Metabolism of Oat Leaves during Senescence

Abstract

A comparison was made of the progress of senescence in the 1st leaf of 7 day old oat plants (Avena sativa cv. ''Victory'') in darkness and in white light. Light delays the senescence, and intensities not over 100-200 ft-c (1000-2000 lx) suffice for the maximum effect. In such intensities, chlorophyll loss and amino acid liberation still go on in detached leaves at 1/3 to 1/2 the rate observed in darkness; however, when the leaves are attached to the plant, the loss of chlorophyll in 5 days is barely detectable. Transfer of the leaves from 1 or 2 days in the low intensity light to darkness, or vice versa, shows no carryover of the effects of the preceding exposure, so that such treatment affords no evidence for the photoproduction of a stable substance, such as cytokinin, inhibiting senescence. Light causes a large increase in invertase-labile sugar and a smaller increase in glucose, and application of 100-300 mM glucose or sucrose in the dark maintains the chlorophyll, at least partially. Short exposure to high light intensity, which increased the sugar content, had a moderate effect in maintaining the chlorophyll. However, 3-(3,4-dichlorphenyl)-1,1-dimethylurea (DCMU) completely prevents the increases in sugars and yet does not prevent the effect of light on senescence, whether determined by chlorophyll loss or by protein hydrolysis. Light causes a 300% increase in the respiration of detached oat leaves, and kinetin lowers that only partly, but unlike the increased respiration associated with senescence in the dark, the increase in the light is fully sensitive to dinitrophenol and cannot be ascribed to respiratory uncoupling. The increased respiration in light is prevented by DCMU, parallel with the prevention of sugar formation. It is ascribed to the accumulation of soluble sugars, acting as respirable substrate. L-Serine does not antagonize the light effect. Apparently, the action of light is not mediated by photosynthetic sugar formation or by photoproduction of a cytokinin. Possibly light exerts its effect by photoproduction of ATP. The action of sugars is ascribed to the same mechanism but by way of respiratory ATP. This hypothesis unifies most of the observed phenomena of the senescence process in oat leaves, and helps to explain some of the divergent findings of earlier workers.