Drought response in forest trees: From the species to the gene

Forest tree species, considering their long lifespan, symbolize one of the best biological examples of adaptation to a frequently changing harsh terrestrial environment. The adaptation to environments with water scarcity was the first challenge in the evolution of terrestrial photosynthetic organisms, and prompted the development of strategies and mechanisms to cope with drought. In this respect, the particular evolution and life history of forest tree species have brought about a plethora of specific adaptations to dry environments. The presence of a hydraulic system for long distance water transport and the need of maintaining functional tissues and organs for long periods of time are two important characteristics making forest tree species singular organisms within the plant kingdom. Selective pressure has prompted a variety of strategies in the control of water losses to maintain the functionality of the hydraulic system without compromising the carbon balance of the plant. These and other physiological responses focussed to increase the dehydration tolerance of tissues (e.g., osmotic adjustment) have played an important role in the development of specific adaptations under water limiting conditions. The adaptive changes are observable at different scales: from the population to the species, from the individual to the gene. The advance of high-throughput technologies will enable to unveil the complex interplay between phenotype and genotype. Genomic, proteomic, transcriptomic, and metabolomic approaches are beginning to bring light to the molecular basis of adaptation to drought in forest tree species. These new technologies, combined with more traditional approaches, will improve our current knowledge of the functional and molecular basis underlying adaptation and evolution of forest tree species living under dry environments. In this respect, this chapter covers some aspects of adaptation to drought at different integrative levels, from an ecophysiological perspective to a molecular-based point of view.