Effects of Previous Water Stress Treatment on the Leaf Photosynthesis and Related Factors in Oats (C_3) and Maize (C_4)

The acclimation effect of previous water stress on the photosynthetic characteristics was clarified using oats (C_3) and maize (C_4) . On the plants previously water stressed (pretreatment), the leaf photosynthetic rate, stomata1 response, water potential and the related factors were measured during the following water stress application (posttreatment) to be compared to those of non stressed (control) plants. The posttreatment was imposed to the plants by withholding water supply (Experiment 1, a lenient stress) or also by stem cut (experiment 2, a quick stress) The results obtained were as follows : Experiment 1. Oats which had been recovered from the previous water stress showed higher values for leaf photosynthetic rate, stomata1 conductance and mesophyll conductance compared to the control when the plants were placed under the posttreatment. Also lower osmotic potential (ψπ) and higher pressure potential (ψp) were observed when the leaf water potential (ψl) of the pretreated Oats reduced. The osmotic adjustment was effective in maintaining leaf photosynthetic rate of oats but such an effect was not detected in maize. Experiment 2. A quick water stress (posttreatment) was applied to the plants, oats and maize, recovered from the previous stress. A similar osmotic adjustment to that observed in the experiment 1 was obtained for oats, but the pretreatment was ineffective or adversely effective for maintenance of ψp in maize