Qualitative and quantitative effect of light on leaf senescence in a submerged aquatic weed, hydrilla verticillata

Incubation of isolated leaves of Hydrilla verticillata (L.F.) Royle, a submerged aquatic weed, under different qualities of light given in a daily cycle showed a faster decline in chlorophyll and protein contents under red light, while a slower decline of the same under far-red and blue light than dark control. When the photon fluence rate of daily light cycle was varied (0 – 62.5 μmol m⁻² s⁻¹), senescence-induced loss of chlorophyll and protein was found to be fluence rate-dependent suggesting a quantitative role for light. Changing duration of daily photoperiod (0 – 24 h), instead of fluence rate, also had a quantitative effect on senescence, particularly chlorophyll loss. In another experiment, where light was given in different combinations of photon fluence rate and duration (in daily cycle) having total fluence (fluence rate x duration) constant, chlorophyll loss was almost fluence-dependent except when the duration was too long (20 h). However, senescence-induced protein loss was greater at higher photon fluence rates compared to dark control.