Female-biased nectar production is associated with the Darwin's inflorescence syndrome

Plant reproduction is influenced not only by individual flower characteristics but also by the arrangement of flowers within inflorescences. In bee-pollinated plants with protandrous flowers in vertical acropetal inflorescences - where male fertile flower structures mature before female ones and basal flowers open first (i.e., Darwin's inflorescence configuration) - male-phase flowers are positioned above female-phase flowers. This arrangement benefits from the upward foraging motion of bees, reducing pollen transfer between flowers of the same plant and increasing pollen export. Additionally, female-biased nectar production in these species could reinforce upward bee movement, further reducing self-pollen transfer. We conducted a comparative study of 112 hermaphrodite angiosperm species with temporal separation of male and female reproductive functions (i.e., dichogamy) to explore whether female-biased nectar production is common among bee-pollinated species with Darwin's inflorescence configuration. Our results revealed female-biased nectar production in bee-pollinated species with Darwin's inflorescence configuration. In the absence of bee pollination, species with this type of inflorescence exhibit male-biased nectar production. Gender-biased nectar production is absent in species lacking this inflorescence type. Female-biased nectar production in vertical acropetal inflorescences with protandrous flowers likely evolved independently in different bee-pollinated species to enhance pollen export. Our study emphasizes the need for microevolutionary research on how nectar production patterns adapt to bee pollination in species with Darwin's inflorescence configuration. It also calls for investigation into the developmental biases influencing nectar production in plants with dichogamous flowers.