Increased solar radiation and soil moisture determine flower colour frequency in a mountain endemic plant population

Flower colour is a fascinating trait that has been of interest to biologists for its utility in understanding variation in natural populations and its role in floral evolution. Here, we investigated whether the co-occurring white and pink flowers of individual plants of the Drakensberg near-endemic taxon, Rhodohypoxis baurii (Baker) Nel. var. confecta Hilliard & Burtt (Hypoxidaceae) are an example of phenotypic plasticity or of flower colour polymorphism and what environmental factors may drive observed changes. We used both field and growth chamber studies to test the relationship between environmental variables and the shift in the proportion of the two flower colours over the flowering season. We found that single flowers do not change colour over time, but some individual plants are potentially responding to changes in environmental conditions by producing pigmented flowers later in the flowering season, which suggests that the trait could be plastic rather than a true polymorphism. The field data showed that soil moisture along with an interaction between ultraviolet (UV) radiation and temperature best explained the change in the number of pigmented flowers over the flowering season but none of our treatments in the growth chambers had a significant effect on the change in the number of pigmented flowers. Given the relationship between anthocyanin production and environmental stress, our field findings suggest that soil moisture plays an important role in facilitating stress tolerance and that R. baurii var. confecta may produce anthocyanins to prevent tissue damage from increased temperature and UV later in the flowering season.