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

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Abstract
The control of photosynthetic starch/sucrose formation in leaves of soybean (G. max L. Merr.) cultivars was studied in relation to stage of plant development, photosynthetic photoperiod and N 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 3 of the 4 varieties tested, nodulated plants had lower leaf starch levels and higher activities of sucrose phosphate synthetase (SPS) and isolated mesophyll cells incorporated more C (% total 14CO2 fixed) into sucrose and less into starch as compared to nonnodulated (nitrate-dependent) plants. The variation among cultivars and N treatments observed in the activity of SPS in leaf extracts was positively correlated with labeling of sucrose in leaf extracts in isolated cells and negatively correlated with whole leaf starch content. Evidently increased demand for assimilates by nodulated roots may be accommodated by greater partitioning of C into sucrose in the mesophyll cells. Photoperiod affects partitioning of fixed C into starch. Within 2 days of transfer of nodulated soybean ''Ransom'' plants from a 14-h to a 7-h photoperiod, leaf starch accumulation rates doubled; 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. Four nodulated soybean cultivars were grown to maturity in a greenhouse. Fully expanded leaves at the top of the canopy were sampled during vegetative growth (45 days), at flowering (79 days) and at mid-podfill (120 days). In general, activities of SPS and uridine-5''-diphosphatase were highest during vegetative growth and they decreased during reproductive development, whereas activity of sucrose synthase and leaf starch content tended to increase. Leaf starch was negatively correlated with levels of SPS. Apparently, sucrose-P synthetase is a key control point regulating the photosynthetic formation of sucrose and hence starch.