Auxins are known to regulate xylem development in plants, but their effects on water transport efficiency are poorly known. Here we used tomato plants with the diageotropica mutation (dgt), which has impaired function of a cyclophilin 1 cis–trans isomerase involved in auxin signaling, and the corresponding wild type (WT) to explore the mutation’s effects on plant hydraulics and leaf gas exchange. The xylem of the dgt mutant showed a reduced hydraulically weighted vessel diameter (D_h) (24–43%) and conduit number (25–58%) in petioles and stems, resulting in lower theoretical hydraulic conductivities (K_t); on the other hand, no changes in root D_h and K_t were observed. The measured stem and leaf hydraulic conductances of the dgt mutant were lower (up to 81%), in agreement with the K_t values; however, despite dgt and WT plants showing similar root D_h and K_t, the measured root hydraulic conductance of the dgt mutant was 75% lower. The dgt mutation increased the vein and stomatal density, which could potentially increase photosynthesis. Nevertheless, even though it had the same photosynthetic capacity as WT plants, the dgt mutant showed a photosynthetic rate c. 25% lower, coupled with a stomatal conductance reduction of 52%. These results clearly demonstrate that increases in minor vein and stomatal density only result in higher leaf gas exchange when accompanied by higher hydraulic efficiency.

Annual crops commonly experience production losses due to soil water limitation and increased vapor pressure deficit (VPD). Crop species and genotypes vary in their ability to sustain production during drought, which reflects variations in drought resistance mechanisms. In this review, we discuss the critical influence of water transport (hydraulic conductances and stomatal traits) on the ability of crops to avoid and tolerate drought, thus sustaining yield. We also summarize scientific gaps to be addressed in the future. Limited transpiration traits, including reduced stomatal density/conductance and increased stomatal sensitivity to soil drought and high VPD, constitute important drought avoidance mechanisms. Drought avoidance is suggested to result in soil water conservation for the critical reproductive stage and yield stability under moderate and terminal droughts. As crop fields experience increasingly drier soils and greater VPD, tolerance mechanisms might become critical to crop production. Osmotic adjustment stands as an important tolerance mechanism that improves crop production during severe droughts. Preventing xylem embolism and/or refilling embolized xylem upon rehydration represent drought tolerance mechanisms critical for plant survival during drought, but their contribution to crop production during droughts is unknown. Time for hydraulic failure combines drought avoidance and tolerance, and its importance for crop production during moderate and severe droughts should be assessed.

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.