Whole-plant hydraulics, water saving, and drought tolerance: A triptych for crop resilience in a drier world

A global increase in atmospheric vapour pressure deficit (VPD) or ‘atmospheric drought’ is driving productivity losses across ecosystems and agrosystems. Fortunately, over the last 15 years, substantial progress has been made in developing cultivars of major crops that take advantage of increases in evaporative demand to increase yields under drought. Such progress has been achieved thanks to knowledge leveraged from plant hydraulics to identify water-saving traits enabling increased soil moisture during the critical seed-fill phase (i.e. a yield-based drought-tolerance, in opposition to plant survival). Such promising traits include enhancing crop transpiration rate sensitivity to increasing VPD or to soil drying and decreasing night-time transpiration, especially in areas with high nocturnal VPD. Despite a surge of literature on the topic of drought-tolerance over the same period, an integrative, organismal understanding of the processes underlying these water-saving traits is still, surprisingly, limited. This hampers much-needed efforts to fast-track breeding of drought-tolerant cultivars and to increase accuracy of models predicting vegetation response to climate change. In this article, we provide an overview of the triptych defined by whole-plant hydraulics, water-saving traits, and yield-based drought-tolerance by recruiting basic concepts in plant-water relations, crop physiology, and recent developments in plant hormonal biology and functional ecology, at scales ranging from cellular to whole-plant levels. We discuss knowledge gaps, trade-offs, and outline general guidelines for future, integrative research efforts.