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

Light intensity () is the most dynamic and significant environmental variable affecting photosynthesis ( ), stomatal conductance ( ), transpiration ( ), and water-use efficiency (WUE). Currently, studies characterizing leaf-scale WUE- 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; WUE) and instantaneous WUE (WUE; WUE) at the corresponding saturation irradiances ( and )? This study developed WUE- and WUE- models sharing the same non-asymptotic function with previously published - and - models. Observation-modeling intercomparison was conducted for field-grown plants of soybean (C) and grain amaranth (C) to assess the robustness of our models versus the non-rectangular hyperbola models (NH models). Both types of models can reproduce WUE- curves well over light-limited range. However, at light-saturated range, NH models overestimated WUE and WUE and cannot return and 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 and WUE increased rapidly within low range of , driven by uncoupled photosynthesis and stomatal responsiveness. Initial response rapidity of WUE was higher than WUE because the greatest increase of and occurred at low . C species showed higher WUE and WUE than C species-at similar and . Our intercomparison highlighted larger discrepancy between WUE- and WUE- responses in C than C species, quantitatively characterizing an important advantage of C photosynthetic pathway-higher gain but lower cost per unit of change. Our models can accurately return the wealth of key quantities defining species-specific WUE- responses-besides - and - responses. The key advantage is its robustness in characterizing these entangled responses over a wide 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.