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

USDA-ARS, Route 2 Box 2953-A, Prosser, WA 99350-9687.
Washington State Univ. Res. and Ext. Center, Route 2 Box 2953-A, Prosser, WA 99350-9687.
USDA-ARS, Route 2 Box 2953-A, Prosser, WA 99350-9687.

^* Corresponding author

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_aVPD) 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.

Received for publication September 1, 1989.