The formation of ozone in Los Angeles type (photochemical) smog was recognized by Haagen-Smit et al (1) in 1952. It soon became apparent that this compound could cause plant lesions identical to those which were seen on economic crops growing in many areas of the Los Angeles Basin. A “stipple” of grape leaves which occurred in midsummer in the field and which became progressively more severe with the season, was shown by Richards et al (4) to be very similar to lesions produced by ozone fumigation. Additional work with conifers (5) showed that a severe needle mottle of Pinus ponderosa and related species was caused by ozone in photochemical smog. Peroxy-acyl nitrates and oxides of nitrogen axe present in this mixture and are toxic to plants, but the separate effects of these pollutants have not been studied on grapes in detail.
Three-year-old `Valencia' orange [Citrus sinensis (L.) Osbeck] trees were exposed to air pollutants for 4. years in open-top field chambers to determine the chronic effects of ambient oxidants (primarily ozone) or sulfur dioxide (SO2) on fruit yield and quality and tree growth. Ozone concentrations averaged 0.012,0.040, and 0.075 ppm for 0800 to 2000 hr during April to October for filtered, half-ambient, and full ambient oxidant chambers. Sulfur dioxide was applied continuously at 0.09 ppm. Oxidant and SO2 effects were only marginally significant, as there was considerable variability in response among individual trees and between years. Across two “on” production years, yields were 31% lower with ambient oxidants, 11% lower with half-ambient oxidants, and 29% lower with sulfur dioxide compared to filtered air. Number of fruit per tree was reduced by ambient oxidants and SO2. Individual fruit weights were reduced by ambient oxidants, but no other fruit quality characteristics showed definite responses to ambient oxidants or SO2. Ambient oxidants had no effect on yield or quality of fruit during one “off' production year. Neither ambient oxidants nor SO, affected tree growth.