The fate of pollen in two morphologically contrasting buzz-pollinated Solanum flowers

Abstract

Study premise: Pollen transfer efficiency (PTE) and pollen deposition patterns on a pollinator's body significantly influence plant reproductive success. However, studies on pollen fates (i.e., the destination of pollen grains after being released) in animal-pollinated species, particularly those offering pollen as the sole reward, are limited. Here, we investigated pollen fates in two nectarless, buzz-pollinated Solanum species with contrasting floral morphology. Methods: We conducted experimental trials involving one pollen donor and four recipient flowers of Solanum rostratum and S. dulcamara, using captive Bombus terrestris as pollinator. After each trial, we assessed the amount of pollen remaining in the anthers, deposited on stigmas, placed on the pollinator, and falling to the ground. We then estimated pollen fates and PTE, and modelled their pollen deposition curves. Key results: We found that S. rostratum produced more pollen but had a more restricted pollen dispensing schedule compared to S. dulcamara. Although PTE was similar between species (0.72% vs. 1.07%, for S. rostratum and S. dulcamara, respectively), pollen loss mainly occurred due to falling to the ground in S. rostratum and bee grooming in S. dulcamara, potentially explained by their different floral architectures. Both species exhibited a typical exponential decay pattern in pollen deposition, with the first visited flowers receiving the most outcross pollen. Conclusions: Our results suggest that PTE in pollen-rewarding, buzz-pollinated species is quantitatively similar to that in nectar-rewarding flowers with single pollen units (PTE = 1-2%), and that different buzz-pollinated flower architectures achieve similar PTE although through different pollen-loss pathways.