Accumulation of Heavy Metals in Blueberry Floral Rewards and Their Effects on Reproductive Fitness and Bumblebee Pollination Behavior

Heavy metals in soil can be translocated to floral rewards via specific physiological pathways, potentially altering pollinator behavior and reducing plant reproductive fitness. However, the comparative effects of different heavy metals on floral rewards and pollination remain poorly understood. Here, blueberries were used as a model system to examine the accumulation of four heavy metals (Zn, Pb, Cu, and Ni) in floral rewards and assess their impacts on plant reproductive performance and bumblebee visitation behavior under controlled conditions. Heavy metals accumulated in floral rewards in a concentration-dependent manner, with strong positive relation to soil metal levels; pollen generally contained higher concentrations than nectar. Increasing soil heavy metal concentrations significantly shortened bumblebee visit duration to individual flowers, with the greatest reduction under high Pb treatment (26.32%), followed by Zn (21.49%), Ni (17.70%), and Cu (15.40%). Visit frequency also declined under medium to high concentrations. All heavy metal treatments reduced pollen viability (1.62–24.48%), pollen deposition on stigmas (9.91–40.59%), and single-fruit weight (16.25–32.51%), with effect magnitudes varying by metal (generally Pb > Cu > Zn > Ni). Overall, heavy metal accumulation in floral rewards negatively affects pollinator behavior and compromises plant reproductive fitness, with metal-specific effect strengths.