A unified framework of plant adaptive strategies to drought: Crossing scales and disciplines

Plant adaptation to drought has been extensively studied at many scales from ecology to molecular biology across a large range of model species. However, the conceptual frameworks underpinning the definition of plant strategies, and the terminology used across the different disciplines and scales are not analogous. 'Drought resistance' for instance refers to plant responses as different as the maintenance of growth and productivity in crops, to the survival and recovery in perennial woody or grassland species. Therefore, this paper aims to propose a unified conceptual framework of plant adaptive strategies to drought based on a revised terminology in order to enhance comparative studies. Ecological strategies encapsulate plant adaptation to multidimensional variation in resource variability but cannot account for the dynamic and short-term responses to fluctuations in water availability. Conversely, several plant physiological strategies have been identified along the mono-dimensional gradient of water availability in a given environment. According to a revised terminology, dehydration escape, dehydration avoidance, dehydration tolerance, dormancy, and desiccation tolerance are clearly distinguishable. Their sequential expression is expressed as water deficit increases while cavitation tolerance is proposed here to be a major hydraulic strategy underpinning adaptive responses to drought of vascular plants. This continuum of physiological strategies can be interpreted in the context of the ecological trade-off between water-acquisition vs. water-conservation, since growth maintenance is associated with fast water use under moderate drought while plant survival after growth cessation is associated with slow water use under severe drought. Consequently, the distinction between 'drought resistance' and 'drought survival', is emphasized as crucial to ensure a correct interpretation of plant strategies since 'knowing when not to grow' does not confer 'drought resistance' but may well enhance 'drought survival'. This framework proposal should improve cross-fertilization between disciplines to help tackle the increasing worldwide challenges that drought poses to plant adaptation.