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Tropospheric ozone is considered to be the most phytotoxic air pollutant because of its oxidizing power. The main objective of this study was to analyze the effect of intra- and interspecific competition between Triticum aestivum L. and Trifolium repens L. on the responses to high concentrations of ozone of both species, and the role of the symbiotic relationship Rhizobium - T. repens on the abovementioned responses. Monocultures and mixtures of both species in different densities were sown. Pots were transferred to open top chambers either with 90-120 ppb ozone or without ozone. Ozone had an overall negative impact on leaf area and biomass production per individual plant. These responses were dependent on species and sowing density in monocultures, but were not changed by species proportion in the mixtures. There was a positive relationship between Rhizobium nodules and plant biomass, with a tendency for smaller plants to present lower number of nodules under ozone. These results suggest that competitive and mutualistic interactions could have a greater role in determining responses to novel air pollutants than species sensitivity to the xenobiotic, per se. [ABSTRACT FROM AUTHOR]

Neotyphodium endophyte fungi are vertically transmitted symbionts of cool-season grasses. The seed phase of the grass’ life cycle appears to be critical for the persistence of the fungus. Endophyte viability decreases faster than seed viability, but little is known of the effects of this endophyte on seed viability. The endophyte could affect seed viability through changes in water content. Here, we assessed the effects of the endophyte on seed viability, the differential survival of endophyte and seed, and the effects of infection on seed water content. Viability of endophyte-infected and noninfected seeds and endophyte were evaluated over a period of 729 d under 12 controlled environmental conditions. Seed viability was reduced by the infection at high temperature and high relative humidity, but not under other conditions. Moreover, endophyte viability decreased faster than seed viability only under high humidity or high temperature. Seed water content was not affected by endophyte presence. The proportion of viable infected seeds was mainly affected by the loss in endophyte viability and secondly by the differential survival of infected and noninfected seeds. Knowledge on the relative importance of these processes is critical to understand the factors affecting the efficiency of endophyte vertical transmission and the frequency of endophyte-infected plants. [ABSTRACT FROM AUTHOR]