Factors determining the midday depression of photosynthesis in trees under monsoon climate

Field studies of gas exchange of Populus deltoides, Prosopis juliflora and Acacia auriculiformis showed large diurnal changes in net photosynthesis (A) and stomatal conductance (g(s)) during autumn. P. deltoides and P. juliflora undergo pronounced midday depression in A and g(s) while A. auriculiformis showed a one-peak response. Several factors indicative of photosynthetic performance were found to be reversibly affected during afternoon decline. These include (i) decrease in initial slope of the CO2 response curve (carboxylation efficiency), (ii) substantial increase in CO2 compensation point and (iii) decrease in overall quantum yield of photosystem II. The phenomenon can be duplicated in potted plants by simulating a typical daily pattern of PPFD and VPD. It is found that high VPD induces significant decline in A and g(s) at moderate temperature and saturating PPFD (800 mu mol m(-1) s(-1)) whereas these parameters are only marginally affected at high PPFD and low VPD. Fluorescence data show that the tree species under study have a high capacity for safe dissipation of excessive excitation energy. The activation of photorespiration, as evident from an increase in CO2 compensation point, maintains constant internal CO2 concentration (C-i) which may aid in minimizing photoinhibition during stomatal closure at midday. In case of P. deltoides and P. juliflora the stomata seem to be quite sensitive to the changes in humidity whereas this does not appear to be essential in case of A. auriculiformis because of its phyllode structure that endows it with mechanisms for conserving water without undergoing large-scale stomatal changes.