Direct Adverse Effects of SO2on Seed Set in Geranium carolinianum L.: A Consequence of Reduced Pollen Germination on the Stigma

Exposure of Geranium carolinianum L flowers to 0.6 ppm SO2 at 90% relative humidity (RH) for 8 h during anthesis reduced the number of germinated pollen grains per stigma, the percentage of pollen germination, and the number of seeds per flower (seed set). Similar exposures at 70% and 80% RH had no effect Regardless of RH, exposure to SO2 had no effect on the total number of pollen grains transferred to the stigma (self-pollination) or on pollen-tube growth in the style. A 6-h exposure to 0.6 ppm SO2 at 90% RH begun immediately after hand pollination reduced seed set by 20%, but a similar exposure begun 30 min after pollination had no effect on seed set. The results suggest that SO2 adversely affects sexual reproduction in G carolinianum by the inhibition of pollen germination on the stigmatic surface, which causes a subsequent reduction of seed set