AMBIENT OZONE (O3) AND ADVERSE CROP RESPONSE - A UNIFIED VIEW OF CAUSE AND EFFECT

This paper presents a cohesive view of the dynamics of ambient O3 exposure and adverse crop response relationships, coupling the properties of photochemical O3 production, flux of O3 from the atmosphere into crop canopies and the crop response per se. The results from two independent approaches ((a) statistical and (b) micrometeorological) were analyzed for understanding cause-effect relationships of the foliar injury responses of tobacco cv Bel-W3 to the exposure dynamics of ambient O3 concentrations. Similarly, other results from two independent approaches were analyzed in: (1) establishing a micrometeorological relationship between hourly ambient O3 concentrations and their vertical flux from the air into a natural grassland canopy; and (2) establishing a statistical relationship between hourly ambient O3 concentrations in long-term, chronic exposures and crop yield reductions. Independent of the approach used, atmospheric conditions appeared to be most conducive and the crop response appeared to be best explained statistically by the cumulative frequency of hourly ambient O3 concentrations between 50 ppb and 90 ppb (100 and 180 mug m-3). In general, this concentration range represents intermediate or moderately enhanced hourly O3 values in a polluted environment. Further, the diurnal occurrence of this concentration range (often approximately between 0900 and 1600 h in a polluted, agricultural environment) coincided with the optimal CO2 flux from the atmosphere into the crop canopy, thus high uptake. The frequency of occurrence of hourly O3 concentrations > 90 ppb (180 mug m-3) appeared to be of little importance and such concentrations in general appeared to occur during atmospheric conditions which did not facilitate optimal vertical flux into the crop canopy, thus low uptake. Alternatively, when > 90 ppb (180 mug m-3) O3 concentrations occurred during the 0900-1600 h window, their frequency of occurrence was low in comparison to the 50-90 ppb (100-180 mug m-3) range. Based on the overall results, we conclude that if the cumulative frequency of hourly ambient O3 concentrations between 50-62 ppb (100-124 mug m-3) occurred during 53% of the growing season and the corresponding cumulative frequency of hourly O3 concentrations between 50-74 ppb (100-148 mug m-3) occurred during 71% of the growing season, then yield reductions in sensitive crops could be expected, if other factors supporting growth, such as adequate soil moisture are not limiting.