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

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Abstract
The activity of photosystems one and two (PS I and PS II) was measured in chloroplasts isolated from the primary leaves of Phaseolus vulgaris. During foliar senescence, the rates of electron transport through PS I and PS II declined by approximately 25% and 33% respectively. These losses of activity could not account for the decrease of 80% in the rate of coupled, non-cyclic electron transport during senescence. It is therefore suggested that an impairment of electron flow between the photosystems limited non-cyclic electron transport in chloroplasts from older leaves. In this study the activity of PS II was measured using oxidized p-phenylenediamine as the electron acceptor, and trifluralin as an inhibitor of electron transport between PS II and PS I. In chloroplasts from young leaves the reduction of ferricyanide was a measure of non-cyclic electron transport, but in preparations from older leaves ferricyanide received a large proportion of electrons from PS II.