Why are many Melastomataceae flowers pollinated by bees even though they lack nectar?

Most species within the Melastomataceae family have evolved nectarless flowers that are specifically adapted for pollination by pollen-collecting bees.1

Because these flowers do not offer nectar as a reward, they rely on the bees' need for pollen to facilitate visitation. To attract these pollinators, the flowers often feature bright colors and stamens with conspicuous, sometimes yellow, appendages. These floral elaborations may enhance the visual attractiveness of the flower and provide structures that make the stamens easier for bees to grasp.1

Furthermore, many plants that require specific vibration frequencies to release their pollen utilize a process known as buzz pollination, or sonication. In this interaction, bees vibrate their flight muscles to trigger the release of a cloud of pollen from the flower, which they then collect. This specialized relationship ensures that the bees obtain the necessary protein-rich food source for their larvae while simultaneously effecting pollination for the plant.234