Quantifying Light Response of Leaf-Scale Water-Use Efficiency and Its Interrelationships With Photosynthesis and Stomatal Conductance in C3 and C4 Species

Abstract

Light intensity (<i>I</i>) is the most dynamic and significant environmental variable affecting photosynthesis (<i>A</i> _n), stomatal conductance (<i>g</i> _s), transpiration (<i>T</i> _r), and water-use efficiency (WUE). Currently, studies characterizing leaf-scale WUE-<i>I</i> responses are rare and key questions have not been answered. In particular, (1) What shape does the response function take? (2) Are there maximum intrinsic (WUE_i; WUE_i-max) and instantaneous WUE (WUE_inst; WUE_inst-max) at the corresponding saturation irradiances (<i>I</i> _i-sat and <i>I</i> _inst-sat)? This study developed WUE_i-<i>I</i> and WUE_inst-<i>I</i> models sharing the same non-asymptotic function with previously published <i>A</i> _n-<i>I</i> and <i>g</i> _s-<i>I</i> models. Observation-modeling intercomparison was conducted for field-grown plants of soybean (C₃) and grain amaranth (C₄) to assess the robustness of our models versus the non-rectangular hyperbola models (NH models). Both types of models can reproduce WUE-<i>I</i> curves well over light-limited range. However, at light-saturated range, NH models overestimated WUE_i-max and WUE_inst-max and cannot return <i>I</i> _i-sat and <i>I</i> _inst-sat due to its asymptotic function. Moreover, NH models cannot describe the down-regulation of WUE induced by high light, on which our models described well. The results showed that WUE_i and WUE_inst increased rapidly within low range of <i>I</i>, driven by uncoupled photosynthesis and stomatal responsiveness. Initial response rapidity of WUE_i was higher than WUE_inst because the greatest increase of <i>A</i> _n and <i>T</i> _r occurred at low <i>g</i> _s. C₄ species showed higher WUE_i-max and WUE_inst-max than C₃ species-at similar <i>I</i> _i-sat and <i>I</i> _inst-sat. Our intercomparison highlighted larger discrepancy between WUE_i-<i>I</i> and WUE_inst-<i>I</i> responses in C₃ than C₄ species, quantitatively characterizing an important advantage of C₄ photosynthetic pathway-higher <i>A</i> _n gain but lower <i>T</i> _r cost per unit of <i>g</i> _s change. Our models can accurately return the wealth of key quantities defining species-specific WUE-<i>I</i> responses-besides <i>A</i> _n-<i>I</i> and <i>g</i> _s-<i>I</i> responses. The key advantage is its robustness in characterizing these entangled responses over a wide <i>I</i> range from light-limited to light-inhibitory light intensities, through adopting the same analytical framework and the explicit and consistent definitions on these responses. Our models are of significance for physiologists and modelers-and also for breeders screening for genotypes concurrently achieving maximized photosynthesis and optimized WUE.