CHANGES IN PHOTOSYSTEM-II FUNCTION DURING SENESCENCE OF WHEAT LEAVES

Analyses of chlorophyll fluorescence were undertaken to investigate th e alterations in photosystem II (PSII) function during senescence of w heat (Triticum aestivum L. cv. Shannong 229) leaves. Senescence result ed in a decrease in the apparent quantum yield of photosynthesis and t he maximal CO, assimilation capacity. Analyses of fluorescence quenchi ng under steady-state photosynthesis showed that senescence also resul ted in a significant decrease in the efficiency of excitation energy c apture by open PSTI reaction centers (F-v'/F-m') but only a slight dec rease in the maximum efficiency of PSII photochemistry (F-v/F-m). At t he same time, a significant increase in non-photochemical quenching (q (N)) and a considerable decrease in photochemical quenching (q(P)) wer e 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. T he decrease in both F-v'/F-m' and q(P) explained the decrease in the a ctual quantum yield of PSII electron transport (Phi(PSII)). We suggest that the modifications in PSII function, which led to the down-regula tion of photosynthetic electron transport, would be in concert with th e lower demand for ATP and NADPH in the Calvin cycle which is often in hibited in senescing leaves.