Light intensity (I) is the most dynamic and significant environmental variable affecting photosynthesis (A _n), stomatal conductance (g _s), transpiration (T _r), and water-use efficiency (WUE). Currently, studies characterizing leaf-scale WUE-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-sat and I _inst-sat)? This study developed WUE_i-I and WUE_inst-I models sharing the same non-asymptotic function with previously published A _n-I and g _s-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 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-sat and 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, driven by uncoupled photosynthesis and stomatal responsiveness. Initial response rapidity of WUE_i was higher than WUE_inst because the greatest increase of A _n and T _r occurred at low g _s. C₄ species showed higher WUE_i-max and WUE_inst-max than C₃ species-at similar I _i-sat and I _inst-sat. Our intercomparison highlighted larger discrepancy between WUE_i-I and WUE_inst-I responses in C₃ than C₄ species, quantitatively characterizing an important advantage of C₄ photosynthetic pathway-higher A _n gain but lower T _r cost per unit of g _s change. Our models can accurately return the wealth of key quantities defining species-specific WUE-I responses-besides A _n-I and g _s-I responses. The key advantage is its robustness in characterizing these entangled responses over a wide 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.

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