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.