EFFECTS OF LIGHT ON DEGRADATION OF CHLOROPHYLL AND PROTEINS DURING SENESCENCE OF DETACHED RICE LEAVES

Regulatory effects of light on senescence of rice leaves were investigated by measuring degradation of chlorophyll and proteins in leaf segments which had been kept in the dark or under illumination with light of different intensities and colors. When leaves had been left in total darkness for three days at 30-degrees-C, there was an initial long lag that lasted for one whole day and then chlorophyll was rapidly degraded in the second and third days. Breakdown of chlorophyll was strongly retarded by continuous illumination with white light of intensity as low as 0.5 mumol photons m-2 s-1 but the effect of light decreased at intensities above 10 mumol photons m-2 s-1. The initial lag and subsequent degradation of chlorophyll in the dark were little affected by illumination with red or far red light at the beginning of dark treatment. However, a brief illumination with red light at the end of the first and/or second day significantly suppressed degradation of chlorophyll during subsequent dark periods and the effect of red light was nullified by a short irradiation with far red light. Thus, degradation of chlorophyll is regulated by phytochrome. Thylakoid membrane proteins and soluble proteins were also largely degraded during three days in the dark. Degradation of membrane proteins such as the apoproteins of light-harvesting chlorophyll a/b proteins of photosystem II and chlorophyll a-binding proteins of reaction center complexes showed a long lag and was strongly suppressed by illumination with weak white light. Thus, the loss of chlorophyll can be correlated with degradation of chlorophyll-carrying membrane proteins. By contrast, light had only a weak protecting effect on soluble proteins and ribulose-1,5-bisphosphate carboxylase/oxygenase rapidly disappeared under illumination with weak white light. Thus, breakdown of thylakoid membrane and soluble proteins are differently regulated by light. Artifacts which would be introduced by detachment of leaves were also discussed.