Ozone effects on plant species mixtures could depend on the characteristics of the species involved, their mixing ratio, or on environmental conditions. Predicting long-term effects on the dynamics of plant communities requires an understanding of the interactions involved. The present experiment was designed to determine the effects of ozone on grassland species in relation to mixing ratio and soil water content (irrigation) using binary mixtures. The grass Trisetum flavescens was grown in potted replacement-series mixtures with Centaurea jacea (Experiment A) or Trifolium pratense (Experiment B). The plants were exposed to three concentrations of ozone in open-top chambers in two irrigation treatments. Total above-ground dry weight over three growth periods was measured. The competitive ability of T. flavescens was expressed as the competitive ratio (CRT). In Experiment B, total above-ground dry weight was reduced by elevated ozone and by reduced soil moisture, and significant interactions were found for ozone × irrigation and ozone × ratio. In Experiment A these effects were not significant. Under well watered conditions, CRT tended to be reduced by elevated ozone in Experiment A, but increased significantly in Experiment B, indicating the importance of the competing species in modifying the ozone effect on T. flavescens. In both experiments reduced irrigation decreased the magnitude of ozone effects on biomass production, which could be related to observed reductions in specific leaf conductance. The results suggest that under well watered conditions the effect of elevated ozone on the competitive balance between species depends on the species mixture, but that the mixing ratio is less important.

Effects of ozone on spring wheat (Triticum aestivum L. cv. Satu) were studied in an open-top chamber experiment during two growing seasons (1992–1993) at Jokioinen in south-west Finland. The wheat was exposed to filtered air (CF), non-filtered air (NF), non-filtered air+35 nl l−1 ozone for 8 h d−1 (NF+) and ambient air (AA). Each treatment was replicated five times. Two wk after anthesis, after 4 wk of ozone treatment (NF+, 45 nl l−1 1000–1800 hours, seasonal mean) the net CO2 uptake of wheat flag leaves was decreased by c. 40% relative to CF and NF treatments, both initial and total activity of Rubisco and the quantity of protein-bound SH groups were decreased significantly. Added ozone also significantly accelerated flag leaf senescence recorded as a decrease in chloroplast size. The effect was significant 2 wk after anthesis, and senescence was complete after 4 wk. In the CF and NF treatments senescence was complete 5 wk after anthesis. The significant effect of ozone on the chloroplasts and net CO2 uptake 2 wk after anthesis did not affect the grain filling rate. However, since the grain filling period was shorter for ozone fumigated plants, kernels were smaller. The decrease in 1000-grain weight explained most of the yield reduction in the plants under NF+ treatment. The results indicate that wheat plants are well buffered against substantial decrease in source activity, and that shortened flag leaf duration is the major factor causing ozone-induced yield loss.

Potted specimens of 27 Swedish native herbs and grasses were exposed to three different ozone concentrations, CF (charcoal filtered air), NF (non-filtered air) and NF+ (1·5 × non-filtered air) in open-top chambers from 2 July to 5 August 1994. The species represented a wide range of different plant strategies according to the C-S-R model. The results show that the stress-tolerators, S, had a smaller mean relative growth rate, R¯, during the exposure period and also a smaller accumulation of biomass compared with other strategies. The species with the intermediate strategy, CSR, had a R¯ similar to that of the species with a large component of C, the competitor strategy, and/or R¯ the ruderal strategy, whereas the net accumulation of biomass was smaller in the CSR group. This difference between R¯ and net growth for the CSR species compared to the species of other strategies, can be explained by a slower growth for the CSR group during the establishment phase. In the present investigation the response to ozone was very small regardless of plant strategy, although there was a weak trend towards slower growth with higher ozone concentration, 18 out of 27 species having a greater growth in CF than in NF+. The growth of one typical stress-tolerant species, Festuca ovina L., was stimulated significantly by ozone. Visible injury, most likely caused by ozone, was noticed in three species, Dactylis glomerata L., Dactylis aschersoniana Graebn. and Phleum alpinum L. Although there were no major restrictions to plant growth in terms of water, nutrients or space in the present open-top chamber experiment, the variability in all climatic factors was as large as in the ambient air. This might harden the plants and make them less sensitive to ozone compared to plants grown in the laboratory under controlled and optimal conditions with low variability.