Describing heterostyly, pollen, and sexual organ reciprocity in Rourea chrysomalla (Connaraceae), a rare and threatened species from the Brazilian Cerrado

Rourea chrysomalla is a rare and endemic plant species from the Brazilian Cerrado, presumably threatened and lacking records within conservation units. In this study, we recorded and characterized, for the first time, the floral and pollen dimorphism, as well as the reciprocity of sexual organs among different morphs of R. chrysomalla. For this purpose, fertile individuals were observed over three years in two natural populations and supplemented by herbarium data. This species is distylous and distinctly exhibits two types of flowers, each characterized by stamens arranged in two whorls of unequal lengths: L-styled and S-styled morphs, both occuring in similar proportions in the two populations. Cross-pollination among anthers of any stamen whorl in L-styled flowers and stigmas in S-styled flowers is more precise in the two morphs, while anthers in S-styled flowers are less reciprocal with the stigmas in L-styled flowers. S-styled flowers exhibit larger pollen grains compared to L-styled flowers, with no differences between the two whorls of stamens. The balanced proportion of floral morphs provides equal opportunities for pollen donation and reception, and reciprocity between morphs is essential for the conservation of R. chrysomalla due to its limited distribution and low population density in the central Brazilian savanna.