Flag leaf senescence in intact rice plant: Effects of hormones on the activities of “senescence-enzymes” during leaf age at the reproductive development

Abstract

Changes of enzyme activities, viz., catalasa, protease, RNase, acid and alkaline pyrophosphatase in. the flag and the second leaves of rice (Oryza sativa L. cv. Jaya) were studied at an interval of 7 days during the process of reproductive development of the plants sprayed with 100 μg/ml aqueous solution of GA3, IAA and kinetin 10 days after flowering. The activities of protease, RNase and acid pyrophosphatase in the flag and second leaf showed a gradual rise with the advance in age, and this rise was much faster in the flag leaf at the 14th day culminating in higher activities in the flag and lower in the second leaf at the 21st day. Kinetin treatment significantly inhibited the rise of these enzyme activities and the onset of their higher activities in the flag leaf compared to the second leaf was much delayed, though the activities remained higher in the flag leaf at the 21st day. Both IAA and GA3 were least effective in this respect. Catalase and alkaline pyrophosphatase activities declined with advance in age and the flag leaf showed more declining activities after the 7th day compared to the second leaf. Kinetin significantly improved their activities and the onset of decline was much delayed in the flag leaf. In the last phase of reproductive development, the activities of these enzymes remained lower in the flag than that of the second leaf. IAA and GA3 had minor effects on these enzymes. Thus, the faster rise and decline in the activities of several “senescence enzymes” in the flag leaf may be associated with earlier senescence of it, and cytokinins are perhaps the most important hormones regulating the activities of these enzymes and consequently flag leaf senescence.