Response of senescing cotyledons of clusterbean to water stress in moderate and low light: Possible photoprotective role of β-carotene

Abstract

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 DI protein or in the level of β-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 β-carotene and a quantitative loss in the DI 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 β-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 β-carotene may contribute to the assembly and stability of the DI protein during senescence and water stress in clusterbean cotyledons.