Morphological and phenological matching shape functional heteranthery

Heteranthery refers to the presence of morphologically distinct stamens within a flower. Previous studies have focused on morphological matching between floral structures and pollinator body size and on the division-of-labour hypothesis (i.e. feeding pollen as a reward and pollinating pollen as a male gametophyte). However, little attention has been paid to when each stamen type presents pollen, despite the importance of phenological matching in determining pollen transport frequency. We hypothesize that morphological and phenological matching affect the qualitative (pollen transfer efficiency) and quantitative (encounter frequency) components of pollen transport, respectively. To test this hypothesis, we first examined male fertility of pollen produced by dimorphic stamens in heterantherous Lagerstroemia indica. Second, we compared morphological matching between floral structures and bee body size and evaluated its consequences for pollen transport efficiency in large carpenter bees and small honeybees. Third, we assessed phenological matching by examining diurnal patterns of anther dehiscence and bee activity and their correspondence. Flowers exhibited clear functional differentiation: feeding pollen contributed less to male success due to lower fertility. Carpenter bees, whose body size matched floral morphology, more effectively transferred pollinating pollen via dorsal deposition, whereas honeybees accumulated feeding pollen ventrally with limited pollination success. Dehiscence of pollinating anthers began early in the morning, coinciding with carpenter bee activity and continued gradually throughout the day. Together, our study reframes heteranthery as multidimensional matching strategy in which phenology governs encounter to pollinators and morphology governs the consequence of pollen transport.