A Physiological Route to Increased Water Use-Efficiency in Alfalfa

Alfalfa is the top-ranked cash crop in New Mexico and the most copious user of irrigation water. The water usage per mass of forage yield might be reduced, that is, the water-use efficiency (WUE) might be increased, because it is under some measure of physiological control.

Working from a knowledge of biophysics and plant physiology, we propose that WUE and yield are strongly controlled by two traits that should be heritable and selectable: (1) the CO2 concentration within the leaf, Ci, and (2) the specific leaf mass. We made a detailed model of alfalfa photosynthesis, transpiration, and growth. We predicted that selection for lower Ci and higher specific leaf mass could improve WUE up to 25% with modest penalties in yield, less than 10%; this indirect selection for WUE appears to have some notable advantages over direct selection. Selection for Ci and specific leaf mass may also be directly extendable to other crop species.

We have grown 80 individual plants of two cultivars, Wilson 9DllA and Mesilla, in controlled environments. We find that (1) within each cultivar there are large ranges in Ci and in specific leaf mass, suitable for breeding, and (2) these two traits are related to WUE and yield substantially as predicted by our model. We will proceed to field trials and heritability tests, as well as to studies of drought tolerance traits.