Arbuscular mycorrhiza formation and its function under elevated atmospheric O 3 : A meta-analysis

We quantitatively evaluated the effects of elevated O3 on arbuscular mycorrhiza (AM) formation and on AM role in promoting plant growth in regard to several moderating variables (O3 levels, O3 exposure duration, plant types, AM fungi family, and additional stress) by means of meta-analysis of published data. The analysis consisted of 117 trials representing 20 peer-reviewed articles and 16 unpublished trials. Relative to non-mycorrhizal controls, AM inoculation did not significantly alter plant growth (shoot biomass, root biomass, total biomass and plant height) when O3 concentration was less than 80 ppb, but at concentrations above 80 ppb symbiosis was associated with increases of 68% in shoot biomass and 131% in root biomass. AM effects on plant growth were affected by the duration of O3 exposure but did not differ much with AM fungi taxa or plant type. AM symbiosis has also led to higher yields under O3 stress, relative to the non-mycorrhizal plants, and the AM effects have been more pronounced as O3 concentration increases. As with biomass, AM effects on yield have been affected by the duration of O3 exposure, with the greatest increase (100%) occurring at 61-90 d. AM-induced promotion of yield differed with fungal species but not with plant type or other abiotic stress. Colonization of roots by AM fungi has been negatively affected by elevated O3 compared to ambient O3; total mycorrhizal colonization rate (MCR), arbuscular MCR, vesicular MCR and hyphal coil MCR declined as O3 levels rose. AM colonization rates were affected by duration of O3 exposure, plant type, AM fungal taxa and other concurrent stresses in most cases. The analysis showed that AM inoculation has the potential to ameliorate detrimental effects of elevated O3 on plant growth and productivity, despite colonization rates being negatively affected by elevated O3.