Relationship between Photosynthesis and Chlorophyll Content during Leaf Senescence of Rice Seedlings

Abstract

Photosynthetic oxygen evolution, chlorophyll contents and chlorophyll a /b ratios of 3rd to 6th leaves of rice seedlings were measured to examine whether or not inactivation of photosynthesis during senescence is related to loss of chlorophyll. Photosynthetic activity decreased more rapidly than chlorophyll content during leaf senescence; as a result, the lower the leaf position, the lower was the rate of oxygen evolution determined on the basis of chlorophyll. Chlorophyll a⊘b ratio also decreased with advancing senescence. Electrophoretic analysis revealed that the decline in chlorophyll a⊘b ratio is due to more rapid degradation of the reaction center complexes than light-harvesting chlorophyll a⊘b proteins of photosystem II and that the photosystem I reaction center disappears in parallel with the inactivation of photosynthesis. A simple method was developed to estimate the amounts of chlorophyll a associated with the reaction center complexes of the two photosystems from the total chlorophyll contents and chlorophyll a⊘b ratios of leaves. Rates of oxygen evolution, determined on the basis of chlorophyll a bound to the reaction center complexes, remained constant throughout the course of senescence. Thus, inactivation of photosynthesis is closely related with loss of the reaction center complexes during leaf senescence of rice seedlings. The results suggest that loss of photosynthesis is mainly caused by loss of a functional unit of photosynthesis or by a decrease in the number of whole chloroplasts.