EFFECT OF DROUGHT STRESS ON NET CO2 UPTAKE BY ZEA LEAVES

The net CO2 assimilation by leaves of maize (Zea mays L. cv. Adonis) p lants subjected to slow or rapid dehydration decreased without changes in the total extractable activities of phosphoenolpyruvate carboxylas e (PEPC), malate dehydrogenase (MDH) and malic enzyme (ME). The phosph orylation state of PEPC extracted from leaves after 2-3 h of exposure to light was not affected by water deficit, either. Moreover, when pla nts which had been slowly dehydrated to a leaf relative water content of about 60% were rehydrated, the net CO2 assimilation by leaves incre ased very rapidly without any changes in the activities of MDH, ME and PEPC or phosphorylation state of PEPC. The net CO2-dependent O-2 evol ution of a non-wilted leaf measured with an oxygen electrode decreased as CO2 concentration increased and was totally inhibited when the CO2 concentration was about 10%. Nevertheless, high CO2 concentrations (5 -10%) counteracted most of the inhibitory effect of water deficit that developed during a slow dehydration but only counteracted a little of the inhibitory effect that developed during a rapid dehydration. In c ontrast to what could be observed during a rapidly developing water de ficit, inhibition of leaf photosynthesis by cis-abscisic acid could be alleviated by high CO2 concentrations. These results indicate that th e inhibition of leaf net CO2 uptake brought about by water deficit is mainly due to stomatal closure when a maize plant is dehydrated slowly while it is mainly due to inhibition of non-stomatal processes when a plant is rapidly dehydrated. The photosynthetic apparatus of maize le aves appears to be as resistant to drought as that of C-3 plants. The non-stomatal inhibition observed in rapidly dehydrated leaves might be the result of either a down-regulation of the photosynthetic enzymes by changes in metabolite pool sizes or restricted plasmodesmatal trans port between mesophyll and bundle-sheath cells.