Changes in photosystem II function during senescence of wheat leaves

Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat (Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO, assimilation capacity. Analyses of fluorescence quenching under steady-state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSTI reaction centers (F-v'/F-m') but only a slight decrease in the maximum efficiency of PSII photochemistry (F-v/F-m). At the same time, a significant increase in non-photochemical quenching (q(N)) and a considerable decrease in photochemical quenching (q(P)) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of Q(A) reduction and an increase in the proportion of Q(B)-non reducing PSII reaction during senescence. The decrease in both F-v'/F-m' and q(P) explained the decrease in the actual quantum yield of PSII electron transport (Phi(PSII)). We suggest that the modifications in PSII function, which led to the down-regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves.