A Matter of Contrast: Yellow Flower Colour Constrains Style Length in Crocus species.

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

A Matter of Contrast: Yellow Flower Colour Constrains Style Length in <i>Crocus</i> species

<div><p>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 <i>Crocus</i> species are a noticeable exception, since yellow-flowering <i>Crocus</i> species–being entirely UV-absorbing–exhibit low colour contrast between yellow reproductive organs and yellow tepals. The elongated yellow or orange-yellow style of <i>Crocus</i> 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 <i>Crocus</i> 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 <i>Crocus</i> species, it was tested whether the style length of <i>Crocus</i> flowers is dependent on the corolla colour. The results show that members of the genus <i>Crocus</i> with yellow tepals have evolved independently up to twelve times in the genus <i>Crocus</i> and that yellow-flowering <i>Crocus</i> species possess shorter styles as compared to violet- and white-flowering ones. The manipulation of flower visitors by anther-mimicking elongated styles in <i>Crocu</i>s flowers is discussed.</p></div

Comparison of reflectance properties of some yellow-coloured <i>Crocus</i> species.

<p>Colour photograph, UV-photograph and relative spectral reflectance of tepals are shown for five exemplary yellow-flowering <i>Crocus</i> species. Note that only in <i>C</i>. <i>korolkowii</i> parts of the tepals are distinctly UV-reflecting.</p

Style length dependent on stamens length in relation to perigone colour.

<p>Analysis of 106 <i>Crocus</i> species regarding perigone colour. Both style length and stamens length are relative to the length of the tepals.</p

Mean relative length of style compared to stamens for <i>Crocus</i> species with different perigone colours.

<p>With standard error; n = 106 species of <i>Crocus</i> comprising 48 violet, 22 violet/white, 19 white and 17 yellow coloured species.</p

Flowering phases and UV-reflectance properties of <i>Crocus</i> species.

<p>(A) Flower of the saffron crocus <i>C</i>. <i>sativus</i> in the early flowering phase with a syrphid fly, <i>Episyrphus balteatus</i>, targeting at the stigma. (B) The same flower in the subsequent flowering phase with a syrphid fly feeding on pollen. (C) Western honeybee, <i>Apis mellifera</i>, landing on a <i>Crocus</i> flower and targeting at the stigma. (D–G) Colour photograph and UV-photograph of violet and yellow garden crocuses.</p

Phylogenetic tree of the genus <i>Crocus</i> with superimposed floral characters.

<p>Phylogenetic tree obtained by Bayesian phylogenetic inference of the nuclear rDNA ITS region. The floral characters indicated are flower colour and relative style length as compared to the relative stamens length and branching of the style for 106 <i>Crocus</i> and 3 <i>Romulea</i> species.</p

Reflectance properties of differently coloured <i>Crocus</i> flowers.

<p>Relative spectral reflectance of tepals in the yellow-flowering <i>C</i>. <i>chrysanthus</i> and <i>C</i>. <i>flavus</i> and the violet- or white-flowering <i>C</i>. <i>vernus</i>.</p