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The effect of long-term low-concentration (0.04-0.065 μ1 1&sup-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. Seeding were grown and exposed to SO2 for six weeks and then pulsed with 14CO2 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-labeled soluble carbohydrates in the shoots, and a greater percentage of the total as 14C-labeled amino- and organic acids in the shots 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. [ABSTRACT FROM AUTHOR]

Two experiments were conducted using controlled environment SO2 fumigation chambers which allowed only the shoots of Phleum pratense to be fumigated. Experiment 1 examined the effect of SO2 fumigation on the ability of vesicular-arbuscular (VA) mycorrhizal fungi to infect roots. Experiment 2 examined the effect of SO2 fumigation on the proliferation of VA mycorrhizas in the roots. There was a significant decrease in the amount of infection in the roots of plants exposed to SO2. Low concentrations (0·05-0·07 μul l-1) of SO2 affected the ability of VA mycorrhizal fungi to colonize and, also, to proliferate within roots. In both experiments, root colonization was the overriding factor affecting the number of tillers, leaf area, and shoot and root dry weights of infected plants. Increased VA mycorrhizal infection resulted in reductions in the plant growth parameters. [ABSTRACT FROM AUTHOR]