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

Authors: Gleason, Sean; Cooper, Mitchell; Wiggans, Dustin; Bliss, Clayton; ROMAY, CINTA - Cornell University; GORE, MICHAEL - Cornell University; MICKELBART, MICHAEL - Indiana University-Purdue University; TOPP, CHRISTOPHER - Danforth Plant Science Center; Zhang, Huihui; DeJonge, Kendall; Comas, Louise

Submitted to: Field Crops Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2019
Publication Date: 4/1/2019
Citation: Gleason, S.M., Cooper, M.S., Wiggans, D.R., Bliss, C.A., Romay, C.M., Gore, M.A., Mickelbart, M.V., Topp, C., Zhang, H., DeJonge, K.C., Comas, L.H. 2019. 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. Field Crops Research. 234:119-128. https://doi.org/10.1016/j.fcr.2019.02.001.
DOI: https://doi.org/10.1016/j.fcr.2019.02.001

Interpretive Summary: Our ability to improve the performance of crops in dry environments is impeded by our poor understanding of what plant traits actually confer drought tolerance. This study aimed to improve our understanding of drought tolerance by measuring many different traits, thought to be important in conferring drought tolerance, and then comparing their effect on plant performance using different statistical approaches. The outcome of these analyses identified a network of plant traits that were strongly connected to one another, and together explained up to 86% of the variation in growth. These traits included the depth of the root system (required to access water at deeper depth), the efficiency of the plant transport tissue (xylem) to transport water (delivered from the root system), and the capacity of leaves to exchange this water for CO2 (stomatal conductance and photosynthesis). These results suggest 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 drought 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.

Technical Abstract: We evaluated 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. Correlation structures were examined using lasso (least absolute shrinkage and selection operator) regression, bivariate line-fitting, and structural equation modeling. Lasso regression revealed that three key traits explained meaningful and independent proportions of variation in total end-of-season biomass under deficit irrigation: 1) the maximal net CO2 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 allocation to deep root system development, facilitated growth (multiple r2 = 0.86). Similarly, 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 under well-watered conditions (multiple r2 = 0.68). Considering the importance of leaf-level gas exchange in this analysis (CO2 and water vapor), we therefore examined root system and xylem traits associated with supplying water to the stomata during water deficit, i.e. to support gas exchange. 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 drought 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.