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.