Abstract
Given that flower size and pigmentation can mediate plant–pollinator interactions, many studies have focused on pollinator-driven selection on these floral traits. However, abiotic factors such as precipitation, temperature, and solar radiation also contribute to geographic variation in floral color, pattern, and size within multiple species. Several studies have described an ecogeographic pattern within species in which high temperature, high ultraviolet (UV) radiation, low precipitation and/or low latitudes are associated with increased floral anthocyanin production, smaller flowers, and/or larger UV-absorbing floral patterns (nectar guides or bullseyes). However, latitude or elevation is often used as a proxy variable to study variation in floral traits associated with a wide range of climatic variables, making the proximate abiotic drivers of variation difficult to identify. In this study, we tested and corroborated several predictions for how the abiotic environment may directly or indirectly shape geographic patterns of floral color, pattern, and size in Clarkia unguiculata (Onagraceae). This study provides the first report of geographic variation in multispectral floral color and pattern in C. unguiculata, while also providing an experimental test of the putative protective role of UV absorption for pollen performance. We quantified geographic variation among greenhouse-raised populations in UV floral pattern size, mean UV petal reflectance, anthocyanin concentration, and petal area in C. unguiculata across its natural range in California and, using 30 year climate normals for each population, we identified climatic and topographic attributes that are correlated with our focal floral traits. In addition, we examined pollen performance under high and low UV light conditions to detect the protective function (if any) of UV floral patterns in this species. Contrary to our expectations, the nectar guide and the proportion of the petal occupied by the UV nectar guide were largest in low solar UV populations. Estimated floral anthocyanin concentration was highest in populations with high solar UV, which does support our predictions. The size of the UV nectar guide did not affect pollen performance in either of the light treatments used in this study. We conclude that, under the conditions examined here, UV-absorbing floral patterns do not serve a direct “pollen protection” function in C. unguiculata. Our results only partially align with expected ecogeographic patterns in these floral traits, highlighting the need for research in a wider range of taxa in order to detect and interpret broad scale patterns of floral color variation.
Highlights
Geographic variation in floral color, pattern, and size is well documented in multiple plant species (Streisfeld and Kohn, 2005; Schemske and Bierzychudek, 2007; Arista et al, 2013; Hopkins and Rausher, 2014; del Valle et al, 2015; Sobral et al, 2015; Tripp et al, 2018)
To contribute toward an understanding of large-scale geographic patterns in these floral features, we quantified UV mean petal reflectance, the proportion of the petal occupied by the UV-absorbing nectar guide, petal area, and estimated floral anthocyanin concentration in greenhouse-raised populations representing eight wild populations of Clarkia unguiculata (Onagraceae) that span a wide latitudinal and climatic gradient (Supplement 1)
This study provides the first report of geographic variation in multi-spectral floral color and pattern in C. unguiculata, while providing an experimental test of the putative protective role of UV absorption for pollen performance
Summary
Geographic variation in floral color, pattern, and size is well documented in multiple plant species (Streisfeld and Kohn, 2005; Schemske and Bierzychudek, 2007; Arista et al, 2013; Hopkins and Rausher, 2014; del Valle et al, 2015; Sobral et al, 2015; Tripp et al, 2018). While it is clear that pollinators, and other biotic factors such as florivores, have shaped floral evolution (see van der Niet and Johnson, 2012 for a review), abiotic factors such as UV (ultraviolet) radiation, temperature and precipitation may drive intraspecific variation in floral color and form (Arista et al, 2013; del Valle et al, 2015; Campbell and Powers, 2015; Koski and Ashman, 2016; Tripp et al, 2018). Anthocyanins (which commonly confer orange, red, blue, or purple coloration to plant tissues) may provide protection against environmental stressors such as extreme (high or low) temperatures, low water availability, high UV light, or other abiotic factors (Lee and Gould, 2002; Lee and Finn, 2007; Landi et al, 2015). Anthocyanins may help to protect developing reproductive tissues from antagonists, while in fully developed flowers, anthocyanins may attract pollinators, and in fruits they may help to attract seed dispersers (Whittall and Strauss, 2006; Landi et al, 2015)
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