DROUGHT-INDUCED EFFECTS ON NITRATE REDUCTASE-ACTIVITY AND MESSENGER-RNA AND ON THE COORDINATION OF NITROGEN AND CARBON METABOLISM IN MAIZE LEAVES

Maize (Zea mays L.) plants were grown to the nine-leaf stage. Despite a saturating N supply, the youngest mature leaves (seventh position on the stem) contained little NO3- reserve. Droughted plants (deprived o f nutrient solution) showed changes in foliar enzyme activities, mRNA accumulation, photosynthesis, and carbohydrate and amino acid contents . Total leaf water potential and CO2 assimilation rates, measured 3 h into the photoperiod, decreased 3 d after the onset of drought. Starch , glucose, fructose, and amino acids, but not sucrose (Suc), accumulat ed in the leaves of droughted plants. Maximal extractable phosphoenolp yruvate carboxylase activities increased slightly during water deficit , whereas; the sensitivity of this enzyme to the inhibitor malate decr eased. Maximal extractable Suc phosphate synthase activities decreased as a result of water stress, and there was an increase in the sensiti vity to the inhibitor orthophosphate. A correlation between maximal ex tractable foliar nitrate reductase (NR) activity and the rate of CO2 a ssimilation was observed. The NR activation state and maximal extracta ble NR activity declined rapidly in response to drought. Photosynthesi s and NR activity recovered rapidly when nutrient solution was restore d at this point. The decrease in maximal extractable NR activity was a ccompanied by a decrease in NR transcripts, whereas Suc phosphate synt hase and phosphoenolpyruvate carboxylase mRNAs were much less affected . The coordination of N and C metabolism is retained during drought co nditions via modulation of the activities of Suc phosphate synthase an d NR commensurate with the prevailing rate of photosynthesis.