Why do bilaterally symmetrical flowers orient vertically? Flower orientation influences pollinator landing behaviour

Flower orientation is an important character influencing plant fitness. Zygomorphic flowers are known to orient vertically. We conducted field experiments in which we changed the flower angle of zygomorphic Commelina communis to determine how flower orientation affects pollinator behaviour. We confirmed that Commelina flowers oriented vertically like other zygomorphic flowers. Then, we artificially prepared control, upward- and downward-oriented flowers and exposed them to natural pollinators (syrphid flies and bumblebees). We found that the frequency of approach by syrphid flies and bumblebees was not influenced by flower angle, but there were fewer landings on downward-oriented flowers than on control and upward-oriented flowers. Moreover, the upward flower orientation increased illegitimate landings (landing on the flower without touching the stigmas or mating-related anthers) compared with controls. Thus, vertical flower orientation in zygomorphic flowers serves to control pollinator landings. Our findings suggest that deviations from vertical orientation may reduce fitness in C. communis by reducing the efficiency of insect-mediated pollen transfer.