Alfalfa Response to Soil Water Deficits. II. Plant Water Potential, Leaf Conductance, and Canopy Temperature Relationships1

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Abstract
Information is needed on the regulation of water vapor loss and on the plant water status of alfalfa (Medicago sativa L.) subjected to soil water deficits. Our objective was to characterize diurnal and seasonal plant water potential (ψp), leaf conductance (G1), and canopy temperature (Tc) relationships for field‐grown alfalfa. Measurements of ψp, G1, and Tc were made hourly from sunrise to sunset on 13 clear days from 10 July to 2 October on alfalfa subjected to a range of soil water levels during vegetative, bud, and bloom growth stages. Plant water potentials of well‐watered plants were −0.1 to −0.4 MPa at sunrise and declined to −0.8 to −1.2 MPa at midday. With soil water depletions, ψp declined rapidly (0.08 to 0.4 MPa/day) and diurnal ψp cycling continued. Under severe plant water stress, ψp were below −2.0 and −4.5 MPa at sunrise and midday, respectively. The G1 under high moisture conditions reached a peak of up to 0.033 m/s at midmorning and decreased gradually until sunset. At moderate plant water deficits, G1 was greatest during low transpirational demand periods (early morning and late afternoon) and depressed at midday. Under extreme plant water stress, G1 remained at low levels (0.001 to 0.003 m/s) the entire day. Stomatal closure at midday resulted in Tc up to 8.5°C higher for water‐stressed vs. well‐watered alfalfa. Leaf conductance declined linearly with decreasing ψp until ψp reached −2.5 MPa. Below a ψp of −2.5 MPa, G1 stabilized at low levels. Regression coefficients (slopes) of the G1 vs. ψp (<−2.5 MPa) linear regression equations were greatest when daily maximum air temperature (MAT) was high and decreased as MAT declined. Under nonlimiting soil moisture conditions, G1 declined linearly as Tc was reduced. Growth stage and daily pan evaporation did not influence the G1 ‐ ψp relationship.