The effect of long-term low-concentration (0.04–0.065 μl l−1) exposure to sulphur dioxide (SO2) on assimilate partitioning in Phleum pratense colonized by vesicular-arbuscular (VA) mycorrhizal fungi from field soil was examined by supplying leaves with 14CO2. Seedlings were grown and exposed to SO2 for six weeks and then pulsed with 4CO2 for 15 min. The distribution of the 14C was analysed after a 12 h chase. SO2 fumigation had little effect on plant dry matter production, however, it had significant effects on the assimilation of 14CO2.
Plants colonized by VA mycorrhizal fungi, both fumigated and controls, had a greater percentage of the total incorporated 14C in the roots, a significantly lower percentage of the total as 14C-labelled soluble carbohydrates in the shoots, and a greater percentage of the total as 14C-labelled amino- and organic acids in the shoots and roots than non-mycorrhizal plants. SO2 exposure reduced root length, root length colonized by VA mycorrhizal fungi, and the length of root infected with arbuscules. It appeared that colonization by VA mycorrhizal fungi in field soil had an overriding effect on carbon assimilation, use, and translocation in the host, and these probably affect host sensitivity to SO2 exposure.