Comparative effects of phosphinothricin on nitrate and ammonium assimilation and on anaplerotic CO 2 fixation in N-deprived barley plants

Abstract

Summary We have analyzed the comparative effect of nitrate and ammonium on the nitrogen-dependent induction of anaplerotic CO 2 fixation in detached leaves of N-limited barley plants. The supply of N, either in the form of NO 3 − or NH 4 + , provoked induction of phosphoenolpyruvate carboxylase activity (PEPCase), whereas the extractable pyruvate kinase, malate dehydrogenase and malic enzyme activities were not affected. The time course induction of PEPCase was dependent on the rate of ammonia assimilation and not on nitrate or ammonium uptake or accumulation. Treatment of N-deprived leaves with phosphinothricin (an inhibitor of glutamine synthetase activity) caused inhibition of ammonia assimilation, resulting in the reversion of ammonia-/nitrate-dependent enhancement of PEPCase. The results indicate that glutamine level controls phosphoenolpyruvate carboxylase activation in both nitrate and ammonium fed plants; consequently, if glutamine synthesis is inhibited, PEPCase activity is not enhanced. Determination of malate content showed that as PEPCase activity increased in response to increasing ammonia assimilation, there was a linear decline in the level of that metabolite. The highest rates of PEPCase enhancement in excised barley leaves when ammonia salt is the N source are in accordance with the highest rates of ammonia assimilation.