Temporal patterns in the mass flowering of Firmiana simplex: A new case of duodichogamy

Premise

Given that insect-pollinated trees are generally characterized by mass flowering, hermaphrodites may benefit from abundant pollinators but suffer the cost of geitonogamy. To reduce such selfing caused by intraplant pollination, duodichogamy (a temporal sexual system involving three alternating sexual phases within an individual) has been suggested to be a common strategy. However, the timing of the female and male phases within individuals throughout floral phenology remains undocumented in any duodichogamous species.

Methods

Over 2 years, we quantified flower sexes at the panicle and the plant level throughout flowering in Firmiana simplex to understand how it minimizes geitonogamy. We studied foraging behavior of diverse insects on staminate and pistillate flowers to examine the anther/pollen “advertising” hypothesis, and hand-pollination treatments were used to examine the breeding system.

Results

Flowers on panicles were highly synchronized within the male, then female, then male phases, with only a few days of overlap between pollen release and stigma receptivity. All examined individuals flowered in a sequence of male-female-male. The absence of days when the population contained only female-phase flowers guaranteed that the “expensive” flowers could be pollinated by diverse bees. Five bee species foraged for nectar but not pollen and did not prefer staminate over pistillate flowers.

Conclusions

Quantitative measurements of daily flower numbers and sexual phases showed that pistillate flowers were highly synchronous and only a few staminate flowers were out of the female-phase days, minimizing reproductive cost in self-incompatible Firmiana simplex. No evidence for the advertising hypothesis of indehiscent anthers was obtained.