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 (gdark) 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 gdark compares with the minimum leaf conductance of wilted leaves (gmin), a trait that has been studied extensively across a broad range of species. We compared gdark to gmin of soybean leaves and found that gdark values were consistently much higher than gmin values measured on the same leaves. Also, across seven soybean varieties known to differ for WUE, gdark but not gmin was correlated with WUE. Thus, gdark and gmin should be considered distinct traits. We measured gdark at two different leaf positions, and found that gdark 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 gdark. The range in gdark 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.