A PROCESS-ORIENTED EXPLANATION OF THE NONLINEAR RELATIONSHIP BETWEEN GRAIN-YIELD WHEAT AND OZONE EXPOSURE

A model for the impact of ozone on grain yield in spring wheat (Tritic um aestivum L.) was constructed. The model was based on the observatio ns in the literature as well as in the present experiment, that the pr oductivity of the above-ground biomass and the carbon partitioning of the above-ground biomass do not respond to ozone above the same thresh old concentrations. The model was applied to a data set in which sprin g wheat was exposed to different ozone concentrations in open-top cham bers. It was concluded that the nonlinearity of a simple dose-response curve for grain yield can be explained in terms of the product of two linear responses: (1) the response of total above-ground biomass to a n ozone dose above 10 nl l(-1), and (2) the response of the harvest in dex to an ozone dose above 40 nl l(-1). The nitrogen content of the gr ain increased linearly with the dose above 40 nl l(-1) It is hypothesi zed that the effect of comparatively small ozone concentrations on tot al above-ground biomass is caused by a reduction of leaf duration by o zone, and that the effect on carbon partitioning is caused by ozone af fecting the sink capacity or activity of the growing grain. The maximu m coefficients of determination (r(2)) for the linear regression betwe en grain yield and accumulated ozone dose above thresholds in the rang e of 0-70 nl l(-1), were never as great as those for total above-groun d biomass and harvest index. Nevertheless, statistically significant l inear regressions were obtained for the thresholds 20, 30 and 40 nl l( -1), the highest correlation being with 30 nl l(-1) Thus, it seems acc eptable to use linear regressions between grain yield and the accumula ted dose, although these do not take into direct account the complexit y of the effects of ozone on wheat.