Iridescence and hydrophobicity have no clear delineation that explains flower petal micro-surface

Abstract

Plant organs including flowers and leaves typically have a variety of different micro-structures present on the epidermal surface. These structures can produce measurable optical effects with viewing angle including shifts in peak reflectance and intensity; however, these different structures can also modulate hydrophobic properties of the surfaces. For some species optical effects have been proposed to act as signals to enhance pollination interactions, whilst the ability to efficiently shed water provides physiological advantages to plants in terms of gas exchange and reducing infections. Currently, little is known about epidermal surface structure of flowering plants in the Southern Hemisphere, and how micro-surface may be related with either hydrophobicity or visual signalling. We measured four Australian native species and two naturalised species using a combination of techniques including SEM imaging, spectral sampling with a goniometer and contact angle measurements. Spectral data were evaluated in relation to published psychophysics results for important pollinators and reveal that potential visual changes, where present, were unlikely to be perceived by relevant pollinators. Nevertheless, hydrophobicity also did not simply explain petal surfaces as similar structures could in some cases result in very different levels of water repellency. The variety and complexity in micro-surfaces (also referred to as micro-sculpture) present on the epidermal surface of different plant organs such as leaves and flowers has long been recognised by plant scientists for taxonomic identification 1. From a sample of about 5,000 plant species including Angiosperms and Gymnosperms, Bartholott identified four main characteristics defining a plant micro-surface: cell morphology, relief of the cell wall, waxes and epicuticular secretions, and multi cellular structures including trichomas (hairs) and glands 1,2. Much effort has been devoted to sample and characterise these microstructures using different microscopy techniques including scanning electron microscopy (SEM), and transmission electron microscopy (TEM) for taxonomic purposes 3 ; however, the underlying cause of such a morphological diversity still remains unclear. One of the first explanations for the origin and diversity of epidermal micro-surfaces is water repellency 4. Excess of liquid due to rainfall or dew can be detrimental to the plant as it reduces photosynthetic activity, promotes the development and spread of fungal infections, facilitates pollutant deposition, and accelerates foliage nutrient leaching 4-6. Considering the negative effects of excess water on plant surface, features that enhance water repellency would be assumed to be favoured over those that do not 4 , and it is expected that different adaptations

Conical flower cells reduce surface gloss and improve colour signal integrity for free-flying bumblebees

“…Interestingly, flat cells on flower petals are less frequently observed in nature, representing only about 10% of tested flowering plant species (Kraaij & van der Kooi 2019), strongly suggesting a selective advantage for having more conical shaped cells. Classic explanations for the evolution of conical cells have included improving grip for insects handling flowers (Whitney et al 2009a(Whitney et al , 2009bAlcorn et al 2012), modulating temperature through optical effects (Comba et al 2000;Whitney et al 2011a;Vignolini et al 2015), modulating wettability (Whitney et al 2011c), hydrophobicity (Garcia et al 2020), increasing of the surface for scent emission (Whitney et al 2011 a;Moyroud & Glover 2016), enabling self-cleaning (Whitney et al 2011a;Moyroud & Glover 2016) and/or visual signalling that may enhance a capacity for discrimination or detection (Whitney et al 2016). Visual signalling seems a very plausible driver of an evolutionary process as it is well established that flowering plants have frequently evolved pigment-based colour signals that are optimally suited to either bee (Chittka and Menzel 1992;Dyer et al 2012), bird (Shrestha et al 2013) or fly vision (Shrestha et al 2016.…”