Andrew D. B. Leakey

Stomata regulate leaf CO 2 assimilation ( A ) and water loss. The Ball–Berry and Medlyn models predict stomatal conductance ( g s ) with a slope parameter ( m or g 1 ) that reflects the sensitivity of g s to A , atmospheric CO 2 and humidity, and is inversely related to water use efficiency (WUE). This study addressed knowledge gaps about what the values of m and g 1 are in C 4 crops under field conditions, as well as how they vary among genotypes and with drought stress. Four inbred maize genotypes were unexpectedly consistent in how m and g 1 decreased as water supply decreased. This was despite genotypic variation in stomatal patterning, A and g s . m and g 1 were strongly correlated with soil water content, moderately correlated with predawn leaf water potential (Ψ pd ), but not correlated with midday leaf water potential (Ψ md ). This implied that m and g 1 respond to long‐term water supply more than short‐term drought stress. The conserved nature of m and g 1 across anatomically diverse genotypes and water supplies suggests there is flexibility in structure‐function relationships underpinning WUE. This evidence can guide the simulation of maize g s across a range of water supply in the primary maize growing region and inform efforts to improve WUE.