Though visitors are often many times stronger than the operative strength of the flowers they pollinate, exceptionally weak bees such as Apis mellifera cannot open the strongest flowers. On the other hand, strong correlations among certain petal morphometric traits (particularly between the keel and wings) give support to the idea that an intrafloral module is associated with the functioning of the mechanism of these legume flowers. In addition, the highly significant correlations found across petals support the view of functional phenotypic integration transcending the ontogenetic organization of flower structure.

Flowers may be interpreted as complex combinations of organs functionally coordinated to attract pollinators and to mechanically interact with the pollinator's body, particularly when flower mechanisms are actively handled by pollinators. Thus, a functional modularity of traits in keel flowers (Fabaceae) was expected because of a compartmentalization between attraction and mechanical functions. To test this hypothesis, we used Collaea argentina, a Fabaceae that exhibits typical keel flowers. The force needed to open keels, the keel displacement angle and floral morphometric traits in 100 plants from a natural population were measured to detect floral characters correlated with the biomechanical variables. Furthermore, we examined the relationships among this functional module, biomechanical variables and female reproductive success to explore whether these traits are the targets of pollinator-mediated phenotypic selection, and used path analysis to examine the causal relationship among these variables. A functional module formed by two morphometric traits of the petals directly involved in the floral mechanism (keel and wings) was found, but no flag trait was involved in this module. Even though the functional module had a positive effect on force and there were significant relationships between the displacement angle and fruit set, no significant effect of force on female reproductive success was detected. These results question whether selection currently plays a role favouring the integration of this module, but this may be consistent with a past stabilizing selection on the force needed to open the keel.

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