Is transpiration efficiency a viable plant trait in breeding for crop improvement?

Increased transpiration efficiency - commonly the ratio of mass accumulation to transpiration - is often suggested as a critical opportunity for genetic improvement for increased crop yields in water-limited environments. However, close inspection of transpiration efficiency (TE) shows that it is a complex term that is explicitly dependent upon both physiological and environmental variables. Physiological variables include leaf photosynthetic capacity, biochemical composition of the plant productions and possible hydraulic limitation on water flow in the plant. Environmental variables include atmospheric CO2 concentration and atmospheric vapour pressure deficit. To complicate the resolution of transpiration efficiency, a weighted integration over the daily cycle and over the dates of interest needs to be resolved. Consequently, it is concluded that transpiration efficiency is not a variable easily resolved for use in many breeding programs. Instead, component traits contributing to TE need to be studied to increase the effective use of available water through the growing season to ultimately maximise growth and yield of the crop.