Divergent strategies of nectar secretion in two bat-pollinated Passiflora species

The secretion dynamics and chemical compositions of flower nectar can vary according to the plant physiology and the pollinator type. Bat-pollinated flowers are commonly associated with nocturnal nectar secretion as well as large amounts of nectar per flower, which is generally diluted and composed mostly of hexose sugars. The nectar traits in bat-pollinated Passiflora species are poorly understood and in this work, we describe these for two such species. We conducted field experiments to assess the nectar secretion dynamics of two species with nocturnal anthesis and similar colours and structures, performed chemical analyses of the sugar components of nectar from these two species, and combined these data with systematic observations of floral visitors. The nectar dynamics of both Passiflora species are similar to those of other bat-pollinated species. In Passiflora mucronata, nectar secretion starts before the flower opens, which occurs around 04h00, with secretion ending after 10h00. Passiflora setacea starts secreting nectar at the time of flower opening, around 22h00, finishing secretion between 04h00 and 06h00. In P. setacea, the final period of anthesis is characterised by active nectar resorption. The different blooming times of these two bat-pollinated passionflowers and the significant differences in their nectar secretion schedules suggest a divergent nectar secretion dynamics: the nectar of P. mucronata is made available only by the end of the bats' foraging activity period, continuing throughout the morning, while P. setacea secretes nectar earlier, with very little overlap in the secretion period between the species. We hypothesise that this segregation in time helps avoid the negative effects from competition for pollinators or pollen interference between flowers with a similar structure, while the outstanding nectar traits and the constant production of flowers throughout the year make these closely related vines important nectar sources for their bat pollinators where they co-occur.