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

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 W U E i = ( C a − C i ) / 1.6 ( C a , atmospheric CO 2 concentration; C i , intercellular CO 2 concentration), may vary from that derived from W U E i = A / g 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 C 3 (soybean and wheat) and two C 4 (maize and grain amaranth) species under changing irradiance ( I ) and CO 2 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 W U E i = ( C a − C i ) / 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 CO 2 -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 CO 2 ( g sc ) can be improved by incorporating an empirical slope ( g 1 ) and residual stomatal conductance ( g 0 ), which can be characterized as A = ( g sc – g 0 ) ( C a – C i ) / g 1 . Consequently, WUE i can be calculated by W U E i = 1 1.6 g 1 ( 1 − 1.6 g 0 g sw ) ( C a − C i ) . It is highlighted that this modified equation can not only more accurately characterize the WUE i in responses to varying I and CO 2 concentration conditions but also yields a remarkably high coefficient of determination ( R 2 > 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. • W U E i = ( C a − C i ) / 1.6 significantly overestimated the observed WUE i in both WUE i – I and WUE i – C i curves for all four species. • Incorporating g 1 and g 0 can improve the relationship between A and g sc . • Modified equation W U E i = 1 1.6 g 1 ( 1 − 1.6 g 0 g sw ) ( C a − C i ) more accurately characterizes WUE i – I and WUE i – C i curves for the four species. • Mathematical interpolation estimated g 0 values much higher than those from the modified equation or commonly assigned values.