Near-optimal response of instantaneous transpiration efficiency to vapour pressure deficit, temperature and [CO2] in cotton (Gossypium hirsutum L.)

The instantaneous transpiration efficiency (ITE, the ratio of photosynthesis rate to transpiration) is an important variable for crops, because it ultimately affects dry mass production per unit of plant water lost to the atmosphere. The theory that stomata optimize carbon uptake per unit water used predicts that ITE should be proportional to the atmospheric [CO2] (C-a), approximately inversely proportional to the square root of the leaf-to-air vapour pressure deficit (D-s), and weakly dependent on leaf temperature (T-leaf). We measured the response of ITE to a range in D-s at constant air temperature (T-air), for two cultivars (DP16 and Sicot 71BRF) of cotton (Gossypium hirsutum L) grown in two C-a (400 and 640 mu l l(-1)) and two T-air (28 and 32 degrees C) treatments. To interpret responses of ITE to these variables, we used a model based on the assumption that stomata are regulated to optimize carbon uptake per unit water used. The measured ITE response to D-s was very close to that predicted by the model, but LIE was overpredicted at low D-s. We found that one model adequately fit all T-air and C-a treatments, and found no significant differences in the single parameter of the model with C-a, T-air, or cultivar. As predicted, ITE increased in proportion to C-a (a 51-64% increase in ITE compared to a 60% increase in C-a). Photosynthesis rate was 16-22% higher in the elevated T-air treatment, which led to a corresponding increase in transpiration rate at a given D-s, again as predicted. The results show that, in cotton, a straightforward framework based on optimal stomatal theory successfully predicted responses of ITE to D-s, T-air, and C-a. These findings greatly simplify modelling of an important component of crop water-use efficiency in response to climate change.