Senescence of Hydrilla leaves in light and darkness

Senescence of isolated leaves of Hydrilla verticillata (L.f.) Royle was studied in both darkness and light (20 μmol m⁻² s⁻¹). Senescence in the dark followed a general pattern of deterioration, i.e., gradual loss of cellular macromolecules like chlorophyll, protein and RNA with a concomitant rise in α-amino nitrogen, protease activity and tissue permeability. In light, however, an accelerated loss of chlorophyll took place although protein loss and increase in protease activity were retarded. A higher level of α-amino nitrogen in leaves in the light than in darkness could be correlated with lower leaching of free amino acids in light. Light decreased tissue permeability, as evidenced by lower conductivity of the incubation medium. In the light, RNA increased over the initial level. Both soluble and insoluble carbohydrates declined in the dark. The decline of insoluble carbohydrate was retarded by light, whereas soluble carbohydrate showed an initial rise and then declined sharply in the light.