Pollination biology and reproductive adaptation of Hibiscus mutabilis Linnaeus (Malvaceae): seasonal variation in floral traits, pollinator assemblages, and breeding system

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Understanding how flowering plants respond to spatiotemporal variation in pollination environments is a central question in floral ecology. Hibiscus mutabilis exhibits a prolonged population-level flowering period that exceeds three months (late summer to late autumn), yet the reproductive strategies underlying this extended phenology remain unclear. Here, we investigated a cultivated population of H. mutabilis located in the Chengdu Botanical Garden, China, to address four questions: (1) do flower lifespan, floral structures, and floral morphology differ among early, middle, and late flowering periods? (2) Do nectar standing crops and secretion rhythms vary across flowering periods? (3) Do the species of flower-visiting insects and their foraging behaviours change during different flowering periods? (4) Does the breeding system shift dynamically across the flowering season? We observed, measured, and recorded the flowering dynamics, floral traits (size, longevity), the pollen/ovule ratio, pollen viability, and stigma receptivity, as well as the nectar secretion volume and sugar concentration. We also documented pollinator assemblages and visitation frequencies across the different flowering periods, supplemented by controlled pollination experiments. The results revealed that the floral longevity of H. mutabilis significantly extended, whereas floral size decreased during the late period compared with those during the early and middle periods. These changes were temporally associated with seasonal declines in temperature and variations in precipitation. Five pollinator species were observed (including four bees (e.g., ( Xylocopa appendiculata , X. dissimilis , Bombus breviceps , and Apis mellifera ) and one hawkmoth (e.g., Macroglossum pyrrhosticta )), and the total pollinator richness remained stable across periods. By contrast, the visitation frequency and sugar concentration decreased significantly during the later period. Although H. mutabilis is self-compatible, dichogamy and herkogamy effectively limited autonomous selfing (self-fertilization/self-pollination) during the early and middle periods. However, late-stage flowers produced fertile seeds through autonomous self-pollination, partially offsetting the decline in pollinator activity. These findings suggest that H. mutabilis coordinates shifts in floral longevity, floral traits, nectar production, and the mating system to maintain reproductive success under variable pollination conditions. We emphasize that our study did not include formal correlation analyses between climate variables and floral traits; thus, the observed associations should be interpreted as preliminary patterns requiring further experimental testing, rather than demonstrating causal effects.

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