Insights of morphology and functional pollen biology of medicinally important endangered plant species Rauvolfia serpentina (L.) Benth. ex Kurz

Abstract Rauvolfia serpentina (L.) Benth. ex Kurz, a critically endangered medicinal shrub of the Apocynaceae family, is renowned for its bioactive indole alkaloids, particularly reserpine. Despite its ethnobotanical significance, the species faces severe threats from habitat loss, overexploitation, and reproductive constraints. This study investigates its floral biology, pollen viability, and in vitro germination to guide conservation and breeding strategies. Floral phenology revealed year-round blooming, with peak flowering during late January–March and May–early July. The species exhibits actinomorphic, pentamerous, insect-pollinated flowers, with anthesis and anther dehiscence occurring between 5:30–8:30 AM. A high pollen-to-ovule ratio (600:1) and nectar secretion support an obligate outcrossing system. Pollen viability, assessed through Fluorescein Diacetate (FDA), Acetocarmine, and 2,3,5-triphenyl tetrazolium chloride (TTC) assays, displayed a pronounced diurnal pattern. Peak viability (≥ 98%) occurred between 5:00–10:00 AM, followed by a sharp decline at post-noon (12:00 PM). FDA and TTC were more reliable indicators of metabolic viability than acetocarmine staining. In vitro pollen germination using Brewbaker and Kwack medium demonstrated sucrose-dependent germination, with optimal performance at 25–35% sucrose. Maximum tube elongation (537.07 ± 66.23 µm) occurred at 35% sucrose. Germination was absent below 10%. Notably, polysiphonous pollen tubes- multiple tubes from a single grain- emerged under high sucrose and boron concentrations, indicating possible in vitro -induced plasticity. This integrated analysis of floral traits, pollen viability, and germination physiology contributes valuable insight for developing targeted conservation, propagation, and genetic improvement strategies. These findings provide critical insights into the reproductive biology of R. serpentina , emphasizing the role of floral timing, metabolic integrity, and nutrient regulation. This knowledge is pivotal for refining conservation protocols and advancing genetic improvement efforts for this threatened status and rising environmental pressures.