Are Corolla Tube Dimensions the Reason for Low Seed Yield in Tetraploid Red Clover?

Abstract

Seed yield is considerably lower in tetraploid than in diploid red clover. The most commonly suggested reason is that tetraploids have longer corolla tubes that prevent short-tongued bumblebee species from reaching the nectar, thereby resulting in deficient pollination. However, recent studies contradict this generally accepted statement. Because there is no consensus on this topic, we investigated the relation between seed yield and corolla tube length on the same plants in two separate field trials with different plant material. One field was sown with the tetraploid cultivar ‘Avanti’, while a second field was planted with tetraploid elite families. No correlation was found between seed yield and corolla tube length in either trial (p = 0.480 and p = 0.430 respectively), which indicates that corolla tube length does not affect seed yield in tetraploids. To explore whether the corolla tube diameter can explain the lack of correlation observed here, we measured this trait in flowers of 25 diploid and 25 tetraploid plants grown in pots. The average corolla tube diameter was 19.0 % wider in tetraploids than in diploids. Possibly, greater corolla tube diameters compensate for longer corolla tubes by allowing bees to insert their heads deeper into the tubes. This hypothesis needs to be validated by direct observations, currently underway at ILVO.