Changes in chlorophyll content and organization during senescence of the primary leaves of Phaseolus vulgaris L. in relation to photosynthetic electron transport

A large decrease was observed in the chlorophyll content of the primary leaves of Phaseolus vulgaris during senescence. Chloroplasts isolated from mature and senescent leaves gave very similar light saturation curves for electron transport reactions involving either PS I or PS II, indicating that the average number of chlorophyll molecules associated with each reaction centre did not change during senescence. It is concluded that the reaction centres ceased to function at the same time as, or perhaps before, their antenna chlorophylls were lost from the thylakoid membrane, and that the percentage decrease in the number of functional reaction centres per leaf was at least as great as the percentage decrease in the leaf chlorophyll content. The chlorophyll-protein composition of thylakoid membrane preparations was examined by electrophoresis of samples treated with sodium dodecyl sulphate. In older leaves a smaller proportion of the chlorophyll applied to polyacrylamide gels was associated with the P700- chlorophyll a-protein complex.. There was also a decline in emission at 734 nm in the 77 °K fluorescence spectrum of intact leaf tissue during senescence. These results indicate that older leaves contained a smaller proportion of chlorophylls associated with PS I, and this is consistent with the decrease observed in the leaf chlorophyll a/b ratio during senescence. The effect of these changes in chlorophyll content on the capacity of the chloroplast to carry out photosynthetic electron transport is discussed.