Effects of low‐concentration sulphur dioxide fumigation and vesicular–arbuscular mycorrhizas on ¹⁴C‐partitioning in Phleum pratense L.

summary The effect of long‐term low‐concentration (0.04–0.065 μl l −1 ) exposure to sulphur dioxide (SO 2 ) on assimilate partitioning in Phleum pratense colonized by vesicular‐arbuscular (VA) mycorrhizal fungi from field soil was examined by supplying leaves with 14 CO 2 . Seedlings were grown and exposed to SO 2 for six weeks and then pulsed with 4 CO 2 for 15 min. The distribution of the 14 C was analysed after a 12 h chase. SO 2 fumigation had little effect on plant dry matter production, however, it had significant effects on the assimilation of 14 CO 2 . Plants colonized by VA mycorrhizal fungi, both fumigated and controls, had a greater percentage of the total incorporated 14 C in the roots, a significantly lower percentage of the total as 14 C‐labelled soluble carbohydrates in the shoots, and a greater percentage of the total as 14 C‐labelled amino‐ and organic acids in the shoots and roots than non‐mycorrhizal plants. SO 2 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 SO 2 exposure.