PLANT AND ENVIRONMENT INTERACTIONS - COTTON YIELD LOSSES AND AMBIENT OZONE CONCENTRATIONS IN CALIFORNIA SAN-JOAQUIN VALLEY

Based on controlled experiments and simulation modeling, ozone (O3) ha s been estimated to cause significant yield losses to cotton. The stud y reported here was conducted to verify losses for 'Acala' cotton (Gos sypium hirsutum L. 'SJ2') along a gradient of ambient O3 concentration s across the San Joaquin Valley in California. Cotton was grown in non filtered (NF) and charcoal-filtered (CF) open-top chambers, and ambien t air (AA) at four sites during the 1988 and 1989 summer growing seaso ns. Cotton yields (weights of mature bolls m-2) were reduced in NF com pared with CF air in general proportion to O3 concentrations across al l sites and years. Greatest cotton yield losses were as Shafter in the southern part of the San Joaquin Valley (20% in 1989), and lowest los ses were at Five Points in the western part of the valley (none in 198 9). Ozone injury symptoms on cotton were most noticeable in areas with greatest yield losses. Linear O3 exposure vs. predicted relative yiel d loss models using four common exposure indices were constructed for each site and year based on the NF, CF, and AA data. All models except the second highest daily maximum concentration (2ndHDM, the current a mbient air quality standard for O3) predicted yield losses comparable to those predicted with previously published models for cotton in the San Joaquin Valley. Ozone exposure indices giving more weight to highe r O3 concentrations or the 7-h daytime mean better predicted cotton yi eld responses over the growing season than 2ndHDM.