Drought-induced senescence is characterized by a loss of antioxidant defences in chloroplasts

The relationship between drought, oxidative stress and leaf senescence was evaluated in field-grown sage (Salvia officinalis L.), a drought-susceptible species that shows symptoms of senescence when exposed to stress. Despite the photoprotection conferred by the xanthophyll cycle, drought-stressed senescing leaves showed enhanced lipid peroxidation, chlorophyll loss, reduced photosynthetic activity and strong reductions of membrane-bound chloroplastic antioxidant defences (i.e. beta -carotene and alpha -tocopherol), which is indicative of oxidative stress in chloroplasts. H2O2 accumulated in drought-stressed senescing leaves. Subcellular localization studies showed that H2O2 accumulated first in xylem vessels and the cell wall and later in the plasma membrane of mesophyll cells, but not in chloroplasts, indicating reactive oxygen species other than H2O2 as direct responsible for the oxidative stress observed in the chloroplasts of drought-stressed senescing leaves. The strong degradation of beta -carotene and alpha -tocopherol suggests an enhanced formation of singlet oxygen as the putative reactive oxygen species responsible for oxidative stress to senescing chloroplasts. This study demonstrates that oxidative stress in chloroplasts mediates drought-induced leaf senescence in sage growing in Mediterranean field conditions.