Nectar dynamics and sugar composition in flowers of Silene and Saponaria species (Caryophyllaceae)

Nectar production in Saponaria officinalis and in five species of Silene (S . alba, S. dioica, S. noctiflora, S. nutans, S. vulgaris) was examined duri ng two consecutive years (May to July 1993, and May to lune 1994) in the Bo tanical Garden of the University of Giessen. Nectar volume and sugar concen tration were studied in relation to time of day, flower sex, flower age, an d flowering stage. Nectar amount in all species studied (except S. dioica) increased in the afternoon or in the evening until midnight (or until the e arly morning in S. nutans). After midnight and until midday, nectar volume in non-visited flowers (except S. dioica) decreased. Nectar volume in non-v isited S. dioica flowers increased constantly with flower age, indicating a stable nectar secretion rate, possibly favouring both day- and night-activ e flower visitors. Even at the time of highest nectar secretion, all specie s studied presented several nectarless flowers. Sucrose dominance in the ne ctar of the nocturnal species S. nutans and Saponaria officinalis fits well with the general syndrome of flowers pollinated by hawkmoths. The syndrome also applies to the nocturnal but regularly selfing, S. noctiflora. The mo re generalistic species S. dioica and S. vulgaris, which are regularly visi ted by bumblebees as well as nocturnal moths, secreted hexose-dominant nect ar. Unexpectedly, Silene alba, the only nocturnal species that strictly exc luded day-active flower visitors by closing flowers during the day, also se creted hexose-dominant nectar. In some cases, nectar volumes and nectar con centration differed significantly between hermaphroditic, male, and female flowers. Female flowers of S. alba, S. dioica, and S. nutans contained sign ificantly less concentrated nectar than male or hermaphroditic (S. nutans) ones. In S. noctiflora and S. vulgaris the difference was not statistically significant but nectar concentration did show the same tendency.