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

Abstract

AbstractA 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 × 1.3, NF × 1.7, and NF × 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 × 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 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.