Flower color variation in Digitalis purpurea: Pollination and soil influences across native and introduced...

Flower color, a key trait influencing plant-pollinator interactions, may be influenced by abiotic factors such as soil. We investigated association between pollinators, soil characteristics, and flower color variations in Digitalis purpurea across native populations in Sweden and introduced populations in Bolivia.

We measured floral traits, reflectance of petals and nectar-guide spots, plant size, pollinator visitation, fruit set, seed production, germination, and soil characteristics.

Individuals were categorized into violet, pink, and white morphs, which were confirmed by spectral measurements and bee vision modelling. Reflectance of inner nectar-guide spots overlapped across morphs, potentially limiting pollinator discrimination. Bumblebees were the main pollinators in all populations. Although visitation varied among morphs, pollinator visits to different color morphs were population specific. In Bolivia, violet flowers were predominant (70-87%), with pink (13-17%) and white (0-13%) at lower frequencies. In Sweden, morph frequencies were more even (violet 20-43%, pink 38-69%, white 11-30%). Morph frequency was not associated with soil composition, despite differences between regions. Reproductive fitness varied across populations but not consistently among morphs: The largest Swedish population had the highest fruit set but the lowest seed set, while germination was lower in Bolivia. Phosphorus was lower in soil beneath violet individuals; other soil variables, plant size, and floral traits did not differ among color morphs.

Floral color variation in D. purpurea was not significantly related to pollinator visitation or soil conditions at the spatial scale examined, suggesting maintenance by local environmental conditions, neutrality, or historical and demographic processes rather than selection.