Which type of reproductive barriers separates two flowering plant species that could interbreed but for the fact that one has a deep flower tube and is pollinated by bumblebees whereas the other has a short, narrow flower tube and is pollinated by honeybees?

Mechanical isolation.

Explanation:

Reproductive isolation may be defined as the behavior and the evolutionary mechanism that are essential for the formation of the speciation. This prevents the fertilization of the unrelated species.

Mechanical isolation is a type of reproductive isolation in which the species are isolated due to the difference in their gametic structure. The deep flower tube has different gametic structure that prevent their mating. Although these flowers are closely related to each other but unable to meet due to the mechanical isolation.

Thus, the answer is mechanical isolation.