Melilotus officinalis seed formation in relation to pollination biology

The phenomenon of “more flowers (ovules) and fewer fruits (seeds)” often exists in angiosperms due to limited pollination, resource or maternal selection. Melilotus officinalis is a widely cultivated and high-quality legume forage. It contains multiple ovules but only 1～2 develop into mature seeds. It remains unclear whether pollination plays a role in seed formation. Therefore, this study was done to investigate the pollination system and seed formation process of M. officinalis. The results showed that there was a mixed mating system type in M. officinalis without apomictic reproduction, which was self-compatible, but not automatically self-pollinated as it requires the aid of pollination insects. It was observed that the population began to bloom in late May, with florescence lasting for approximately 40 d, and the opening of a single flower lasting 1～1.5 d. M. officinalis had a general pollinating system, with the main pollinators being bees, flies and hoverflies. The number of pollens on stigmas were 18.19 in the natural state, and 6.04 in the emasculated state, without pollen restriction. Under artificial cross-pollination conditions, 80% of the ovules at position 1 (observed from the base of pod), and 21.8% at position 2 develop and form seeds. The ovules at positions 3～7 did not develop, and had undergone massive abortions starting on the 4th day after pollination. Therefore, it was inferred that there were no factors restricting seed formation in terms of pollination (pollen source and the intensity of pollination) in M. officinalis, This phenomenon is known as a type of seed selective abortion by maternal regulation, which may be a reproductive strategy to increase seed fitness or seed dispersal ability.