Effect of supplemental NO3 on plant growth and components of photosynthetic carbon metabolism in soybean (Glycine max)

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Abstract
The effect of supplied N (5, 10, or 20 mM .**GRAPHIC**. on plant growth and components of photosynthetic carbon metabolism was determined in soybean plants (Glycine max (L.) Merr. cv. Ransom) inoculated with Rhizobium japonicum strain USDA 110. The N treatments were started at 14 days after transplanting and plants were sampled at 10, 17, 23, and 43 days after initiation of the treatments. Total dry weight, shoot/root ratio, and mean relative growth rate all increased one- to two-fold in response to supplied N. Carbon dioxide exchange rate, assimilate export rate, sucrose phosphate synthase activity, leaf sucrose concentration, and leaf starch mobilization increased in response to supplied N at the first two sample dates, whereas starch accumulation rate was affected to a lesser degree. The response of these parameters to supplied N was diminished at the later two sample dates. Plant age and partial alleviation of the nitrogen stress that occurred during early nodule development were factors associated with the decreased treatment response at the later sampling dates. Assimilate export rate was correlated positively with carbon exchange rate (r=0.84), sucrose phosphate synthase activity (r=0.71), and leaf sucrose concentration (r=0.83). This suggested that the enzymatic capacity for sucrose formation as well as net carbon fixation and leaf sucrose concentration are related to the rate of assimilate export. The results illustrated the interdependence of photosynthetic carbon metabolism and N nutrition, and emphasized that certain photosynthetic components are more sensitive to changes in N nutrition than others.