Senescence of clusterbean (Cyamopsis tetragonoloba L.) cotyledons in moderate light (12 W m-2) brings about a loss in the pigments, enhanced lipid peroxidation and a decline in PS II photochemical activity without any loss either in Dl protein or in the level of beta-carotene. The senescence syndrome is aggravated in the cotyledons of water-stressed seedlings with an increase in thylakoid lipid peroxidation, a decline in the level of beta-carotene and a quantitative loss in the Dl protein. Loss of the protein, however, is arrested in the seedlings experiencing water stress at low light (3 W m-2) intensity that correlates with the stability in the level of beta-carotene and a slow rate of lipid peroxidation. Loss of the protein in moderate light is attributed to water-stress sensitized photoinhibitory damage. The data on changes in the components of xanthophyll cycle suggest the low activity of the cycle both during senescence and water stress. It is, therefore, concluded that beta-carotene may contribute to the assembly and stability of the Dl protein during senescence and water stress in clusterbean cotyledons.

Seedlings of Cyamopsis tetragonoloba were grown on Petri dishes either in water or water plus 3 % PEG-6000 to induce water stress. The senescing cotyledons experiencing the stress exhibited loss in contents of leaf proteins and chlorophyll (Chl) and a decline in oxygen evolution. The effect of PEG treatment was more pronounced at moderate (MI) than low (LI) irradiance. The stress-induced loss in the activity of superoxide dismutase and increase in the thylakoid lipid peroxidation accompanied a change in the physical status of the bilayer membrane as demonstrated by an enhancement of room temperature Chl a fluorescence polarization and decrease in energy transfer efficiency in pigment assembly. This resulted in a sustained decrease in photosystem 2 activity blocking channels of energy utilization. The absorbed quanta, thus unutilized, were excess even at MI, leading to photoinhibitory response.

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