Abstract Irrigated alfalfa (Medicago sativa L.) is a high user of water and has received limited research to improve its water-use efficiency (WUE). Alfalfa is grown on approximately one-half of the cultivated land and has the highest consumptive use of all crops in Wyoming. Alfalfa lines were developed which expressed lighter colored and larger leaves. The water relations and productivity of these lines were evaluated under a series of soil water levels in a controlled environment and under field environments in Wyoming and Montana. In the controlled environment the WUE of the pale and dark leaf genetic lines were 1.47 and 1.22 g dry forage kg-1 water, respectively, when averaged across seven soil moisture levels. Large-leaved alfalfa plants produced 1.43 and small-leaved 1.29 g dry forage kg-1 water. Leaf transpiration per unit leaf area of pale leaves in irrigated field studies ranged from 12 to 17% lower than dark (normal) leaves. Transpiration of large leaves was 10 to 16% lower than small (normal) leaves. In field studies, the forage yield of pale leaves was 25 to 29% higher than dark leaves. The yield of large-leaved plants was 17 to 22% higher than small-leaved. These studies indicate the WUE of alfalfa can be improved through plant breeding.
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