Canopy temperature and growth of differentially irrigated alfalfa

Plant temperature of alfalfa (Medicago sativa L.) grown with different amounts of irrigation water has not been reported. The objective of this experiment was to determine if progressive differences in canopy temperature existed among plots of alfalfa (ev. Cody) subjected to 7 graded watering treatments. Irrigation water (0, 2.5, 5.1, 7.6, 10.2, 12.7, 15.2 cm) was added after each of three harvests in 1980 and 1981. Extremes in weather between the summers of 1980 and 1981 enabled comparison of data from a stressed season (1980) with those from a non-stressed season (1981). Throughout the growth period in both years, canopy temperatures, leaf area, stem dry weight, leaf dry weight and total dry weight were determined. Canopy temperature was measured with an infrared thermometer. The relationship between canopy-minus-air temperature (T_c — T_a) versus vapor-pressure deficit (VPD) was determined on well watered alfalfa for 1980 and 1981.

Differences in canopy temperature, leaf area index, leaf dry weight, stem dry weight, and total dry weight, due to treatments, were evident in the dry year (1980), but not in the wet year (1981). In the dry year, the irrigated plots generally had cooler canopy temperatures and higher dry weights than the dry land plots, but differences due to the level of water added were not apparent. In both the dry year and the wet year, (T_c — T_a) was inversely related to VPD. Also, in both years, and for all treatments, leaf dry weight was about equal to stem dry weight.