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ARS Home » Pacific West Area » Pullman, Washington » WHGQ » Research » Publications at this Location » Publication #378011

Research Project: Genetic Improvement of Wheat and Barley for Environmental Resilience, Disease Resistance, and End-use Quality

Location: Wheat Health, Genetics, and Quality Research

Title: Leaf temperature impacts whole plant water use efficiency independent of changes in leaf level water use efficiency

item SEXTON, THOMAS - Washington State University
item Steber, Camille
item COUSINS, ASAPH - Washington State University

Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2020
Publication Date: 1/5/2021
Citation: Sexton, T.M., Steber, C.M., Cousins, A.B. 2021. Leaf temperature impacts whole plant water use efficiency independent of changes in leaf level water use efficiency. Journal of Plant Physiology. 258-259. Article 153357.

Interpretive Summary: Increasing wheat and other crop yields under drought requires exquisite control of the balance between photosynthesis and water use. Humans rely on plant photosynthesis, directly or indirectly, for all forms of food. More efficient photosynthesis and water use should increase grain yield under stress. During photosynthesis, leaves assimilate carbon from carbon dioxide into high energy molecular bonds, leading to production of the sugars, starches, and fatty acids used as food. Carbon dioxide gas from the atmosphere enters leaves through pores called stomates that can open and close. Stomates also allow water to escape the leaf as it is drawn up from the roots by transpiration. This transpiration of water cools the leaves. When plants are thirsty, they close stomates to conserve water and become warmer because they don’t transpire. Closed stomates also reduce the ability to take up carbon dioxide for photosynthesis limiting grain yield in wheat. More efficient use of water in the leaf should increase carbon fixed for the amount of water lost. This paper found that this leaf water use efficiency (WUE) is not the only trait contributing to better photosynthesis and yield when water is limited. The ability to keep leaves cooler, likely from getting better access to soil water through roots or due to differences in plant body shape, keeps more water in the leaves when the stomates open because water evaporates less at lower temperature. This means that cooler plants are able to get more carbon for photosynthesis while loosing less water through open stomates.

Technical Abstract: Whole plant water use efficiency (above ground biomass over lifetime water loss, WUE_plant) can influence yield in wheat and other crops. Breeding for WUEplant is difficult because it is influenced by many component traits. For example, intrinsic water use efficiency (WUEi), the ratio of net carbon assimilation (Anet) over stomatal conductance contributes to WUE_plant and can be estimated from carbon isotope discrimination ('). However, ' is not sensitive to differences in the water vapor pressure deficit between the air and leaf (VPD_leaf). Alternatively, measurements of instantaneous leaf water use efficiency (WUE_leaf) defined as Anet over transpiration, can be determined with gas exchange but the dynamic nature of field conditions are not represented. Specifically, fluctuations in canopy temperature lead to changes in VPD_leaf impacting transpiration but not Anet, altering WUE_leaf that in turn affects WUE_plant. To test this relationship, WUE_plant was measured in conjunction with WUEi, WUE_leaf, and canopy temperature under well-watered and water-limited conditions in two drought tolerant wheat cultivars that differ in canopy architecture. In this experiment boundary layer conductance was low and significant changes in leaf temperature occurred between cultivars and treatments that were well correlated with WUE_plant, likely due to the effect of canopy temperature on VPD_leaf driving T. However, deviations between, WUEi, WUE_leaf, and WUE_plant were present because measurements made at the leaf level do not account for variation leaf temperature. This uncoupled the relationship of measured WUE_leaf and WUEi from WUE_plant and emphasizes the importance of canopy temperature on carbon uptake and transpired water loss.