Reconciling Water‐Use Efficiencies of Cotton in Field and Laboratory

Stomatal behavior of cotton ( Gossypium hirsutum L.) differs between controlled environments and the field. Two important differences are the much greater stomatal conductance maintained in the field in the absence of water stress, and the reduced response of stomata to water stress in the field. This review paper describes and supports the hypothesis that the differences in stomatal behavior result from effects of temperature upon abscisic acid (ABA) metabolism. The reasoning is based on the following points: (i) controlled‐environment experiments are frequently run at unrealistically low temperatures near the optimum for plant growth (≈ 30 °C); (ii) higher temperatures in controlled environments change stomatal behaviors to resemble those seen in the field, and they promote the metabolic loss of ABA from leaf tissue; (iii) in the field, ABA concentrations leaves are very low at high temperatures, and stomatal responsiveness to CO 2 (an indicator of a role for ABA in mediating stomatal movements is poor; (iv) stomata at high temperatures in the field retain the ability to respond to exogenous ABA; (v) in the field late in the season, when temperatures resemble those of controlled environments, ABA concentrations of leaves and stomatal properties shift to resemble those seen in controlled environments; and finally, (vi) these shifts are unrelated to photosynthetic properties of the mesophyll. Evidence indicates that stomatal behavior in the field may be related to heat tolerance of the crop, with the very high conductances at high temperatures permitting substantial evaporative cooling of the foliage. Both plant physiology and agronomy need controlled environments, because these permit studies of stomatal responses to environmental factors in the absence of confounding variables. Such experiments must use realistic temperatures that simulate the field environment, however, to obtain results relevant to field performance.