THE ROLE OF SUCROSE SYNTHASE IN THE RESPONSE OF SOYBEAN NODULES TO DROUGHT

Experiments were carried out to investigate the effects of drought stress on enzymatic activities related to carbon and nitrogen metabolism in soybean nodules. Gradual drought stress, imposed by withholding water/nutrients, resulted in declines in the water potential of leaves and nodules consistent with a significant decline in N-2 fixation. However, the amounts of nitrogenase components 1 and 2 were virtually unaffected by drought stress. Similarly, no significant changes could be detected in aspartate aminotransferase, phosphoenolpyruvate carboxylase, glutamine synthetase or alkaline invertase activities throughout the experiment. In contrast, sucrose synthase (SS), one of the enzymes involved in sucrose metabolism in legume nodules, declined dramatically in activity and in content within a few days of withholding water. Coincident with this decline in SS activity were significant increases in the nodule contents of sucrose, total free amino acids and ureides. The amounts of proline, however, did not increase until some days later. It is suggested that SS may play a key role in the regulation of nodule carbon metabolism and, therefore, of nitrogen fixation under drought stress conditions.