Occurrence of preanthesis cleistogamy in Richardia brasiliensis suggests it may be more common in Rubiaceae than realised

Abstract

Background: Most Rubiaceae species are hermaphroditic with outcrossing often promoted by distyly and self-incompatibility, as well as by andromonoecy and dioecy in some species. However, cleistogamy has been reported in three genera, including Richardia. Aims: To characterise the reproductive biology of Richardia brasiliensis (Rubiaceae), a ruderal herb native to southern USA, Central America and large parts of South America, and determine if it exhibits preanthesis cleistogamy. Methods: In three natural populations of R. brasiliensis in Argentina, floral processes were monitored in the flower-bud stage and during anthesis until senescence. In addition, stigma receptivity and pollen viability were determined at these different floral stages in two populations, and experimental controlled pollinations were performed to check for spontaneous selfpollination, natural pollination and apomixis. Floral visiting insects were observed and collected for identification. Results: Autonomous self-pollination occurred when flowers were in bud and stigmas were receptive, with ovules fertilised prior to anthesis. These flowers were protogynous and functionally cleistogamous, though later passed through a chasmogamous phase when flowers opened and were visited by diverse insects. Conclusions: Our study demonstrates that preanthesis cleistogamy occurs in flower-buds of R. brasiliensis, indicating that this mode of self-pollination and self-fertilisation may be more common in Rubiaceae than previously realised.