Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/2/2012
Publication Date: 1/15/2013
Citation: Duursma, R., Payton, P.R., Bange, M., Broughton, K., Medlyn, B., Tissue, D. 2013. Near-optimal response of instantaneous transpiration efficiency to vapour pressure deficit, temperature and [CO2] in cotton (Gossypium hirsutum L.). Agricultural and Forest Meteorology. 168:168-176. Interpretive Summary: Water-use efficiency (WUE) is an important concept in crop production because it summarizes total biomass production per unit water used. Particularly in irrigated crops such as cotton (Gossypium sp.), crop water use significantly affects the level of productivity and is a direct cost to the farmer. In recent years, declining water allocation to cotton producing regions across Australia and the U.S. and potential impacts of increased climate variability, has renewed interest in efficient management of water resources, which may potentially be achieved by maximising plant WUE. Global climate change models predict a continued increase in atmospheric carbon dioxide (CO2) concentrations and an accompanied increase on global average air temperatures (Tair) for the near future. A consequence of increased Tair is increased evaporative demand of the atmosphere, since the vapour pressure deficit is a function of Tair. It is crucial that we improve our understanding of future impacts of atmospheric CO2 and Tair on crop WUE such that appropriate cultivars and management practices are developed to adapt to a changing climate. This study examined the response of two commonly grown cotton cultivars to increased CO2, increased Tair, and altered vapour pressure deficit on cotton growth and photosynthesis. To interpret responses of physiological parameters to these variables, we used a model based on the assumption that stomata are regulated to optimize carbon uptake per unit water used. Our results show that, in cotton, a straightforward framework based on optimal stomatal theory successfully predicted responses of plant water use to vapour pressure deficit, Tair, and atmospheric CO2 concentration. These findings greatly simplify modeling of an important component of crop water-use efficiency in response to climate change. Additionally, these results provide a robust framework to understand the balance between carbon uptake and water use in an important global crop species, and improves our ability to predict the effect of climate change on crop water-use efficiency. In future work, we will apply our framework to plants growing in the field, to identify whether our current understanding of the response of ITE to Ca, Tair and D translates to field conditions, and to total crop water-use efficiency.
Technical Abstract: 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] (Ca), approximately inversely proportional to the square root of the leaf-to-air vapour pressure deficit (D), and independent of air temperature (Tair). We measured the response of ITE to a range in D at constant Tair, for two cultivars (DP16 and Sicot 71BRF) of cotton (Gossypium hirsutum L.)grown in two Ca (400 and 640 'l l-1) and two Tair (28 and 32 ºC) 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 was very close to that predicted by the model. We found that one model adequately fit all Tair and Ca treatments, and found no significant differences in the single parameter of the model with Ca, Tair, or cultivar. As predicted, ITE increased in proportion to Ca (a 51-64% increase in ITE compared to a 60% increase in Ca). Photosynthesis rate was 16-22% higher in the elevated Tair treatment, which led to a corresponding increase in transpiration rate at a given D, again as predicted. The results show that, in cotton, a straightforward framework based on optimal stomatal theory successfully predicted responses of ITE to D, Tair, and Ca. These findings greatly simplify modelling of an important component of crop water-use efficiency in response to climate change.