Canopy Temperature and Stomatal Conductance of Water‐stressed Dormant and Nondormant Alfalfa Types

Abstract Alfalfa ( Medicago sativa L.) from diverse origins may respond differently to water stress. Objectives were to assess dormant and nondormant‐type alfalfa response to water stress as inferred from canopy temperature and stomatal conductance. The effects of continuous deficit irrigation produced by a line‐source sprinkler system on ‘Vernal’ (dormant type), ‘Vernema’ (intermediate dormancy type), and ‘CUF 101’ (nondormant type) were investigated at Prosser, WA, on a Warden loam (coarse‐silty, mixed, mesic Xerollic Camborthids). Water stress was expressed as the Crop Water Stress Index ( CWSI ), a canopy‐temperature ( T c ) based indicator of water stress. The Y ‐intercepts of Vernal and CUF 101 canopy‐air temperature/vapor pressure deficit ( T c – T a VPD ) curves were significantly different ( P = 0.05) with CUF 101 0.6 °C (28.5%) higher than Vernal. Mean canopy resistance to vapor transport ( r cp ) was 33% greater for CUF 101 than for Vernal. Slopes of stomatal conductance/ CWSI curves were similar, but stomatal conductance at zero CWSI of CUF 101 was 60% of Vernal. Total water use and yields of Vernal, Vernema, and CUF 101 were not significantly different ( P = 0.05) among cultivars, although CUF 101 mean daily water use was 2.1 mm greater than Vernal water use early in the growth cycle. Higher T c for CUF 101 was consistent with lower CUF 101 stomatal conductance. The differences in T c and stomatal conductance were related to dormancy type. Lower transpiration of CUF 101 at full canopy cover has implications for development of more water‐use efficient alfalfa.