Diurnal and circadian fluctuation of malate levels and its close relationship to nitrate reductase activity in tobacco leaves

In the leaves of nitrate-fed tobacco plants, the malate level increased 3-fold during the day and decreased during the night. These 24-h oscillations continued to occur in plants placed in continuous light conditions. On the other hand, the malate content declined markedly and showed no rhytmic variations when plants were transferred to continuous darkness. The fluctuation pattern of malate level were similar to those of in vitro nitrate reductase (NR, EC 1.6.6.1) activity although they were not parallel. Moreover, when NR activity, and presumably nitrate-reduction, were strongly decreased either in plants grown with tungstate (an inhibitor of NR activity) or in plants starved of nitrogen, leaf malate accumulation during the day was completely suppressed. The supply of nitrate to N-starved plants induced NR activity and leaf malate accumulation unless tungstate was present in the nutrient solution. These suggest that the malate content in leaves is dependent upon the level of nitrate reduction.