INFLUENCE OF A MYCORRHIZAL FUNGUS AND/OR RHIZOBIUM ON GROWTH AND BIOMASS PARTITIONING OF SUBTERRANEAN CLOVER EXPOSED TO OZONE

The influence of soilborne symbionts such as rhizobia or mycorrhizal fungi on plant response to ozone (O sub(3)) has not been well defined. Leguminous plants in the field are infected by both types of organisms, which influence plant nutrition and growth. We studied the effects of infection with Rhizobium leguminosarum biovar trifolii and/or Gigaspora margarita on response of subterranean clover (Trifolium subterraneum L. cv Mt. Barker) to O sub(3). Exposures were conducted in greenhouse CSTR chambers using four O sub(3) concentrations [charcoal-filtered (CF), 50, 100, or 150 ppb; 6 h day super(-1), 5 day wk super(-1) for 12 weeks] as main plots (replicated). Four inoculum types were subplot treatments, i.e., inoculated with one, both, or neither microorganisms. At 2-wk intervals, plants were exposed to super(14)CO sub(2) and harvested 24 h later for determination of biomass and super(14)C content of shoots and roots. Ozone at 100 or 150 ppb suppressed clover growth during the experiment. Inoculation with G. margarita alone suppressed clover growth by the last two harvests, whereas R. leguminosarum alone enhanced growth during this time period. When both symbionts were present, the plants grew similarly to the noninoculated controls. Shoot/root ratios were increased by 100 or 150 ppb O sub(3) compared to that for CF-treated plants. Shoot/root ratios were greater for all inoculated plants compared to noninoculated controls. Under low O sub(3) stress (CF or 50 ppb), plants inoculated with both R. leguminosarum and G. margarita transported a greater proportion of recent photosynthate ( super(14)C) to roots than did noninoculated plants; we attribute this to metabolic requirements of the microorganisms. At the highest level of O sub(3) stress (150 ppb), this did not occur, probably because little photosynthate was available and the shoots retained most of it for repair of injury. Statistically significant interactions occurred between O sub(3) and inoculum types for shoot and total biomass. When averaged across harvests, 50 ppb O sub(3) suppressed biomass in the plants inoculated with G. margarita alone. Apparently, the mycorrhizal fungus is such a significant C drain that even a small amount of O sub(3) stress suppresses plant growth under these conditions.