The effect of drought stress on the sex ratio variation of Silene otites

Under dry environmental conditions the sex ratio of many dioecious plants i s male-biased, which is usually explained by the higher susceptibility of f emales to drought stress. We investigated if spatio-temporal variation in t he sex ratio of Silene otites could be explained by the higher sensitivity of female plants to drought stress as compared to males. Long-term field ob servations, however, did not support this hypothesis. The sex ratio in 34 p atches at the study site in Central Germany changed from slightly female bi ased in 1994 to strongly male-biased in 1997 and 1998, The interannual chan ge in the proportion of plants that were female was positively correlated w ith the number of days with soil-water deficit in the late summer, suggesti ng higher mortality in males than in females under drought stress. In two c losely studied patches, mortality in males was also higher than in females, although this difference could not be related to drought stress. These fie ld observations were supported by an experiment with potted plants in two c limate chambers, in which male mortality was higher during a three-week per iod without water supply. We conclude that the often reported male bias in patches of S. otites is not caused by sexual differences in the sensitivity to drought stress. Field data in this study, however, suggest that male S, otites plants flower earlier than females, which causes a shift in sex rat io to more male bias among flowering plants.