Interactive Effects of CO2 and O3 on the Growth of Trisetum flavescens and Trifolium pratense Grown in Monoculture or a Bi-Species Mixture

Abstract

SUMMARY Golden oat grass (Trisetum flavescens L.) and red clover (Trifolium pratense L.) were grown as monocultures or bi-species mixtures under controlled conditions and exposed to ambient (350 ppm) or elevated (580 ppm) CO2, with or without addition of O3 (diel profile with 150 ppb maximum). Shoot biomass measurements after the initial growth and two re-growth periods were used to determine the specific responses of both species, and the difference in the specific response between monocultures and mixtures. T. pratense was much more responsive to CO2, O3, and their combination, compared to T. flavescens. In the case of O3 but not of CO2, the difference in sensitivity between species was larger in mixture than in monoculture. In contrast to elevated CO2, O3 significantly reduced the root:shoot ratio in the mixture, which could explain the increasing negative effect of O3 on clover with progressing harvests. The relative CO2 stimulation of T. pratense and of the cumulative mixture shoot biomass was larger in the presence than in the absence of O3, which was due to an almost complete protection from O3 stress by elevated CO2. In the mixture, the fraction of T. flavescens was small and increased during the experiment; this increase was most pronounced with O3, but any change in mixture biomass was dominated by the response of T. repens. The results confirm that in grass/legume mixtures legumes are most sensitive to elevated CO2 and O3, but the magnitude of specific responses depends on canopy structure and of plant development. Elevated CO2 minimizes the negative impacts of O3 stress on above- and below-ground plant growth.