Effect of drought stress on net CO₂ uptake by Zea leaves

Abstract

The net CO₂ assimilation by leaves of maize (Zea mays L. cv. Adonis) plants subjected to slow or rapid dehydration decreased without changes in the total extractable activities of phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) and malic enzyme (ME). The phosphorylation state of PEPC extracted from leaves after 2–3 h of exposure to light was not affected by water deficit, either. Moreover, when plants which had been slowly dehydrated to a leaf relative water content of about 60% were rehydrated, the net CO₂ assimilation by leaves increased very rapidly without any changes in the activities of MDH, ME and PEPC or phosphorylation state of PEPC. The net CO₂-dependent O₂ evolution 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 contrast to what could be observed during a rapidly developing water deficit, inhibition of leaf photosynthesis by cis-abscisic acid could be alleviated by high CO₂ concentrations. These results indicate that the 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 leaves appears to be as resistant to drought as that of C₃ 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 transport between mesophyll and bundle-sheath cells.