Same visitors, different outcomes: floral phenotype gates pollinator-vectored nectar microbial establishment

The microbiome of nectar has the potential to modify flower chemistry and plant-pollinator interactions. How its assembly differs between diurnal and nocturnal guilds, and how it depends on the flower they visit, remains poorly resolved in crops. We conducted field assays using two commercial sunflower cultivars (Helianthus annuus) that differ markedly in nectar volume and sugar composition, under four pollinator access treatments. Diurnal pollinator access, whether continuous or daytime-only, resulted in a cultivar-dependent microbiome signature. In one cultivar, nectar communities were characterized by the dominance of specialist taxa such as the yeast Metschnikowia and the bacterium Acinetobacter. However, neither taxa established dominance in the nectar of the other cultivar despite receiving substantial insect visits. Instead, continuous access alone lowered overall fungal diversity without restructuring composition, which was more strongly structured by inter-annual variation than by pollinator access treatment. Profiling of the volatile organic compounds in florets revealed clear differences between cultivars, but different access treatment induced only minor changes in the volatile blends. Our findings establish that pollinator guilds shape sunflower nectar microbiota in a genotype-dependent manner. We propose that diurnal insect visits seed nectar with specialist taxa, but whether those specialists establish depends on nectar availability and composition.