Are corolla tube dimensions the reason for low seed yield in tetraploid red clover?

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.