Abstract
Most flowers display distinct colour patterns comprising two different areas. The peripheral large-area component of floral colour patterns attracts flower visitors from some distance and the central small-area component guides flower visitors towards landing sites. Whereas the peripheral colour is largely variable among species, the central colour, produced mostly by anthers and pollen or pollen mimicking floral guides, is predominantly yellow and UV-absorbing. This holds also for yellow flowers that regularly display a UV bull’s eye pattern. Here we show that yellow-flowering Crocus species are a noticeable exception, since yellow-flowering Crocus species–being entirely UV-absorbing–exhibit low colour contrast between yellow reproductive organs and yellow tepals. The elongated yellow or orange-yellow style of Crocus flowers is a stamen-mimicking structure promoting cross-pollination by facilitating flower visitors’ contact with the apical stigma before the flower visitors are touching the anthers. Since Crocus species possess either yellow, violet or white tepals, the colour contrast between the stamen-mimicking style and the tepals varies among species. In this study comprising 106 Crocus species, it was tested whether the style length of Crocus flowers is dependent on the corolla colour. The results show that members of the genus Crocus with yellow tepals have evolved independently up to twelve times in the genus Crocus and that yellow-flowering Crocus species possess shorter styles as compared to violet- and white-flowering ones. The manipulation of flower visitors by anther-mimicking elongated styles in Crocus flowers is discussed.
Highlights
Flowers are signalling apparatuses that attract pollinators and repel antagonists by means of their combination of visual, olfactory, gustatory and tactile/mechanical signals [1,2]
The particular example of the saffron crocus shows how a long style might facilitate pollen transfer between flowers of the staminate towards flowers of the pistillate flowering phase by displaying the sole ‘floral guide’ at the beginning of the anthesis of each individual flower, when only the style, but not the stamens are visible in the just opening flower; later the stamens replace the style as an attraction signal and landing site (Fig 1A and 1B)
This study addresses the suitability of the style in Crocus species for guiding pollinating insects towards the stigma for pollen deposition
Summary
Flowers are signalling apparatuses that attract pollinators and repel antagonists by means of their combination of visual, olfactory, gustatory and tactile/mechanical signals [1,2]. The overall flower colour is thought to be a main attractant at long distances [3], whereas floral guides, sometimes referred to as nectar guides or honey guides, guide approaching flower visitors at close range [4,5,6,7,8,9]. Pollen eating flower visitors possess innate search images (sensu Menzel [13]) for pollen-bearing stamens and naïve individuals respond to stamens as well as to stamen-mimicking structures [4,6,14,15,16,17].
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