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. β β β β -carotene and α α α α -tocopherol), which is indicative of oxidative stress in chloroplasts. H 2 O 2 accumulated in drought-stressed senescing leaves. Subcellular localization studies showed that H 2 O 2 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 H 2 O 2 as direct responsible for the oxidative stress observed in the chloroplasts of drought-stressed senescing leaves. The strong degradation of β β β β -carotene and α α α α -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.

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This study evaluated the possible role of hydrogen peroxide (H2O2) in the acclimation of a Mediterranean shrub, Cistus albidus L., to summer drought growing under Mediterranean field conditions. For this purpose, changes in H2O2 concentrations and localization throughout a year were analysed. H2O2 changes in response to environmental conditions in parallel with changes in abscisic acid (ABA) and oxidative stress markers, together with lignin accumulation, xylem and sclerenchyma differentiation, and leaf area were also investigated. During the summer drought, leaf H2O2 concentrations increased 11-fold, reaching values of 10 μmol g−1 dry weight (DW). This increase occurred mainly in mesophyll cell walls, xylem vessels, and sclerenchyma cells in the differentiation stage. An increase in ABA levels preceded that of H2O2, but both peaked at the same time in conditions of prolonged stress. C. albidus plants tolerated high concentrations of H2O2 because of its localization in the apoplast of mesophyll cells, xylem vessels, and in differentiating sclerenchyma cells. The increase in ABA, and consequently of H2O2, in plants subjected to drought stress might induce a 3.5-fold increase in ascorbic acid (AA), which maintained and even decreased its oxidative status, thus protecting plants from oxidative damage. After recovery from drought following late-summer and autumn rainfall, a decrease in ABA, H2O2, and AA to their basal levels (∼60 pmol g−1 DW, ∼1 μmol g−1 DW, and ∼20 μmol g−1 DW) was observed.

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Mechanisms of drought stress resistance were studied in Cistus clusii Dunal and Cistus albidus L., two native Mediterranean shrubs that can withstand severe summer drought. While water deficit, solar radiation and temperature increased from winter to summer in the field, C. clusii and C. albidus reduced leaf area, increased root mass per leaf area, and showed diurnal changes in stomatal conductance to minimize water loss. In both species, the consequent reductions in CO2 assimilation were accompanied by reduced efficiency of photosystem II photochemistry, and protection against stress was afforded by enhanced de-epoxidation of violaxanthin in the xanthophyll cycle and increases in alpha-tocopherol and beta-carotene. In addition, hydrogen peroxide (H2O2) accumulation was observed in mesophyll cell walls of both species during the first stages of drought, although no accumulation of H2O2 was observed in chloroplasts or other organelles during the study. Despite these common responses, C. albidus and C. clusii differed in the extent of photo- and antioxidative protection. In response to drought, C. clusii showed a higher de-epoxidation state of the xanthophyll cycle and higher alpha-tocopherol and beta-carotene concentrations than C. albidus. We conclude that several structural and biochemical mechanisms underlie stress resistance in C. clusii and C. albidus, and are indicative of the different degrees of stress resistance of these shrubs.