Relationships Between Leaf Water Potential, Canopy Temperature, and Evapotranspiration in Irrigated and NonIrrigated Alfalfa

A prerequisite to exploiting potential production of any crop is to accumulate a knowledge of plant responses, whether the response is to climate, fertilizer, or water. This paper describes a study on a Waukegan silt loam soil (fine-silty over sandy or sandy skeletal, mixed, mesic Typic Hapludolls) where stress differences between irrigated and nonirrigated alfalfa (Medicago sativa L.) were evaluated during the daytime in order to expand the limited information available on alfalfa water relations. Canopy temperature (CT), evapotranspiration (ET), and leaf water potential (Χ₁ served as indicators of stress and were measured using an infrared thermometer, portable chamber, and pressure chamber, respectively. Canopy temperature and ET did not differ appreciably between irrigated and nonirrigated alfalfa in early morning, but after 0900 h and throughout the afternoon nonirrigated alfalfa had a higher CT and lower ET. Leaf water potential of nonirrigated alfalfa was consistently lower during the day. The maximum difference in CT, ET, and Ψ₁ between irrigated and nonirrigated alfalfa occurred at 1500 h where they equalled 2°C, 0.2 mm/h, and 0.7 MPa, respectively. These stress differences reflected the differences in the plant available water of 280 and 60 mm (corresponding to 117 and 25% of extractable water) in the 1.83-m soil profile of the irrigated and nonirrigated plots, respectively. Nonirrigated alfalfa Ψ declined at a faster rate as the peak stress period (1500 h) was approached; however, after this period a faster rehydration was observed for this treatment, as indicated by the relationship between and ET. Despite the faster rehydration of nonirrigated alfalfa, the degree of hysteresis was greater for this treatment than for irrigated alfalfa. The relationship between CT and ET indicated a larger rate of change in CT prior to the peak stress period for nonirrigated as compared to irrigated alfalfa. After this time and until 1700 h, CT remained constant as ET decreased, indicating a decreasing ratio between ET and net radiation and thus greater restrictions on water movement through the plant, a direct cause of the hysteresis observed.