Investigating the pollination syndrome of the Hawaiian lobeliad genus Clermontia (Campanulaceae) using floral nectar traits

Abstract

Floral nectar sugar compositions have, for several decades, been used to predict a plant species' pollinator guild. Plants possessing a generalist ornithophilous pollination syndrome produce nectar that is dilute (8-12% w/v sugars) with a low sucrose to hexose (glucose and fructose) ratio. The Hawaiian lobeliad genus Clermontia contains 22 endemic species of shrubs and small trees that are believed to have evolved flowers adapted for pollination by now mostly extinct or endangered endemic passerines in the Drepanidinae and Mohoidae. We analyzed the nectar sugar compositions, concentration, and nectar standing crop of 23 taxa to test the assumption that Clermontia taxa have evolved floral traits in response to selection pressures from these avian pollinators. All Clermontia taxa produced nectar with sugar concentrations (mean: 9.2% w/v ± 1.8 SD) comparable to the nectar of other plant species with a generalized bird pollination system. Nectar sugars were overwhelmingly composed of hexoses in all taxa (mean sucrose/hexose ratio: 0.02 ± 0.02). Nectar standing crop volumes varied widely among taxa, ranging from 9.7 µL ± 7.1 to 430.5 µL ± 401.8 (mean volume: 177.8 ± 112.0). Collectively, the nectar traits indicate that Clermontia species possess a generalist passerine pollination syndrome.