Effect of N-Source on Soybean Leaf Sucrose Phosphate Synthase, Starch Formation, and Whole Plant Growth

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Abstract
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.