PROTEASES IN RICE LEAVES——SOME CHEMICAL AND PHYSICAL PROPERTIES AND ACTIVITIES IN RELATION TO SENESCENCE
Wu Guangnan Liu Baoren Zhang Jinyu ( Institute of Crop Genetics and Physiology, Jiangsu Academy of Agricultural Sciences )
Since it has been proved that the delay of leaf senescence could increase the yield of rice, it is important to study its mechanism for further improvement of regulating techniques. Protein degradation was the basic characteristic of leaf senescence and chlorophyll content was the sensitive index for degree of senescence. Identified by inhibitors (IAA and PMSF) and different substrates, there existed more than three subclasses of proteases in rice leaves,namely sulfhydryl proteinase, serine proteinase and carboxypeptide. The former two were endopeptidases and the later one was exopeptidase, but sulfhydryl proteinase was the most active one. The pH optima for both hemoglobin-digested proteinase and carboxypeptidase were 5 and the temperature optima 55℃ and45℃ respectively.Fractionated by ammonium sulfate, both enzymes were precipitated at 30-60% saturation. Chlorophyll and protein contents increased during the elongation stage of rice leaf, reached their peaks just after elongation was accomplished and fell during leaf senescence. The specific activities of hemoglobin-digested proteinase and carboxypeptidase decreased during leaf elongation and increased during its senescence. But the beginning of increasing of specific activities lagged behind the beginning of decreasing of leaf protein content. Therefore, the decreasing of protein content of rice leaves during senescence was not only regulated by the activities of proteases, but also by its acces-sibitity to substrates and the rate of protein synthesis in cell.