Summary: Variation in the stable carbon and oxygen isotope composition (δ¹³C, Δ¹⁸O) of co‐occurring plant species may reflect the functional diversity of water use strategies present in natural plant communities. We investigated the patterns of water use among 10 coexisting plant species representing diverse taxonomic groups and life forms in semiarid southeast Spain by measuring their leaf δ¹³C and Δ¹⁸O, the oxygen isotope ratio of stem water and leaf gas exchange rates. Across species, Δ¹⁸O was tightly negatively correlated with stomatal conductance (g_s), whereas δ¹³C was positively correlated with intrinsic water use efficiency (WUE_i). Broad interspecific variation in Δ¹⁸O, δ¹³C andWUE_iwas largely determined by differences ing_s, as indicated by a strong positive correlation between leaf δ¹³C and Δ¹⁸O across species The 10 co‐occurring species segregated along a continuous ecophysiological gradient defined by their leaf δ¹³C and Δ¹⁸O, thus revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies ranging from ‘profligate/opportunistic’ (highg_s, lowWUE_i) to ‘conservative’ (lowg_s, highWUE_i). Coexisting species maintained their relative isotopic rankings in 2 yr with contrasting rainfall, suggesting the existence of species‐specific ‘isotopic niches’ that reflect ecophysiological niche segregation in dryland plant communities.

In order to understand the role of cytosolic antioxidant enzymes in drought stress protection, transgenic tobacco (Nicotiana tabacum cv. Xanthi) plants overexpressing cytosolic Cu/Zn-superoxide dismutase (cytsod) (EC 1.15.1.1) or ascorbate peroxidase (cytapx) (EC 1.11.1.1) alone, or in combination, were produced and tested for tolerance against mild water stress. The results showed that the simultaneous overexpression of Cu/Znsod and apx or at least apx in the cytosol of transgenic tobacco plants alleviates, to some extent, the damage produced by water stress conditions. This was correlated with higher water use efficiency and better photosynthetic rates. In general, oxidative stress parameters, such as lipid peroxidation, electrolyte leakage, and H₂O₂ levels, were higher in non-transformed plants than in transgenic lines, suggesting that, at the least, overexpression of cytapx protects tobacco membranes from water stress. In these conditions, the activity of other antioxidant enzymes was induced in transgenic lines at the subcellular level. Moreover, an increase in the activity of some antioxidant enzymes was also observed in the chloroplast of transgenic plants overexpressing cytsod and/or cytapx. These results suggest the positive influence of cytosolic antioxidant metabolism on the chloroplast and underline the complexity of the regulation network of plant antioxidant defences during drought stress.

Potted tomato plants (Lycopersicon esculentum Mill. cv. Amalia) were submitted to three different treatments: control (C) plants were maintained at day/night temperature of 25/18 °C; preconditioned plants (PS) were submitted to two consecutive periods of 4 d each, of 30/23 and 35/28 °C before being exposed to a heat stress (40/33 °C lasting 4 d) and non-preconditioned (S) plants were maintained in the same conditions as the C plants and exposed to the heat stress. The inhibition of plant growth was observed only in PS plants. Heat stress decreased chlorophyll content, net photosynthetic rate and stomatal conductance in both PS and S plants. However, PS plants showed good osmotic adjustment, which enabled them to maintain leaf pressure potential higher than in S plants. Furthermore, at the end of the recovery period PS plants had higher pressure potential and stomatal conductance than in S plants.