THE RESPONSE OF SPRING WHEAT (TRITICUM-AESTIVUM L) TO OZONE AT HIGHER ELEVATIONS .2. CHANGES IN YIELD, YIELD COMPONENTS AND GRAIN QUALITY IN RESPONSE TO OZONE FLUX

The effect of ozone on the yield, yield components and grain composition of spring wheat (Triticum aestivum L., cv. Albis) was investigated in 1989 and 1990 at 900 m above sea level. Plants were grown from the three-leaf stage until harvest in open-top chambers ventilated with charcoal-filtered air (CF), unfiltered air (UF) or unfiltered air with one of two levels of O3 added when global radiation exceeded 400 J m-2 s-1 (O3 - 1, O3 - 2). Ambient plots (AA) without chambers were used as a reference. Mean 7-h d-1 (09.00-16.00 h) O3 concentrations in the CF, O3 - 1 and O3 - 2 treatment, respectively, was approximately 0.5, 1.4 and 1.8 times the concentration in the UF treatment, which was 70-mu-g m-3 in 1989 and 76-mu-g m-3 in 1990. The OTC environment was characterized by a warmer, more humid microclimate and reduced soil moisture, compared with the open field. In 1989, this effect of the chambers was more pronounced than in 1990. When compared with AA plots, chamber enclosure significantly reduced grain and straw yield in 1989, while in 1990 no difference was observed. Grain yield decreased with increasing seasonal mean O3 concentration as a result of reductions in the weight of individual grains and in the number of grains per head. A small decrease in the ratio between grain weight and total above-ground biomass (harvest index) indicated a shift in biomass allocation. With increasing O3, small or no changes were observed in protein, starch and K concentrations, and in protein quality (Zeleny value) of the grain. Relationships between different measures of O3 exposure and relative grain yield were constructed with a quadratic model. With mean concentrations (7-h d-1 mean or radiation-weighted mean) or with a cumulative index (SUM06 = sum of concentrations above 60-mu-l l-1) to describe exposure, the response of grain yield to increasing O3 was smaller in 1990 than in 1989. Nearly identical exposure-response functions were obtained for both seasons, with the mean O3 flux used to estimate the absorbed dose of O3. Quadratic exposure-response functions suggest that ambient levels of O3 reduced grain yield by 9.5 % and 11-6 % in 1989 and 1990, respectively. Using Weibull exposure-response functions, a comparison with results from an earlier study at 485 m above sea level indicates reduced sensitivity at the higher elevation.