Floral dynamics in the protandrous flowers of Bauhinia variegata (Fabaceae): phase-specific morphology, scent, and nectar production

Abstract Main conclusion Protandry in Bauhinia variegata is expressed through coordinated changes in floral morphology, scent composition, secretory activity, and nectar presentation, whereas petal reflectance remains stable; molecular docking further indicates that key phase-associated volatiles are structurally compatible with a bee odourant-binding protein, supporting mechanistic plausibility of olfactory detectability without demonstrating behavioral attraction. Abstract Bauhinia variegata exhibits protandrous flowers in which floral morphology, scent production, and nectar presentation change through anthesis. Here, we examined anthesis, stigma receptivity, petal reflectance, volatile organic compound (VOC) composition across floral organs and phases, secretory structures, nectar dynamics, and the structural compatibility of selected phase-discriminant VOCs with a honeybee odourant-binding protein. Flowers exhibited prolonged anthesis, opening at night and remaining functional for 2–3 days. Pollen release predominated during the 1st day (staminate phase), whereas maximal stigma receptivity occurred on the 2nd day (pistillate phase), accompanied by elongation and curvature of the style. Petal reflectance remained stable across phases. Floral scent was spatially heterogeneous among organs and temporally dynamic across anthesis, with terpene-rich bouquets dominated by α-pinene and β-pinene and with phase- or organ-informative compounds including nonanal, limonene, trans-caryophyllene, and 6-methyl-5-hepten-2-one. Cavitated secretory trichomes and sepals showed evidence of terpene-associated secretion. Nectar volume and total sugar content peaked during the pistillate phase, coinciding with strongest stigma receptivity. Molecular docking with AmelOBP14 showed that key phase-associated VOCs are structurally compatible with a bee odourant-binding protein, providing complementary mechanistic support for their potential olfactory detectability. Together, these results show that protandry in B. variegata is expressed through coordinated changes in morphology, scent, secretory activity, and nectar presentation, indicating a complex signalling strategy; however behavioural responses of floral visitors remain to be tested.