Sexual organs of Sarcotheca macrophylla Blume (Oxalidaceae), an endemic species of Borneo with two floral types confirmed as heterostyly

ABSTRACT: Sarcotheca macrophylla Blume (Oxalidaceae), an endemic plant species to Borneo, consists of short-styled (S-morph) and long-styled (L-morph) flowers, referred to as heterostyly. However, the character states related to heterostyly have not been fully disclosed in S. macrophylla. Additionally, S. macrophylla has a reproductive issue due to a lack of seeds in fruits. This research aims to examine the floral characteristics and detail of sexual organs (androecium and gynoecium) in this species and to analyze the consequences of heterostyly on its natural pollination. We collect flowers from 34 individuals and investigate the inaccuracy index and all of floral character states. We used light and stereo microscopes to evaluate the heterostyly syndrome through morpho-anatomical observations of the flowers. Our results reveal that the two floral types (S-morph and L-morph) of S. macrophylla possess reciprocal herkogamy with ancillary pollen polymorphism in palynological characters, differences in pollen number, and pollen viability. These characters suggest that S. macrophylla has distyly type. Furthermore, the reproductive organs of the floral morphs appear to contribute differently to reproductive fitness. The S-morph flowers demonstrate enhanced male function through higher pollen production and viability, while the L-morph flowers exhibit enhanced female function through floral architecture that promotes effective pollen reception.