Comparison of Levers and Internal Barriers in Flowers

Comparing the lever and other internal barriers in terms of forces needed to release or to overcome them points to the potential importance of the staminal lever as a mechanical barrier. In S. glutinosa, the force needed to trigger the staminal lever is lower by about 40% than the force necessary to pass the artificial proboscis through the additional internal barrier, built in this species by a hairy ring in the narrow part of the flower tube. This indicates that in terms of forces, the lever does not represent the main device to exclude insects from gaining access to the nectar, and, more likely, the narrow corolla tube with its hairy ring has this function in S. glutinosa.

The situation in S. sclarea is different. In this species (even after exclusion of the peak due to sticking events), the staminal lever is the main barrier in terms of the force required to protrude the artificial proboscis to the nectar at the flower base.

ClaBen-Bockhoff et al. [21] measured the forces that are necessary to trigger the levers of eight Salvia species; three of these species were also included in this analysis. The previously published values are identical to those found in our present studies [in brackets] for two species: S. pratensis 2.98 ± 2.43 mN (n = 1140) [2.7 ± 0.3 mN (n = 28)] and S. glutinosa 1.47 ± 1.05 mN (n = 780) [1.6 ± 0.8 mN (n = 10)]. The only exception is S. sclarea for which a tentative value of 31.8 mN based on two measurements was given previously by ClaBen-Bockhoff et al. [21]. This is much higher than the value of 5.2 ± 3.0 mN (n = 43) for the more detailed measurements presented in this paper. This difference might be due to the possibility that the actual release of the staminal lever is very difficult to distinguish from the sticking event at the sculptured sterile lever base that caused the large force peak in the present measurements.

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