SHORT-TERM EFFECTS OF NITRATE ON SUCROSE SYNTHESIS IN WHEAT LEAVES

Experiments were carried out with fully expanded leaves from three-week-old seedlings of wheat (Triticum aestivum L.) raised without NO3-. Nitrate was supplied to the leaves through the transpiration stream in the light. Uptake of NO3- was linear with NO3- concentrations from 0 to 80 mM in the solution. Net sucrose synthesis showed inverse relationships versus nitrate uptake, assimilation, and accumulation, with correlation coefficients close to 1. By contrast, no alteration in sucrose synthesis was observed when KCl was substituted for KNO3 in the uptake solution. Sucrose synthesis was not affected by nitrate in seedlings treated with tungstate which absorbed but did not reduce NO3-. After 10 h, the final amount of sucrose in the tissues was only slightly decreased in the presence of NO3-, indicating that the effect of NO3- did not result from an altered sucrose-storage capacity. Comparison of the carbon skeleton and energy reductant necessary for NO3- and CO2 assimilation is consistent with the hypothesis that the processes of NO3- assimilation and sucrose synthesis compete for photosynthetic energy and carbon.