Abstract Ozone (O 3 ) and soil water deficit are two environmental stresses that significantly affect the growth and yield of alfalfa ( Medicago sativa L). However, little is known of the responses of field‐grown alfalfa to O 3 , and the effects of the interaction between O 3 and water stress on canopy temperature and water relations of alfalfa have not been previously reported. The objective of this 2‐yr study was to determine the interactive effects of O 3 and soil water deficits on canopy temperatures, water use, and water use efficiency (WUE) of alfalfa. Alfalfa (cv. WL‐514) was grown in 30‐3‐ by 5.5‐m plots on Wasco sandy loam (coarse‐loamy, mixed, nonacid, thermic Typic Torriorthents) in Shafter, CA, and was exposed in open‐top chambers to five levels of O 3 for 12 h daily, from March to October of 1984 and 1985. Ozone treatments ranged from charcoal‐filtered air (CF) to twice ambient O 3 concentrations. Each plot received either normal amounts of irrigation (NI) or 30% less than normal (WS). Canopy temperature‐air temperature differentials (T c ‐T a ) were significantly reduced by water stress an average of 27.9% in 1984 and 44.0% in 1985. Ozone also significantly reduced T c ‐ T a by 31% in NI and 37% in WS plots in 1984, but in 1985 O 3 had no effect on T c ‐ T a . Water use, rate of soil water depletion, or depth of effective rooting zone were not affected by O 3 , hereas water stress significantly reduced all three. Water use efficiency was significantly reduced by O 3 , averaging 12% lower in nonfiltered compared with CF plots. The effects of O 3 on WUE were attributed to premature senescence and abscission of older alfalfa leaves.
Temple et al. (Sun,) studied this question.