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Combined Effects of Ozone and Water Stress on Alfalfa Growth and Yield

Air Pollution Res. Ctr., Univ. of California, Riverside, CA 92521-0312.

^* Corresponding author.

ABSTRACT

A 2-yr study (1984 and 1985) designed to determine the interactive effects of ozone (O₃) and water stress on alfalfa (Medicago sativa L. cv. WL-514) was conducted in Shafter, CA. The objectives of this study were to develop O₃ dose-yield response functions for alfalfa, to determine how water stress could alter these functions, and to describe the cumulative effects of multiple-year exposures to O₃ on this perennial crop. Field-grown alfalfa, raised under normally irrigated (NI) or water-stressed (WS) conditions in open-top chambers, was exposed to five O₃ treatments including charcoal-filtered (CF), nonfiltered (NF), NF plus an additional 33, 67, or 100% of ambient O₃ concentrations (NF x 1.3, NF x 1.7, and NF x 2.0, respectively). Ambient O₃ concentrations averaged 0.049 µL L^–1 in 1984 and 0.042 µL L^–1 in 1985 for the seasonal 12-h (0900–2100) means from April to October. Water stress reduced total seasonal yield about 10% in 1984 and 27% in 1985. Ozone significantly reduced yields in both years, and the interaction between O₃ and water stress was statistically significant in 1985 and for combined 1984 and 1985 years. Seasonal yields for 1985 in NI plots were reduced 0, 14.9, 24.3, and 24.9% in NF, NF x 1.3, NH x 1.7, and NF x 2.0 treatments, respectively. Comparable seasonal yield reductions in WS plots were 5.9, 7.2, 13.5, and 18.1%. Ozone dose-alfalfa yield loss functions were homogeneous between 1984 and 1985 and no evidence of a cumulative effect of multiple-year exposure to O₃ was found on top growth. However, crown (underground stem) weights were significantly reduced by O₃, suggesting that continued exposure to O₃ could shorten the productive life of alfalfa stands, in addition to its effect on yield.

Key Words: Medicago sativa L. • Dose-response functions • Air pollution

Contribution of the Statewide Air Pollution Res. Ctr., Univ. of California, Riverside, CA 92521.

Received for publication July 22, 1986.