Dark-adapted leaf conductance, but not minimum leaf conductance, predicts water use efficiency of soybean ( Glycine max L. Merr.)

Walden-Coleman, A. E., Rajcan, I. and Earl, H. J. 2013. Dark-adapted leaf conductance, but not minimum leaf conductance, predicts water use efficiency of soybean (Glycine max L. Merr.). Can. J. Plant Sci. 93: 13–22. The conductance to water vapor of dark-adapted leaves (g dark ) has been shown to be negatively correlated with whole-plant water use efficiency (WUE) in soybean, but the physiological basis of this relationship is unknown. It is also not clear how g dark compares with the minimum leaf conductance of wilted leaves (g min ), a trait that has been studied extensively across a broad range of species. We compared g dark to g min of soybean leaves and found that g dark values were consistently much higher than g min values measured on the same leaves. Also, across seven soybean varieties known to differ for WUE, g dark but not g min was correlated with WUE. Thus, g dark and g min should be considered distinct traits. We measured g dark at two different leaf positions, and found that g dark measured at the lower leaf position (two main stem nodes below the youngest fully expanded leaf) was best correlated with WUE. We then used this method to screen a selection of current commercial soybean varieties adapted to Ontario, Canada, for variation in g dark . The range in g dark among the commercial varieties was as broad as that measured previously among more diverse genotypes, suggesting that Ontario soybean varieties might also vary widely for WUE.