Why Does Dark Adapated Leaf Conductance Predict Water Use Efficiency of Soybean?.

Increasing water use efficiency (WUE, defined as the amount of plant dry matter produced per unit of water transpired), has long been considered as a potential means of improving crop growth and yield under conditions where soil water availability is limiting.  It has recently been demonstrated that another trait, the conductance to water vapour of dark-adapted leaves (g_dark), is negatively correlated with WUE in several crop species, including soybean.  However, the physiological basis of this relationship is unclear, since existing theory to explain intraspecific differences in WUE does not invoke an obvious role for g_dark.  Our objectives were i) to verify that the correlation between WUE and g_dark exists among commercial soybean varieties adapted to Ontario, Canada; and ii) to investigate the physiological basis of this relationship.  Twelve commercial soybean varieties previously shown to differ for g_dark were grown under both water-replete and cyclic water stress conditions in a greenhouse experiment.  Six weeks after planting, just prior to destructive harvest for WUE determinations, measurements were made of leaf net CO₂ assimilation rate (A_n), stomatal conductance (g_s), leaf internal CO₂ concentration (C_i), stomatal density (Den_s), stomatal aperture length (Len_s), and g_dark.  Genotypes varied (p < 0.0001) for WUE with a range of 12%.  As in past studies, WUE was negatively correlated with g_dark (r = -0.70, p < 0.01).  g_dark was not correlated with Den_s or Len_s, but surprisingly was highly positively correlated with C_i (r = 0.83, p < 0.001) and g_s (r = 0.91, p < 0.001).  Well-established theory indicates that WUE should be negatively correlated with C_i.  We therefore conclude that the correlation between WUE and g_dark arises from the unexpected strong correlation between conductance to water vapour of leaves when they are dark adapted, and C_i of those same leaves during steady-state photosynthesis.