DO SEX-RELATED DIFFERENCES IN RESPONSE TO ENVIRONMENTAL VARIATION INFLUENCE THE SEX-RATIO IN THE DIOECIOUS RUMEX-ACETOSELLA

Populations of the dioecious herb Rumex acetosella show variation in sex ratio along an old-field successional gradient in central France, where female-biased or 50:50 ratios occur in young populations and male-biased ratios occur in old populations. In order to test the hypothesis that differences in environmental sensitivity and/or reproductive biology account for sex related differences in habitat occupation of this species, two complementary experiments were performed. First, the environmental sensitivity (or reaction norms) of biomass allocation for male and female plants was examined over an experimental gradient of density and nutrients to test whether the sexes differ in habitat preference. Aboveground vegetative biomass and floral biomass of females was greater than that of males under more productive environmental conditions (i.e. reduced competition on mineral rich soils). Under less productive conditions, these differences between male and female plants were markedly reduced. This provided evidence for a difference in the response of the two sexes to environmental variation. Males also showed reduced total aboveground and floral biomass when grown in pots containing a high density of female plants. The percentage of flowering plants was significantly influenced by environmental conditions, but the sex ratio of flowering individuals was balanced over the whole gradient after one season of growth. Neither competition nor environmental conditions induced a differential rate of flowering in the two sexes. Second, we examined whether the two sexes show a different cost of flowering by quantifying (a) differences in resource allocation patterns in flowering and non-flowering individuals and (b) the effect of flowering on subsequent growth. Clonal growth of females that did not flower in the first year of study was significantly higher than that of females that had flowered and borne fruit. The latter invested less biomass per sprout than both flowering males and non-flowering individuals of both sexes. This indicates that the direct cost of resource allocation to sexual reproduction, especially for female plants, has the delayed effect of reducing subsequent vegetative propagation. Patterns of resource allocation in male and female plants thus provide insights into the mechanisms responsible for biased sex ratios in this species.