Stomatal Aperture and the Senescence of Leaves

Abstract

The senescence of isolated leaves of the seedling of Victory oats has been under study in our laboratory for some eight years. When the leaves are detached and placed in the dark, proteolysis begins in about 6 hours and after 20–24 hours the breakdown of chlorophyll begins. By 72 hours (at 24°C) almost 60% of both the chlorophyll and the protein have disappeared (Tetley and Thimann, 1974). In white light the process is generally similar but much slower; after 72 hours in 200 foot-candles of fluorescent lights such leaves have lost only about 20% of their protein and 10% of their chlorophyll (Thimann et al., 1977). At first this delaying effect of light was ascribed to the photosynthetic production of reducing sugar, and it was found that infusing the leaves with glucose or sucrose did delay senescence, as had been reported by earlier workers (Marré et al., 1966). However, three facts mitigated against this explanation. First, the action of light reached saturation at 150 f-c, or 1500 lux, while photosynthesis is barely saturated at 10 times this intensity. Second, the concentration of glucose needed to delay senescence comparably to the effect of light was several times higher than that actually present in the leaf. Third, and most critical, DCMU¹ at 10⁻⁵ M totally inhibited the photosynthetic production of sugars, yet had no influence at all on the delaying effect of light. This observation not only proved that sugar formation was not the cause of the light effects but it raised acutely the problem of how light is acting.