Stomatal conductance, xylem water transport, and root traits underpin improved performance under drought and well-watered conditions across a diverse panel of maize inbred lines

We evaluated sixteen traits related to water acquisition and transport, stomatal conductance, and photosynthesis within a diverse panel of maize inbred lines, founders of the U.S. maize nested association mapping (NAM) population, with the aim to determine which traits confer improved growth under water deficit and well-watered conditions. Lasso regression revealed that three key traits explained meaningful and independent proportions of variation in total end-of-season biomass under deficit irrigation (multiple r ² = 0.86): 1) the maximal net CO ₂ assimilation rate (P = 0.007), 2) the achievable stomatal conductance during the hottest part of the day (P = 0.005), and 3) the width-to-depth ratio of the root system at the seedling stage (P = 0.060), i.e., initial deep root system development facilitated growth. Under well-watered conditions, maximal stomatal conductance in the morning (P = 0.014) and the width-to-depth ratio of the root system (P = 0.043) were identified as key traits contributing to improved performance (multiple r ² = 0.68). Structural equation models revealed that growth under water deficit was linked more strongly to stomatal conductance occurring during the middle of the day (std coef = 0.75; P = 0.006), rather than the maximal stomatal conductance (std coef = −0.25; P = 0.368). In turn, the maintenance of stomatal conductance through the middle of the day depended on the capacity of the xylem tissue to supply water (per unit cross-sectional area) (std coef = 0.48; P = 0.046). Aligned with the transport of water to the stomata and growth, root system depth (r = 0.77; P = 0.003) and width-to-depth ratio (r = −0.55; P = 0.064) at seedling stages were also correlated with the capacity of the xylem to transport water, thus suggesting close coordination between root, xylem, and stomatal traits to achieve greater growth under water deficit and well-watered conditions. We propose that maize performance under the drought conditions considered here, could likely be improved via lower stomatal sensitivity to hydraulic and atmospheric cues, greater xylem conductivity, and a deeper, but not necessarily more extensive, root system.