Heliotropism in Trillium grandiflorum provides increased reproductive success

Heliotropism of flowers is thought to provide a reproductive advantage to individuals of many species; yet, there are few clear demonstrations of a fitness advantage provided by heliotropism, most of which are almost completely restricted to plants from the arctic or alpine regions where a heat gain provides an advantage to the plants. In this study, we report heliotropism in the white trillium, an early spring blooming species from the temperate forests of eastern North America. The white trillium shows horizontal seasonal heliotropism with flowers mainly facing south throughout the summer, but it also exhibits daily horizontal and vertical heliotropism with the orientation of the flower tracking the sun during the day. The study of 479 tethered plants indicated that seasonal heliotropism provides a reproductive advantage to the white trillium, with south-facing individuals showing a 12% increase in fertilized ovules compared to north-facing plants. The ecology of this long-lived perennial is, in many aspects, similar to that of alpine and arctic heliotropic species. Indeed, white trilliums are insect-pollinated and they flower when the weather is cold, before the trees leaf out. This suggests that there are potentially more species for which heliotropism is advantageous amongst early spring flowers from the temperate forest. Key words: heliotropism, white trillium, fertilized ovules, fitness, plant reproduction On pense que l'heliotropisme des fleurs procure un avantage reproductif aux individus de nombreuses especes. Pourtant, il existe peu de demonstrations claires de l'avantage de 1'heliotropisme sur le plan de la valeur adaptative, et ces demonstrations sont presque toutes limitees aux plantes des regions arctiques ou alpines, ou un gain de chaleur procure un avantage aux plantes. Dans cette etude, nous decrivons 1'heliotropisme chez le trille blanc, une espece a floraison printaniere precoce des forets temperees de Test de l'Amerique du Nord. Le trille blanc montre un heliotropisme saisonnier horizontal avec des fleurs principalement orientees vers le sud tout au long de l'ete, mais il presente egalement un heliotropisme horizontal et vertical quotidien avec l'orientation de la fleur suivant le soleil pendant la journee. L'etude de 479 plantes attachees a indique que 1'heliotropisme saisonnier procure un avantage reproductif au trille blanc, les individus orientes vers le sud presentant une augmentation de 12 % d'ovules fecondes par rapport aux plantes orientees vers le nord. L'ecologie de cette plante vivace a longue duree de vie est, sous de nombreux aspects, similaire a celle des especes heliotropes alpines et arctiques. En effet, les trilles blancs sont pollinises par les insectes et ils fleurissent lorsque le temps est froid, avant l'apparition du feuillage des arbres. Cela suggere qu'il y a potentiellement plus d'especes pour lesquelles 1'heliotropisme est avantageux parmi les fleurs printanieres precoces de la foret temperee. [Traduit par la Redaction] Mots-cles : heliotropisme, trille blanc, ovules fecondes, valeur adaptative, reproduction vegetale
Introduction The heliotropism of flowers has attracted curiosity and interest amongst evolutionary ecologists for more than 50 years (Hocking and Sharplin 1965; Hocking 1968; Kevan 1972, 1975). Heliotropism is the [...]