There are currently two grand hypotheses to account for stomatal responses to environment: (1) the optimization hypothesis, stating that stomata open and close to optimize water-use efficiency in any environment; and (2) the thermal kinetic window (TKW) hypothesis, stating that stomata open and close to optimize leaf temperature. In the high-temperature cotton-growing regions of Arizona, transpiration is extremely high, and stomata do little to limit it. This behavior appears to be consistent with the TKW hypothesis. If the crop is grown with a chronic mild water stress, though, there is a shift toward more stomatal control of water loss. This can be interpreted as optimization of gas exchange. As originally envisioned, optimization requires a "messenger" compound to coordinate the activities of the leaf mesophyll and the stomata. Several lines of evidence indicate that the messenger is abscisic acid (ABA). High temperature decreases leaf ABA level below a critical threshold for functioning, but water stress restores ABA-like features of the stomatal control system. The two hypotheses may model simply whether ABA synthesis and degradation are balanced to maintain a high or low level of the compound in leaves.

The purpose of this symposium is to examine the gas exchange properties of cotton from the broadest possible perspective, the perspective of the plant's fitness for its environment. In this case fitness has to do with two interrelated and potentially growth-limiting environmental parameters: temperature and water availability. When water is the limiting factor, then the problem of maximizing growth becomes one of balancing opposing priorities: carbon gain can occur only while water is being lost. Thus the plant best fitted for this environment is one that maximizes its water-use efficiency (WUE).