COMPARATIVE EFFECTS OF PHOSPHINOTHRICIN ON NITRATE AND AMMONIUM ASSIMILATION AND ON ANAPLEROTIC CO2 FIXATION IN N-DEPRIVED BARLEY PLANTS

We have analyzed the comparative effect of nitrate and ammonium on the nitrogen-dependent induction of anaplerotic CO2 fixation in detached leaves of N-limited barley plants. The supply of N, either in the form of NO3- or NH4+, provoked induction of phosphoenolpyruvate carboxylas e 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 assim ilation and not on nitrate or ammonium uptake or accumulation. Treatme nt of N-deprived leaves with phosphinothricin (an inhibitor of glutami ne synthetase activity) caused inhibition of ammonia assimilation, res ulting in the reversion of ammonia-/nitrate-dependent enhancement of P EPCase. The results indicate that. glutamine level controls phosphoeno lpyruvate carboxylase activation in both nitrate and ammonium fed plan ts; consequently, if glutamine synthesis is inhibited, PEPCase activit y is not enhanced. Determination of malate content showed that as PEPC ase activity increased in response to increasing ammonia assimilation, there was a linear decline in the level of that metabolite. The highe st rates of PEPCase enhancement in excised barley leaves when ammonia salt is the N source are in accordance with the highest rates of ammon ia assimilation.