Water use efficiency is highest for

Water use efficiency is the ratio of water used in plant metabolism to water lost by the plant through transpiration.
Water use efficiency (WUE) is a measure of the amount of biomass produced per unit of water used by a plant. It is based on the concept introduced 100 years ago by Briggs and Shantz in 1913 showing a relationship between plant productivity and water use.
WUE is highest for the Finger millet (F. millet) as it adapted to hot and semi-arid conditions. It can withstand cultivation at altitudes over 2000 m above sea level, because of its high drought tolerance and the long storage time of the grains.