The relationship between floral pigments and floral transpiration in Leptosiphon bicolor (Polemoniaceae)

Flower color is an important cue for attracting pollinators.  Yet, flower color polymorphisms are maintained in several species.  Pigments associated with flower color, such as anthocyanins, facilitate tolerance to abiotic stress.  The fact that many plant species maintain both white and colored flowers in nature means that there may be some alternative advantage associated with the lack of pigment.  The objective of this research was to explore one possible advantage of white flower color in Leptosiphon bicolor (Polemoniaceae).  Transpiration rates from white flowers may be lower than from colored, and therefore be provide a physiological advantage. Pink L. bicolor flowers would seem to be at a disadvantage due to their dark pigments which absorb more solar radiation, making them warmer, and possibly leading to higher transpiration rates. In a greenhouse experiment, we compared transpiration rates and floral sizes of white and pink flowers in L. bicolor. While white flowers transpired 21.9% more per unit area than pink flowers, this difference was not statistically significant (P = 0.07).  The corolla tube lengths were greater in pink flowers than in the white (P < 0.001). The corolla diameters were also greater in the pink than in the white (P = 0.003). However, in spite of having smaller flowers, plants with white flowers transpired ~50% more water per flower than plants with pink flowers.  Our results do not support the hypothesis that a flower color polymorphism is maintained in L. bicolor due to differences in floral transpiration rates between the color morphs.