Photosynthetic Electron Transport During Senescence of the Primary Leaves ofPhaseolus vulgarisL.

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Abstract
The kinetics of 685 nm chlorophyll fluorescence emission were measured at 20 °C following illumination of primary leaves of P. vulgaris. During foliar senescence, a large reduction was observed in the maximal level of fluorescence emission (P) of the induction curve, normalized with respect to the minimal level (O), and in the time taken to reach P. This suggests that fewer plastoquinone (PQ) molecules were able to accept electrons from each photosystem two (PS II) reaction centre in older leaves. Measurements of fluorescence emission at 77 °K indicated that the primary photochemical quantum yield of the PS II reaction centres remained constant during senescence. The redox state of the PQ pool was estimated throughout the induction curve at 20 °C. In both mature and senescent leaves PQ was highly reduced at P. There followed a reoxidation of PQ in the mature leaves, but in the old leaves the PQ pool remained reduced. This indicates that the rate of electron flow from PQ to photosystem one (PS I) decreased considerably during senescence. Fluorescence was quenched from P to a steady state level (T) in leaves of all ages, and this was associated with a redistribution of excitation in favour of PS I. Since, in senescent leaves, changes in the redox state of PQ were absent, it is suggested that quenching resulted from the generation of proton and ion gradients across the thylakoid membranes, and the synthesis of ATP.

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