Two long-term effects of light that control the stability of proteins related to photosynthesis during senescence of rice leaves. Plant and Cell Physiology 39

creased during senescence, with no or small changes in the initial slope of the light-response curve ($), an index of light-harvesting and light-utilization capacity. Senescence of rice leaves was associated with shading of the leaves by upper younger leaves and evidence indicated that a major modulator of the light-response curve during senescence is the light environment of leaves. Chi and proteins in leaves were largely degraded during several days in darkness, and proteins that are related to Pmax and fa were stabilized by light at different intensities. A decline in Pma, during senes-cence was related to degradation of Rubisco, Cyt/and the P subunit of ATPase that was completely suppressed only at about 300/anol quanta m ~ 2 s- 1 and, hence, proceeded in the darkened environments caused by shading. By con-trast, 0, remained relatively constant during senescence be-cause Chi and proteins related to the light-harvesting Chi proteins and the reaction center complexes were effectively stabilized by very weak light. Evidence was obtained in-dicating that phytochrome is involved in the stabilization of Chi, whereas the photoprotection of Rubisco is a com-plex reaction involving photosynthesis and a signal func-tion^) of light. Physiological or ecophysiological signifi-cance of the two long-term effects of light that differentially stabilize the two groups of proteins are discussed. Key words: Chloroplast proteins — Leaf senescence — Light-harvesting capacity — Long-term effect of light