Improving Crop Water Use.

Crop assimilation rate (A) is intimately linked to crop transpiration rate (T): A/T = k/VPD where k is defined by several physical and physiological variables and VPD is atmospheric vapor pressure deficit. The k term includes leaf interior CO2 concentration and atmospheric CO2 concentration. Leaves commonly maintain leaf interior CO2 concentration at fairly stable values. However, atmospheric CO2 concentration is increasing and this leads directly to a predictable increase in k, and hence an increase in A/T. However, VPD is a variable that potentially can have a very large influence on A/T. Often it is assumed that VPD will increase with temperature increase under climate change, but this is not yet resolved. Greater increases in night temperature than day temperature will tend to stabilize VPD, which thus far has been the observed tendency. Of course, the effective VPD can also be under regulation by the plant. For plants that can sustain transpiration only up to a maximum rate, midday stomata closure under high VPD decreases the contribution of the midday VPD values to the overall integrated daily VPD. Consequently, those plants with the maximum transpiration trait have a lower integrated daily VPD, and consequently a greater A/T than those plants that do not express a maximum transpiration rate. Given the predicted possibility of greater incidence of water deficits in the U.S. under climate change, development of crops with the maximum transpiration trait appears to be highly desirable.