Metabolism of Oat Leaves during Senescence

Abstract

A comparison has been made of the progress of senescence in the first leaf of 7-day-old oatplants (Avena sativa cv. Victory) in darkness and in white light. Light delays the senescence, and intensities not over 100 to 200 ft-c (1000-2000 lux) suffice for the maximum effect. In such intensities , chlorophyll loss and amino acid liberation stiOl go on in detached leaves at one-third to one-half the rate observed in darkness; however, when the leaves are attached to the plant, the loss of chlorophyH 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 cytoki-nin, inhibiting senescence. Light causes a large increase in invertase-labile sugar and a smaler increase in glucose, and application of 100 to 300 mm glucose or sucrose in the dark maintains the chlorophyll, at least partially. Correspondingly, short exposure to high lght 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 chloro-phyOl 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 therefore cannot be ascribed to respiratory uncoupling. The increased respiration in light is prevented by DCMU, parallel with the prevention of sugar formation. It is therefore ascribed to the accumulation of soluble sugars, acting as respirable substrate. Also, L-serine does not antagonize the light effect. For all of these reasons, it is concluded that the action of light is not mediated by photosynthetic sugar formation, nor by photo-production of a cytokinin. Instead, we propose that 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. The four preceding papers of this series (4, 22-24) have been concerned with the senescence of the first leaf of the 7-day-old oat plant, held in darkness, either detached or attached to the whole plant. However, as a practical matter, leaves do senesce even in full light, and a few investigators (e.g. 2, 7, 25) have studied the process in daylight. senescence in leaves of the oat (3, 25, 26), wheat (8), potato (14), tobacco (20), rice (16), and Hibiscus (17); in the last instance, a long photoperiod was found more effective than a short one. In rice, red light was effective in delaying senescence and far red had the opposite effect (16). In the presence of EDTA, however, white light was reported to promote bleaching (11), and although this effect has twice been confirmed (9, 23), its relationship to normal senescence in light is not clear. Recently , Takegami (20) has found light to prevent the decrease in total RNA which occurs in senescing tobacco leaves. The effect differed in the different fractions, the fall in 16S and 23S components being somewhat inhibited, whereas the 18S and 25S fractions were actually increased by white light. Several investigators have ascribed these effects of light to the photosynthetic production of sugars. Applying sucrose to detached leaves did delay senescence, at least for 48 hr, and to about the same extent as light (5, 14); it also inhibited the hydrolysis of RNA (5, 25). Our own experiments, reported below, show a moderate effect of glucose, or of endogenous sugar, on the senescence of the first leaves of oats, and glucose plus nitrate has been reported to exert some retarding effect on senescence of the fourth leaf of the same cultivar (27). However, the effects of sugars on senescing bean and wheat leaves were small (3, 7); indeed both in wheat and in Xanthium leaves, sucrose tends to promote the bleaching of Chl in the light (7, 10). In 1937, Vickery et al. (26) found that glucose was much less effective than light in decreasing protein loss in tobacco leaves. In detached cucumber cotyledons, in which light delays the Chl loss, 3% glucose imitates the effect of light on Chl loss but not its effect on proteolysis (12). Also, in the somewhat artificial system of wheat leaf segments treated with aminotria-zole, light delayed Chl loss, and neither sugars nor DCMU had much influence (8). Thus, to explain the action of light on senescence as being due to sugar formation is certainly open to question. The present series of experiments has been carried out to make a more thorough study of the differences between senescence in light and in darkness and to elucidate the somewhat confusing comparisons between the effects of white light and those of sugars. Some interactions between attachment to the plant, exposure to light, and treatment with cytokinin have also been explored, and an attempt has been made to bring the effects of light into harmony with the concepts of the senescence process arrived at in preceding papers and by other workers. As a result, a general hypothesis is proposed to explain the influence of light and of some other factors on senescence. MATERIALS AND METHODS The experimental material consisted of the first leaves of oat seedlings (Avena sativa cv. Victory) being grown for 7 days under cool white fluorescent light (200 ft-c at the base of the plants), as used in the preceding papers (22-24). In most experi-448