Snap bean genotypes (Phaseolus vulgaris L.) with different ozone (O 3 ) sensitivities (line S156: O 3 -sensitive; line R123: O 3 -tolerant) were grown for 70 days with or without inoculation of arbuscular mycorrhizal (AM) fungi under ambient (C AMB = 20 nanolitres (nl)/l and elevated (C MID = '50 nl/l and C HIG = 80 nl/l) O 3 . Sequential determinations (leaf injury, pigment concentration, chlorophyll fluorescence, photosynthesis, etc) were carried out during plant growth to evaluate mycorrhizal influence on the photosynthetic function of the two genotypes under elevated O 3 . Inoculation with AM fungi alleviated leaf injury in both genotypes and delayed time of injury manifestation (TIM) in the R123 line at the blooming stage (growth stage (GS): 61-65 (Zadoks scale, Zadoks et al. 1974), 30-35 days after the onset of O 3 fumigation), but mycorrhizal effect was slight at the initial growth stage (GS 11-13, 0-5 days after onset of O 3 fumigation). Relative to the non-mycorrhizal plant, AM fungi inoculation increased the concentrations of chlorophyll (Chl) a, Chl b and carotenoids in S156 plants, regardless of O 3 levels, while in R123 plants a similar effect was observed only in the C AMB treatment. At the blooming (GS 61-65) and the pod filling (GS 71-77, 45-50 days after starting O 3 fumigation) stages, photosynthetic rate, stomatal conductance and transpiration rate for the two genotypes decreased with elevated O 3 in all treatments, although the effect was reduced in C AMB and C MID treatments in AM-inoculated plants; however, the mycorrhizal effect was slight in the C HIG treatment. Intercellular carbon dioxide concentration increased with elevated O 3 regardless of AM fungi inoculation, but it was lower in the mycorrhizal plants than in the non-mycorrhizal plants, in most cases. Furthermore, AM fungi inoculation significantly increased the maximum quantum yield of photosystem II (PS II) photochemistry (Fv/Fm) and electron transport rate in both genotypes in the C HIG treatment. The present study indicated that in some cases, AM fungi inoculation can enhance plant tolerance to elevated O 3 through improving plant photosynthetic function, but the effect was reduced by serious O 3 stress.
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