Morphological dynamics of flower development and sex differentiation in monoecious soapberry (Sapindus mukorossi Gaertn.)

Soapberry ( Sapindus mukorossi Gaertn.) is a monoecious species with unisexual flowers, valued for its fruit oil and saponins, used in biofuels, detergents, and pharmaceuticals. However, its industrial potential is limited by low fruit yield caused by an imbalanced ratio of female to male flowers. The developmental processes underlying flower formation and the critical stages of sex differentiation in this species remain poorly understood. This study investigated the cymose panicle structure and the spatial distribution of male and female flowers, and conducted detailed morphological and cytological analyses across 15 developmental stages. The observations revealed that both male and female flowers initially passed through a bisexual phase before one set of sexual organs was arrested and underwent abortion during a broad developmental window. Ovule abortion in male flowers primarily occurred during the pre-meiosis to the pre-mitosis stage (S10–S11), while in female flowers, pollen abnormalities appeared at the post-meiosis stage, specifically during the first mitotic division (mitosis Ⅰ, S11), preceding the arrest of the styles and filaments. Distinct features, including abnormal cell plate degradation and formation, pollen degradation, calcium oxalate crystal-associated anther dehiscence, extensive programmed cell death (PCD) and other abnormalities were observed in the sterile anther of female flowers. These results identified megaspore mother cell (MMC) pre-meiosis (S10c) as the point of sex determination, and MMC meiosis and pollen mitosis I (S11) as the onset of visible sex differentiation. This developmental framework provides new insights into sex regulation in soapberry flowers and lays a foundation for targeted breeding programs aimed at enhancing fruit yield, oil production, and saponin output. • Mixed FF and MF arrangement without consistent pattern in cymose panicles. • Ovule abortion in male flowers primarily occurred during meiosis (S10–S11). • Female flower pollen exhibited mitotic defects at S11 before filament arrest.