A Technique for Measuring Petal Gloss, with Examples from the Namaqualand Flora

Abstract

The degree of floral gloss varies between species. However, little is known about this distinctive floral trait, even though it could be a key feature of floral biotic and abiotic interactions. One reason for the absence of knowledge is the lack of a simple, repeatable method of gloss measurement that can be used in the field to study floral gloss. A protocol is described for measuring gloss in petal samples collected in the field, using a glossmeter. Repeatability of the technique is assessed. We demonstrate a simple yet highly accurate and repeatable method that can easily be implemented in the field. We also highlight the huge variety of glossiness found within flowers and between species in a sample of spring-blooming flowers collected in Namaqualand, South Africa. We discuss the potential uses of this method and its applications for furthering studies in plant-pollinator interactions. We also discuss the potential functions of gloss in flowers.

Iridescence as Camouflage

“…If, on the other hand, survival from the bird experiment is due to aposematism or neophobia, we predicted that the iridescent prey should be easy to detect in the human experiment. Finally, as the surface of these iridescent beetles also produces specular reflection of white light (hereafter, gloss, e.g., [19]), we also predicted an interactive effect of background gloss: iridescent beetles on glossy leaves should have a lower signal-to-noise ratio and thus be less detectable [20].…”

“…The glossmeter measures gloss by recording the light reflected at 20, 60 and 85 away from the perpendicular to a surface. However, previous literature has shown that the recommended angle to use for small surfaces such as petals is 60 as it only requires a 4.7 3 2 mm aperture, and this is the measurement we use throughout [19]. The glossmeter was calibrated using the black polished glass standard that was supplied with the meter.…”

The physics of pollinator attraction

“…However, gloss is only expected to be effective as an attractant if the leaves are not also glossy, and, in the case of Ranunculus , not moving. Glossiness is hard to assess in a reproducible manner, and therefore hard to compare between leaves and petals, although recent attempts have been made (Whitney et al ., ; Papiorek et al ., ). There are inherent limitations to measuring gloss, such as the multidimensional nature of gloss perception and the fact that glossiness varies hugely with ambient light conditions (Chadwick & Kentridge, ).…”

“…Having a smooth, lustrous surface is an efficient way of being conspicuous and gloss is another optical effect, produced by physical means, which has been described in flowers of multiple species (Fig. 1c) (Parkin, 1928;Gaisterer et al, 1999;Vignolini et al, 2012a,c;Whitney et al, 2012;Papiorek et al, 2014). Petal gloss can be defined as the specular reflection of light from the surface of the petal and this effect is maximized when the petal surface is flat (Gaisterer et al, 1999;Whitney et al, 2012).…”

“…1c) (Parkin, 1928;Gaisterer et al, 1999;Vignolini et al, 2012a,c;Whitney et al, 2012;Papiorek et al, 2014). Petal gloss can be defined as the specular reflection of light from the surface of the petal and this effect is maximized when the petal surface is flat (Gaisterer et al, 1999;Whitney et al, 2012). However, most flowers do not have flat cells in the visually active part of their petal epidermis and are thus not expected to be very glossy (Kay et al, 1981;Papiorek et al, 2014).…”