Induction and suppression of RNase activity by growth regulators in senescing ragi leaves

Abstract

Activity of RNase was studied in attached and detached leaves of 7‐day‐old ragi (Eleusine coracana Gaertn. cv PR 202) plants during senescence using crude enzyme extracts. The RNase activity was relatively constant in attached leaves. In excised leaves incubated in the dark there was a rapid rise in enzyme activity up to 48 h, followed by a decline. No such decrease was observed in the light. Benzimidazole and gibberellic acid suppressed the activity of RNase up to 48 h in the dark and 96 h in the light. Both the growth regulators also prevented the post‐48 h decline in RNase activity of dark incubated excised leaves. Decline in the levels of chlorophyll and RNA in the illuminated excised leaves was not affected by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethyIurea, but the inhibitor prevented the photo‐induced rise in RNase activity. Cycloheximide and actinomycin D could completely prevent both detachment (increase in enzyme activity after the leaf is excised) and photo‐induced rise in RNase activity. Benzimidazole and gibberellic acid prevented the rise in the activity of RNase on one hand and maintained it on the other by their influence on its biosynthesis. Photoinduction of RNase and photo‐induced retardation of senescence are concluded to be two different processes.

Senescence of Hydrilla leaves in light and darkness

“…The increased level of RNA in Hydrilla leaves during senescence in the light is in contrast with earlier observations in ragi (Eleusine coracana) leaves (Kumar and Khan 1983). This could be ascribed not only to a lower rate of degradation but also to a higher rate of synthesis of RNA.…”

“…The decline in aamino nitrogen during the later period of senescence might be due to consumption of amino acids as substrates of respiration since the sugar is then almost completely depleted in the tissue (Yemm 1937). A slow but gradual decrease in RNA content was observed during senescence in the dark, an observation made by several authors in land plants (Smillie and Krotkov 1961, Osborne 1967, Kumar and Khan 1983.…”

“…Changes in nucleic acids and carbohydrate levels also occur during leaf senescence (Smlllie and Krotkov 1961, Tetley and Thimann 1974, Naito et al 1978, Nooden 1980. Light delays the senescence of leaves by retarding these changes (Takegami 1975, Choe and Thimann 1977, Kumar and Khan 1983. However, there are instances where light enhances chlorophyll loss during senescence (Goldthwaite andLaetseh 1967, Pjon 1981).…”