DIFFERENCES BETWEEN SEXES IN FLORAL MORPHOLOGY, NECTAR PRODUCTION AND INSECT VISITS IN A DIOECIOUS SPECIES, SILENE DIOICA

Abstract

Summary Silene dioica (L.) Clairv. was studied in several wild populations near Swansea and in an experimental population to determine the extent of differences between the sexes in floral morphology, nectar production, and insect visits to flowers, and the effects of these differences on seed set.Male flowers are larger than female flowers and are borne more densely over a longer period. Male flowers have longer and more frequently white corona scales which may mimic the projecting white styles of female flowers. Male and female flowers were observed to secrete similar quantities of nectar sugar on the first morning of opening, but female flowers secreted more than males subsequently, and the total volumes of nectar secreted by female flowers were considerably greater. Male plants varied more than females in the total quantity of nectar sugar secreted per flower. Nectar concentration was nearly always greater in male flowers than in female flowers.Bumblebees, particularly Bombus hortorum (L.), B. pascuorum (Scop.) and B. terrestris (L.), were the most important visitors to the. flowers of S. dioica in the study sites, with some honeybees [Apis mellifera (L.)], butterflies (mainly Pieridae) and hoverflies (Syrphidae). Some visitors, including those that were most numerous at some sites, made most or all of their visits illegitimately. In the field in 1980 many visitors were observed to discriminate in favour of male flowers, which were much more numerous than female flowers. In the experimental population in 1981, many visitors discriminated in favour of female flowers, which were more numerous than in the field populations but were still substantially outnumbered by male flowers. Some individual bumblebees showed a change in preferences during the day, disciminating in favour of female flowers only during the afternoon, when the nectar resources available from female flowers were proportionately greater. The total numbers of visits that were observed to female and male flowers in the experimental population corresponded closely with the total quantities of nectar sugar available from each sex. The importance of the different insect visitors and of their discrimination between sexes in pollination is discussed. Seed set did not appear to be reduced by discrimination or insufficiency of pollinator visits in closely spaced plants, but was reduced in plants isolated by distances of 15 m or greater.