Fast stomatal responses require anisohydric stomatal behaviour and lower water use efficiency in sorghum

Intrinsic water use efficiency (iWUE), the ratio of leaf carbon assimilation (A_n) to stomatal conductance to water vapour (g_s), is an important crop trait. The effect of environmental fluctuations, such as light transitions, on iWUE are mediated via the stomatal kinetic responses to these fluctuations. We screened for variation in stomatal kinetic response to changes in light intensity in a sorghum population containing different haplotypes, selected based on variation in aquaporin (AQP) alleles. We assessed the role of stomatal anatomy in determining iWUE. Furthermore, we grew plants under water stress to reveal the trade-offs between water use and conservation underlined by stomatal kinetics and behavior. The results showed that iWUE measured from steady state A_n/g_s and iWUE calculated from A_n and g_s responses to transient light change (dynamic iWUE) correlated, propelled by prolonged stomatal opening time (k_open) but independent of stomatal closing time (k_close). Stomatal size and density played a minor role in determining those responses but still might have an indirect effect. Leaf width correlated instead with faster stomatal opening and increased anatomical conductance. “Faster” stomata were significantly determined by decreased regulation of leaf water potential (increased anisohydry) because they had reduced sensitivity to changes in leaf water status, while “slower” stomata were characterized by water conservation and heightened sensitivity to changes in leaf water potential. We found those two sets of behaviors can be distinguished by an AQP-associated haplotype (SbTIP3.2), opening the door for newer genetic determinants of iWUE that can incorporate possible trade-offs.