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 s ex ratio along an old-field successional gradient in central France, w here female-biased or 50:50 ratios occur in young populations and male -biased ratios occur in old populations. In order to test the hypothes is that differences in environmental sensitivity and/or reproductive b iology account for sex related differences in habitat occupation of th is species, two complementary experiments were performed. First, the e nvironmental 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 pref erence. Aboveground vegetative biomass and floral biomass of females w as greater than that of males under more productive environmental cond itions (i.e. reduced competition on mineral rich soils). Under less pr oductive 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 showe d reduced total aboveground and floral biomass when grown in pots cont aining a high density of female plants. The percentage of flowering pl ants was significantly influenced by environmental conditions, but the sex ratio of flowering individuals was balanced over the whole gradie nt after one season of growth. Neither competition nor environmental c onditions induced a differential rate of flowering in the two sexes. S econd, we examined whether the two sexes show a different cost of flow ering by quantifying (a) differences in resource allocation patterns i n flowering and non-flowering individuals and (b) the effect of flower ing on subsequent growth. Clonal growth of females that did not flower in the first year of study was significantly higher than that of fema les that had flowered and borne fruit. The latter invested less biomas s per sprout than both flowering males and non-flowering individuals o f both sexes. This indicates that the direct cost of resource allocati on to sexual reproduction, especially for female plants, has the delay ed effect of reducing subsequent vegetative propagation. Patterns of r esource allocation in male and female plants thus provide insights int o the mechanisms responsible for biased sex ratios in this species.