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

Intrinsic water use efficiency (WUEi) is a critical parameter that encapsulates the equilibrium between carbon assimilation and the concomitant water expenditure. Enhancing the WUEi 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 WUEi 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 WUEi, traditionally calculated by WUEi=(Ca−Ci)/1.6(Ca, atmospheric CO2 concentration; Ci, intercellular CO2 concentration), may vary from that derived from WUEi=A/gsw(A, net photosynthetic rate; gsw, 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 WUEi values calculated by different equations and the observed values. The results showed that WUEi=(Ca−Ci)/1.6 significantly overestimated the calculated WUEi values than their corresponding observations by at least 17.78 %, 23.20 %, 9.07 %, and 14.26 % in light-response of WUEi (WUEi–I) and by at least 23.28 %, 22.02 %, 13.44 %, and 12.59 % in CO2-response of WUEi (WUEi–Ci) curves for soybean, wheat, maize, and grain amaranth, respectively. However, the relationship between net photosynthetic rate (A) and stomatal conductance to CO2 (gsc) can be improved by incorporating an empirical slope (g1) and residual stomatal conductance (g0), which can be characterized asA=(gsc–g0)(Ca–Ci)/g1. Consequently, WUEi can be calculated by WUEi=11.6g1(1−1.6g0gsw)(Ca−Ci). It is highlighted that this modified equation can not only more accurately characterize the WUEi 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.