Linking water-use strategies with drought resistance across herbaceous crops

Woody plants minimize xylem embolism formation during drought essentially by closing stomata at higher water potentials and/or by increasing the xylem resistance to embolism. Both of these mechanisms result in a higher stomatal safety margin (SSM), which is the water potential difference between stomatal closure and embolism formation. Here, we investigated whether increasing SSM represents a drought resistance mechanism for herbaceous plants and how the different water-use strategies impact their survival. For that, we exposed four herbaceous crops with contrasting drought resistance to severe water deficit to assess drought-induced damage and mortality. Unlike woody species, SSM was not associated with plant survival for herbaceous crops. Soybean, which presented the largest SSM across the four crops (1.67 MPa), exhibited the earliest mortality of leaves and whole plants as well as the highest rate of plant mortality (100%) at the end of the drought period. Cowpea, with an SSM of 0.63 MPa, was the most drought-resistant species, with the latest leaf damage and the highest plant survival (100%). The most effective traits ensuring survival in herbaceous crops under severe drought were those related to drought avoidance mechanisms such as (1) early stomatal closure, (2) very low residual transpiration post-stomatal closure, and (3) high capacitance pre- and post-turgor loss.