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 f ungi on plant response to ozone (O-3) has not been well defined. Legum inous plants in the field are infected by both types of organisms, whi ch influence plant nutrition and growth. We studied the effects of inf ection with Rhizobium leguminosarum biovar trifolii and/or Gigaspora m argarita on response of subterranean clover (Trifolium subterraneum L. cv Mt. Barker) to O-3. Exposures were conducted in greenhouse CSTR ch ambers using four O-3 concentrations [charcoal-filtered (CF), 50, 100, or 150 ppb; 6 h day(-1), 5 day wk(-1) for 12 weeks] as main plots (re plicated). Four inoculum types were subplot treatments, i.e., inoculat ed with one, both, or neither microorganisms. At 2-wk intervals, plant s were exposed to (CO2)-C-14 and harvested 24 h later for determinatio n of biomass and C-14 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 n oninoculated controls. Shoot/root ratios were increased by 100 or 150 ppb O-3 compared to that for CF-treated plants. Shoot/root ratios were greater for all inoculated plants compared to noninoculated controls. Under low O-3 stress (CF or 50 ppb), plants inoculated with both R. l eguminosarum and G. margarita transported a greater proportion of rece nt photosynthate (C-14) to roots than did noninoculated plants; we att ribute this to metabolic requirements of the microorganisms. At the hi ghest level of O-3 stress (150 ppb), this did not occur, probably beca use little photosynthate was available and the shoots retained most of it for repair of injury. Statistically significant interactions occur red between O-3 and inoculum types for shoot and total biomass. When a veraged across harvests, 50 ppb O-3 suppressed biomass in the plants i noculated with G. margarita alone. Apparently, the mycorrhizal fungus is such a significant C drain that even a small amount of O-3 stress s uppresses plant growth under these conditions.