THE SHORT-TERM EFFECT OF NO3- AND NH3 ASSIMILATION ON SUCROSE SYNTHESIS IN LEAVES

The short-term regulation of sucrose synthesis during nitrogen assimilation has been studied during NO3- and NH4+ assimilation in NO3- depleted wheat leaves, and during NH3 assimilation in barley leaf segments and spinach leaf discs. In all cases, provision of inorganic nitrogen led to increased production of amino acids and an inhibition of sucrose synthesis. The increased anapleurotic carbon flux to amino acids was not accompanied by depletion of organic acids, but was always associated with a decrease in phosphoenolpyruvate, showing that phosphoenolpyruvate carboxylase and/or pyruvate kinase are activated. The decreased flux to sucrose was not accompanied by a depletion of UDP-glucose or hexose-phosphates. Triose-phosphates only decreased slightly, and glycerate-3-phosphate was unaltered. Fructose-2,6-bisphosphate increased by 50-100 % in all cases. Sucrose-phosphate synthase activity declined in wheat and spinach. It is argued that regulation mechanisms, which include an increase in fructose-2,6-bisphosphate, are involved in regulating carbon metabolism during the assimilation of nitrogen.