Influences of residual stomatal conductance on the intrinsic water use efficiency of two C3 and two C4 species

Abstract

Intrinsic water use efficiency (WUE_i) is a critical parameter that encapsulates the equilibrium between carbon assimilation and the concomitant water expenditure. Enhancing the WUE_i of crops is not only instrumental in bolstering their resilience to drought but also enables higher carbon fixation efficiency under conditions of scarce water resources. Improving the WUE_i of crop varieties has become a major goal because water has become a critical limiting factor in crop productivity within the context of global change. The WUE_i, traditionally calculated by $WU{E}_i=\left(C_a-C_i\right)/1.6$(C_a, atmospheric CO₂ concentration; C_i, intercellular CO₂ concentration), may vary from that derived from $WU{E}_i=A/g_{\mathrm{sw}}$(A, net photosynthetic rate; g_sw, stomatal conductance to water vapor). In the study, the LI-6400 portable photosynthesis system was used for monitoring the leaf gas exchange of two C₃ (soybean and wheat) and two C₄ (maize and grain amaranth) species under changing irradiance (I) and CO₂ concentration conditions. One paired-sample t test was used to compare the significant differences between WUE_i values calculated by different equations and the observed values. The results showed that $WU{E}_i=\left(C_a-C_i\right)/1.6$ significantly overestimated the calculated WUE_i values than their corresponding observations by at least 17.78 %, 23.20 %, 9.07 %, and 14.26 % in light-response of WUE_i (WUE_i–I) and by at least 23.28 %, 22.02 %, 13.44 %, and 12.59 % in CO₂-response of WUE_i (WUE_i–C_i) curves for soybean, wheat, maize, and grain amaranth, respectively. However, the relationship between net photosynthetic rate (A) and stomatal conductance to CO₂ (g_sc) can be improved by incorporating an empirical slope (g₁) and residual stomatal conductance (g₀), which can be characterized as$A=\left(g_{\mathrm{sc}}–g_0\right)\left(C_a–C_i\right)/g_1$. Consequently, WUE_i can be calculated by $WU{E}_i=\frac{1}{1.6g_1}\left(1-\frac{1.6g_0}{g_{\mathrm{sw}}}\right)\left(C_a-C_i\right)$. It is highlighted that this modified equation can not only more accurately characterize the WUE_i in responses to varying I and CO₂ concentration conditions but also yields a remarkably high coefficient of determination (R² > 0.989) for the four species. These findings will provide plant physiologists and agronomists with a precise calculation tool to better understand and optimize crop water use efficiency in the face of environmental challenges.