Diurnal variation in plant-pollinator interactions in Bumblebee-pollinated Impatiens oxyanthera at two altitudes

Spatio-temporal matching between plants and pollinators is critical for their interactions, yet small-scale patterns are understudied. We explored these using the endemic Impatiens oxyanthera (Balsaminaceae) and its bumblebee pollinators, comparing diurnal variations in floral traits, pollinator size, visit frequency, and reproductive success across two altitudes. High-altitude flowers exhibited greater corolla tube temperature gradients (warmer mornings, cooler afternoons), more nectar, and were visited more frequently by longer-tongued bumblebees than low-altitude counterparts. Morning size matching (lower height differences between corolla tubes and pollinator bodies) was better at high altitudes, alongside more ovules and seeds. Diurnally, corolla temperature gradients peaked at 8:00, with stable nectar volume and sugar composition (except sucrose) throughout the day. High-altitude flowers showed stronger diurnal shifts in corolla-pollinator height matching (better in mornings) than low-altitude ones. At high altitudes, bumblebee visits to female-phase flowers were lowest at 8:00 but increased by 10:00 with rising temperatures. Pollen deposition peaked at 18:00 (with more unfertilized ovules) and reached a trough at 16:00 or 14:00 (fewer unfertilized ovules) across altitudes. Redundancy analysis highlighted corolla temperature and pollination frequency as key drivers of seed number and unfertilized ovules, particularly at high altitudes. Altitude and time significantly influenced these interactions, with more pronounced diurnal variations in interactions and reproductive success at high altitudes, suggesting plant sexual reproduction adapts to temporal environmental heterogeneity.