Oscillating flower colour changes of Causonis japonica (Thunb.) Raf. (Vitaceae) linked to sexual phase changes

Flower colour change may represent an ‘honest signal’ for pollinators, denoting flowers with good conditions for rewards and pollination. All previously reported flower colour changes are unidirectional, except for an incomplete case in one Fabaceae species. In this study, we discovered a very rare example of complete oscillating flower colour change associated with sexual phase changes in Causonis japonica (Vitaceae). More specifically, flower discs of C. japonica exhibit an orange colour in the initial male phase then soon fade into pink with desorption of the stamens. Several hours later in the daytime of the same or the following day, with stigma maturation and style elongation, the orange flower disc colour is recovered before fading into pink again. Importantly, we found that the colour change is caused by the accumulation and the degradation of carotenoids. Moreover, nectar secretion was roughly correlated with the abovementioned colour changes. This is the first example of an apparent oscillating colour change mediated by carotenoid content alteration in flowers.