A signal-like role for floral humidity in a nocturnal pollination system

Abstract

Previous studies have considered floral humidity to be an inadvertent consequence of nectar evaporation, which could be exploited as a cue by nectar-seeking pollinators. By contrast, our interdisciplinary study of a night-blooming flower, Datura wrightii, and its hawkmoth pollinator, Manduca sexta, reveals that floral relative humidity acts as a mutually beneficial signal in this system. The distinction between cue- and signal-based functions is illustrated by three experimental findings. First, floral humidity gradients in Datura are nearly ten-fold greater than those reported for other species, and result from active (stomatal conductance) rather than passive (nectar evaporation) processes. These humidity gradients are sustained in the face of wind and are reconstituted within seconds of moth visitation, implying substantial physiological costs to these desert plants. Second, the water balance costs in Datura are compensated through increased visitation by Manduca moths, with concomitant increases in pollen export. We show that moths are innately attracted to humid flowers, even when floral humidity and nectar rewards are experimentally decoupled. Moreover, moths can track minute changes in humidity via antennal hygrosensory sensilla but fail to do so when these sensilla are experimentally occluded. Third, their preference for humid flowers benefits hawkmoths by reducing the energetic costs of flower handling during nectar foraging. Taken together, these findings suggest that floral humidity may function as a signal mediating the final stages of floral choice by hawkmoths, complementing the attractive functions of visual and olfactory signals beyond the floral threshold in this nocturnal plant-pollinator system.

Nepenthespitchersversusthermogenic flowers: Thermal patterns and their role in prey capture and pollination

“…Recent studies demonstrated that bees are sensitive to floral humidity (von Arx et al 2012; von Arx 2013; Harrap et al 2021; Dahake et al 2022), and on a much larger scale, variations in environmental humidity influence insect behavioral responses (Chown et al 2011; Enjin 2017). Rands and Whitney (2008) stated that temperature receptors in bee’s antennae are sensitive to temperature variations, and can sense differences as low as 0.25°C.…”

Chemistry, biosynthesis and biology of floral volatiles: roles in pollination and other functions

“…252 Several aliphatic ketones (e.g., 2-undecanone, 2-tridecanone, 2-pentadecanone) and terpenoids [e.g., (E)-b-bergamotene (163) , (E,E)farnesol (164)] elicited physiological responses in antennae of the moths. In elegant studies, von Arx et al 253 and Dahake et al 254 demonstrated that approaching and landing behaviors of foraging H. lineata and M. sexta hawkmoths are inuenced by oral humidity. Floral humidity gradients are due to passive (nectar evaporation) and/or active (stomatal conductance) processes and allow the moths to sense nectar availability from a distance and to forage efficiently.…”

“…Floral humidity gradients are due to passive (nectar evaporation) and/or active (stomatal conductance) processes and allow the moths to sense nectar availability from a distance and to forage efficiently. 253,254 Finally, Agrius convolvuli responded to various compounds of its Gladiolus longicollis host plant, among them aromatic compounds [e.g. benzaldehyde (88), benzyl acetate ( 90) phenylacetaldehyde (83), eugenol (87)] and nitrogen-bearing compounds [3-methylbutylaldoxime (114), 2-phenylacetonitrile (92), 2-phenylnitroethane ( 93)].…”

Flowers are leakier than leaves but cheaper to build

“…Thus, pollinator selection on volatile organic compound emissions may be tightly coupled to the mechanisms of flower water balance. Or, water loss from the flower may itself be used as a signal for pollinators (Dahake et al, 2022). Regardless of which explanation may be true, both mechanisms suggest that climate change may alter the selection dynamics on floral hydraulic traits.…”