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

Abstract

Abstract A 2‐yr study (1984 and 1985) designed to determine the interactive effects of ozone (O 3 ) and water stress on alfalfa ( Medicago sativa L. cv. WL‐514) was conducted in Shafter, CA. The objectives of this study were to develop O 3 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 O 3 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 O 3 treatments including charcoal‐filtered (CF), nonfiltered (NF), NF plus an additional 33, 67, or 100% of ambient O 3 concentrations (NF × 1.3, NF × 1.7, and NF × 2.0, respectively). Ambient O 3 concentrations averaged 0.049 µ L L −1 in 1984 and 0.042 µ L 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 O 3 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 × 1.3, NH × 1.7, and NF × 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 O 3 was found on top growth. However, crown (underground stem) weights were significantly reduced by O 3 , suggesting that continued exposure to O 3 could shorten the productive life of alfalfa stands, in addition to its effect on yield.