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 N(2)-dependent or NO(3) (-)-supplied conditions, and the partitioning of photosynthate and dry matter was characterized. Although no treatment effects on photosynthetic rates were observed, NO(3) (-)-supplied plants in both cultivars had lower starch accumulation rates than N(2)-dependent plants. Leaf extracts of NO(3) (-)-supplied plants had higher activities of sucrose phosphate synthase (SPS) and cytoplasmic fructose-1,6-bisphosphatase (FBPase) than N(2)-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 NO(3) (-)-supplied plants than N(2)-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.