Bee pollination improves fruit quality in a crop with multi‐pistillate flowers by increasing stigmatic pollen loads and the fraction of pistils receiving enough pollen

Abstract Bee pollination increases crop yield quantity and quality. However, pollination requirements for pollen quantity and quality to maximise yield components remain unknown for most crops. Moreover, the role of pollen distribution within multi‐pistillate flowers that produce aggregate fruits in determining fruit quality has remained unexplored. We assessed the effects of pollen quantity and quality on fruit set and fruit quality, using raspberry ( Rubus idaeus , variety: Autumn Bliss) as a model crop. Across 50 censuses, we recorded 964 visits to raspberry flowers, with Bombus terrestris (66%) and Apis mellifera (30%) as the main flower visitors, followed by native bees (4%). We conducted five pollination treatments (hand self‐pollination, hand cross‐pollination, spontaneous self‐pollination, open pollination, and pollen supplementation), estimating pollen receipt, its distribution among stigmas, and fruit set and quality. Flowers isolated from pollinators (i.e., spontaneously self‐ pollinated) set 26% fewer fruits and 74% fewer ‘high‐quality’ fruits than those visited by pollinators (i.e., open‐pollinated). On average, flowers visited by pollinators also set heavier and better‐shaped fruits with more drupelets than isolated flowers. The number of drupelets per fruit increased with both the average pollen grains per stigma and the proportion of stigmas receiving at least 15 pollen grains, a measure of pollen distribution within the multi‐pistillate flowers. Thus, bee pollination increases fruit quality by optimizing the quantity and distribution of pollen grains among stigmas in multi‐pistillate flowers.