carbon export rates. Export during the illuminated periods was neither highly correlated with photosynthesis nor was export during the illuminated periods significantly different among the treatments. In contrast, the rate of subsequent nocturnal carbon export from these leaves varied widely and was found to be highly correlated with leaf starch content at the end of the illumination period (r = 0.934) and with nocturnal leaf respiration (r = 0.954). Leaves which had accumulated the highest levels of starch (about 275 micrograms per square centimeter) by the end of the illumination period exhibited nocturnal export rates very similar to those during the daylight hours. Leaves which accumulated starch to only 50 to 75 micrograms per square centimeter virtually ceased nocturnal carbon export. For leaves with starch accumulations of between 50 and 275 micrograms per square centimeter, nocturnal export was directly proportional to leaf starch at the end of the illumination period. After the nocturnal export rate was established, it continued at a constant rate throughout the night even though leaf starch and sucrose contents declined.     

Sucrose phosphate synthase activity and the co-ordination of carbon partitioning during sucrose and amino acid accumulation in desiccation-tolerant leaf material of the C4 resurrection plant Sporobolus stapfianus during dehydration

Both sucrose and amino acids accumulate in desiccation-tolerant leaf material of the C(4) resurrection plant, Sporobolus stapfianus Gandoger (Poaceae). The present investigation was aimed at examining sucrose phosphate synthase (SPS) activity and various metabolic checkpoints involved in the co-ordination of carbon partitioning between these competing pathways during dehydration. In the initial phase of dehydration, photosynthesis and starch content declined to immeasurable levels, whilst significant increases in hexose sugars, sucrose, and amino acids were associated with concomitant significant increases in SPS and pyruvate kinase (PK) activities, and maximal activity levels of phosphoenolpyruvate carboxylase (PEPCase), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and NADH-dependent glutamate synthase (NADH-GOGAT). The next phase of dehydration was characterized by changes in metabolism coinciding with net hexose sugar phosphorylation. This phase was characterized by a further significant increase in sucrose accumulation, with increased rates of net sucrose accumulation and maximum rates of SPS activity measured under both saturating and limiting (inhibitory) conditions. SPS protein was also increased. The stronger competitive edge of SPS for carbon entering glycolysis during hexose phosphorylation was also demonstrated by the further decrease in respiration and the simultaneous, significant decline in both PEPCase and PK activities. A decreased anabolic demand for 2-oxoglutarate (2OG), which remained constant, was shown by the co-ordinated decrease in GOGAT. It is proposed that the further increase in amino acids in this phase of dehydration may be in part attributable to the breakdown of insoluble proteins.     

Alterations in leaf carbohydrate metabolism in response to nitrogen stress

A series of experiments was conducted to characterize alterations in carbohydrate utilization in leaves of nitrogen stressed plants. Two-week-old, nonnodulated soybean plants (Glycine max [L.] Merrill, `Ransom'), grown previously on complete nutrient solutions with 1.0 millimolar NO₃⁻, were transferred to solutions without a nitrogen source at the beginning of a dark period. Daily changes in starch and sucrose levels of leaves were monitored over the following 5 to 8 days in three experiments. Starch accumulation increased relative to controls throughout the leaf canopy during the initial two light periods after plant exposure to N-free solutions, but not after that time as photosynthesis declined. The additional increments of carbon incorporated into starch appeared to be quantitatively similar to the amounts of carbon diverted from amino acid synthesis in the same tissues. Since additional accumulated starch was not degraded in darkness, starch levels at the beginning of light periods also were elevated. In contrast to the starch effects, leaf sucrose concentration was markedly higher than controls at the beginning of the first light period after the N-limitation was imposed. In the days which followed, diurnal turnover patterns were similar to controls. In source leaves, the activity of sucrose-P synthase did not decrease until after day 3 of the N-limitation treatment, whereas the concentration of fructose-2,6-bisphosphate was decreased on day 2. Restricted growth of sink leaves was evident with N-limited plants within 2 days, having been preceeded by a sharp decline in levels of fructose-2,6 bisphosphate on the first day of treatment. The results suggest that changes in photosynthate partitioning in source leaves of N-stressed plants resulted largely from a stable but limited capacity for sucrose formation, and that decreased sucrose utilization in sink leaves contributed to the whole-plant diversion of carbohydrate from the shoot to the root.     

Reversibility of Photosynthetic Inhibition in Cotton after Long-Term Exposure to Elevated CO(2) Concentrations

Cotton (Gossypium hirsutum L. cv Stoneville 213) was grown at 350 and 1000 microliters per liter CO₂. The plants grown at elevated CO₂ concentrations contained large starch pools and showed initial symptoms of visible physical damage. Photosynthetic rates were lower than expected based on instantaneous exposure to high CO₂.

A group of plants grown at 1000 microliters per liter CO₂ was switched to 350 microliters per liter CO₂. Starch pools and photosynthetic rates were monitored in the switched plants and in the two unswitched control groups. Photosynthetic rates per unit leaf area recovered to the level of the 350 microliters per liter CO₂ grown control group within four to five days. To assess only nonstomatal limitations to photosynthesis, a measure of photosynthetic efficiencies was calculated (moles CO₂ fixed per square meter per second per mole intercellular CO₂). Photosynthetic efficiency also recovered to the levels of the 350 microliters per liter CO₂ grown controls within three to four days.

Recovery was correlated to a rapid depletion of the starch pool, indicating that the inhibition of photosynthesis is primarily a result of feedback inhibition. However, complete recovery may involve the repair of damage to the chloroplasts caused by excessive starch accumulation. The rapid and complete reversal of photosynthetic inhibition suggests that the appearance of large, strong sinks at certain developmental stages could result in reduction of the large starch accumulations and that photosynthetic rates could recover to near the theoretical capacity during periods of high photosynthate demand.