Plants are often subjected to periods of soil and atmospheric water deficit during their life cycle. The frequency of such phenomena is likely to increase in the future even outside today’s arid/semi‐arid regions. Plant responses to water scarcity are complex, involving deleterious and/or adaptive changes, and under field conditions these responses can be synergistically or antagonistically modified by the superimposition of other stresses. This complexity is illustrated using examples of woody and herbaceous species mostly from Mediterranean‐type ecosystems, with strategies ranging from drought‐avoidance, as in winter/spring annuals or in deep‐rooted perennials, to the stress resistance of sclerophylls. Differences among species that can be traced to different capacities for water acquisition, rather than to differences in metabolism at a given water status, are described. Changes in the root : shoot ratio or the temporary accumulation of reserves in the stem are accompanied by alterations in nitrogen and carbon metabolism, the fine regulation of which is still largely unknown. At the leaf level, the dissipation of excitation energy through processes other than photosynthetic C‐metabolism is an important defence mechanism under conditions of water stress and is accompanied by down‐regulation of photochemistry and, in the longer term, of carbon metabolism.

Potted cuttings of three Eucalyptus globulus Labill. clones (AR3, CN44, MP11) were either well watered or subjected to one of two soil water deficit regimes for six months in a greenhouse. Reductions in lateral branching, leaf production and leaf expansion were the leading contributors to the large differences observed in biomass production between well-watered and water-stressed plants. Although no significant differences among clones were observed in dry matter accumulation or in the magnitude of the response to soil water deficits, sensitivity of lateral branching, leaf initiation and whole-plant foliage to water stress was significantly lower in CN44 than in AR3 and MP11. When the confounding effect of differences in plant size resulting from the different watering regimes was removed, allometric analysis indicated that the genotypes differed in biomass allocation patterns. In addition to a drought-induced reduction in leaf number, water deficits also resulted in smaller leaves because leaf expansion was inhibited during dehydration events. Resumption of leaf expansion following stress relief occurred in all of the clones, but was particularly evident in severely stressed plants of Clone AR3, possibly as a result of the osmotic adjustment observed in this genotype.

Predicted future climatic changes for the Mediterranean region give additional importance to the study of photooxidative stress in local economic species subjected to combined drought and high-temperature conditions. Under this context, the impact of these stresses on photosynthesis, energy partitioning, and membrane lipids, as well as the potential ability to attenuate oxidative damage, were investigated in Ceratonia siliqua L. Two thermal regimes (LT: 25/18°C; HT: 32/21°C) and three soil water conditions (control, water stress, and rewetting) were considered. HT exacerbated the adverse effects of water shortage on photosynthetic rates (P _N) and PSII function. The decrease in P _N was 33% at LT whereas at HT it was 84%. In spite of this, the electron transport rate (ETR) was not affected, which points to an increased allocation of reductants to sinks other than CO₂ assimilation. Under LT conditions, water stress had no significant effects on yield of PSII photochemistry (Φ_PSII) and yields of regulated (Φ_NPQ) and nonregulated (Φ_NO) energy dissipation. Conversely, drought induced a significant decrease of Φ_PSII and a concomitant increase of Φ_NO in HT plants, thereby favouring the overproduction of reactive oxygen species (ROS). Moreover, signs of lipid peroxidation damage were detected in HT plants, in which drought caused an increase of 40% in malondialdehyde (MDA) content. Concurrently, a marked increase in proline content was observed, while the activities of catalase (CAT) and ascorbate peroxidase (APX) were unaffected. Despite the generation of a moderate oxidative stress response, C. siliqua revealed a great capability for photosynthetic recovery 36 h after rewatering, which suggests that the species can cope with predicted climate change.