Quantifying Light Response of Leaf-Scale Water-Use Efficiency and Its Interrelationships With Photosynthesis and Stomatal Conductance in C(3) and C(4) Species

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 (C(3)) and grain amaranth (C(4)) 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). C(4) species showed higher WUE(i–max) and WUE(inst–max) than C(3) 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 C(3) than C(4) species, quantitatively characterizing an important advantage of C(4) 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.