Review. When water runs short: How plant hydraulic traits function and matter most

Long-standing puzzles have perplexed scientists about the physiological tradeoff between drought resistance and crop production, and this review brings the pieces together. What, exactly, allows a crop to keep producing when water runs short? Should breeding focus on early stomatal closure and limited transpiration? Does stronger sensitivity to high vapor pressure deficit conserve soil water without reducing yield? When do tolerance traits such as osmotic adjustment, resistance to xylem embolism, or hydraulic refilling make a real difference? Cardoso and colleagues explain how plant hydraulics, including root and leaf conductance, stomatal traits, and water storage capacity, shape both drought avoidance and drought tolerance. Avoidance conserves water by lowering stomatal density and conductance, strengthening responses to soil drying and high vapor pressure deficit, and reducing nighttime and residual water loss. Tolerance keeps plants functional at lower water potentials through osmotic adjustment and by preventing or refilling xylem embolism. The authors propose an integrated measure, the “time for hydraulic failure,” that combines both strategies. They conclude that avoidance is often most important under moderate drought, while tolerance becomes essential as stress intensifies beyond turgor loss. Genetic differences and nutrient management can shift the point where plants begin to limit transpiration, and some genotypes naturally cap transpiration once the air becomes sufficiently dry. Because photosynthesis responds nonlinearly to stomatal conductance, modest reductions in conductance can improve intrinsic water-use efficiency with little penalty to carbon gain. The review also highlights nighttime and residual leaf conductance as overlooked sources of water loss that could be addressed through breeding. Although many crops have xylem that are less resistant to embolism than wild relatives, useful variation exists, root pressure may help restore water transport after drought, and the direct effect of embolism on yield remains uncertain. (Summary by Dr. Hao Chen @ Auburn CFWE) Plant Physiol. 10.1093/plphys/kiaf521