Are nectar guide colour changes a reliable signal to pollinators that enhances reproductive success?

Background: Ageing and post-pollination changes in floral colour occur widely in flowering plants, but it remains an open question as to whether or not colour changes in nectar guides are associated with the quantity of floral rewards that ultimately influence pollinator visitations and reproductive success. Aims: To examine whether nectar guide changes should be considered as a reliable signal to pollinators and to assess the effects of nectar guide changes on reproductive success. Methods: We studied the process and adaptive value of colour changes in the nectar guides of Arnebia szechenyi whose flowers typically display conspicuous nectar guides at the onset of anthesis, after which they begin to fade, and disappear completely on the second day. Results: Changes in nectar guide colour in A. szechenyi were intrinsic and age-dependent, although pollination somewhat accelerated the change. By the time that the nectar guides disappeared completely, floral rewards were reduced almost to zero. Artificial removal of nectar guides decreased both fruit set and pollen export. Flowers without nectar guides do not appear to increase the overall attractiveness of the plants. Conclusions: Nectar guides and their changes represent reliable signals to pollinators and enhance both male and female reproductive success.