Modifications in photosystem II photochemistry in senescent leaves of maize plants

Analyses of CO₂ exchange and chlorophyll fluorescence were carried out to assess photosynthetic performance during senescence of maize leaves. Senescent leaves displayed a significant decrease in CO₂ assimilatory capacity accompanied by a decrease in stomatal conductance and an increase in intercellular CO₂ concentration. The analyses of fluorescence quenching under steady-state photosynthesis showed that senescence resulted in an increase in nonphotochemical quenching and a decrease in photochemical quenching. It also resulted in a decrease in the efficiency of excitation energy capture by open PSII reaction centres and the quantum yield of PSII electron transport, but had very little effect on the maximal efficiency of PSII photochemistry. The results determined from the fast fluorescence induction kinetics indicated an increase in the proportion of Q(B)-non-reducing PSII reaction centres and a decrease in the rate of Q(A) reduction in senescent leaves. Theoretical analyses of fluorescence parameters under steady-state photosynthesis suggest that the increase in the non-photochemical quenching was due to an increase in the rate constant of thermal dissipation of excitation energy by PSII and that the decrease in the quantum yield of PSII electron transport was associated with a decrease in the rate constant of PSII photochemistry. Based on these results, it is suggested that the decrease in the quantum yield of PSII electron transport in senescent leaves was downregulated by an increase in the proportion of Q(B)-non-reducing PSII reaction centres and in the non-photochemical quenching. The photosynthetic electron transport would thus match the decreased demand for ATP and NADPH in carbon assimilation which was inhibited significantly in senescent leaves.