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photosynthetic water-use efficiency (also called instantaneous water-use efficiency), which is defined as the ratio of the rate of carbon assimilation (photosynthesis) to the rate of transpiration, and
water-use efficiency of productivity (also called integrated water-use efficiency), which is typically defined as the ratio of biomass produced to the rate of transpiration.
Increases in water-use efficiency are commonly cited as a response mechanism of plants to moderate to severe soil water deficits, and has been the focus of many programs that seek to increase crop tolerance of drought. However, there is some question as to the benefit of increased water-use efficiency of plants in agricultural systems, as the processes of increased yield production and decreased transpirational water loss (that is, the main driver of increases in water-use efficiency) are fundamentally opposed. If there existed a situation where water deficit induced lower transpirational rates without simultaneously decreasing photosynthetic rates and biomass production, then water-use efficiency would be both greatly improved and a desired trait in crop production.
^Bacon, M. Water Use Efficiency in Plant Biology. Oxford: Blackwell Publishing Ltd., 2004. ISBN1-4051-1434-7. Print.
Tambussi, E. A.; Bort, J.; Araus, J. L. (2007). "Water use efficiency in C3 cereals under Mediterranean conditions: a review of physiological aspects". Annals of Applied Biology. 150 (3): 307–321. doi:10.1111/j.1744-7348.2007.00143.x.