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

Experiments were carried out to investigate the effects of drought str ess on enzymatic activities related to carbon and nitrogen metabolism in soybean nodules. Gradual drought stress, imposed by withholding wat er/nutrients, resulted in declines in the water potential of leaves an d nodules consistent with a significant decline in N-2 fixation. Howev er, the amounts of nitrogenase components 1 and 2 were virtually unaff ected by drought stress. Similarly, no significant changes could be de tected in aspartate aminotransferase, phosphoenolpyruvate carboxylase, glutamine synthetase or alkaline invertase activities throughout the experiment. In contrast, sucrose synthase (SS), one of the enzymes inv olved in sucrose metabolism in legume nodules, declined dramatically i n activity and in content within a few days of withholding water. Coin cident with this decline in SS activity were significant increases in the nodule contents of sucrose, total free amino acids and ureides. Th e 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 nodu le carbon metabolism and, therefore, of nitrogen fixation under drough t stress conditions.