Morphological and functional sex expression in the Malagasy endemic Tina striata (Sapindaceae)

Knowledge of the functional sex expression of flowers is critical to identify the breeding system and potential for outcrossing in individuals but difficult to determine based solely on floral morphology. Individuals of the Malagasy endemic Tina striata (Sapindaceae) have both morphologically male (staminate) flowers and hermaphroditic (bisexual) flowers, although many Sapindaceae species have only functionally unisexual flowers. Two populations of T. striata were studied to determine the functional sex expression of flowers and their breeding system.

To determine whether morphologically hermaphroditic flowers have only female function, we compared male flowers and morphologically hermaphroditic flowers for (1) floral size, (2) pollen production and morphology, (3) patterns of anther dehiscence, (4) pollen germination, and (5) floral visitation patterns.

Relative to male flowers, morphologically hermaphroditic flowers had similar pollen grain morphology but smaller androecia and less pollen. Anthers from male flowers dehisced on days 3 and 4 of flowering and then abscised with the male flower. Anthers from hermaphroditic flowers did not dehisce or release pollen and abscised after the fourth day of flowering. Pollen from hermaphroditic flowers was less likely to germinate and produce pollen tubes. Floral visitors were similar for male and hermaphroditic flowers, and no visitation to hermaphroditic flowers was observed after stamen abscission.

Morphologically hermaphroditic flowers of T. striata do not have male function and are functionally female; therefore, T. striata has a monoecious breeding system, with possible adaptations to promote outcrossing.