Fixed non-random orientation to the Sun (conversotropism) in two window-flowered greenhood orchids, Diplodium spp. (Orchidaceae: Pterostylidinae); implications for other window flowers and pollinator behaviour

Abstract

Many plant species orient their flowers to the Sun. Such orientations may be dynamic (heliotropism or solar tracking) or static, adopting fixed non-random azimuths to points on the daily arc of the Sun (conversotropism as defined herein). Two cauline greenhood orchid species, Diplodium reflexum and D. ampliatum (Pterostylidinae), are shown to be negatively conversotropic and non-heliotropic. Cauline greenhoods are trap and window flowers having large translucent light panels that glow when backlit flowers are viewed from the front. Conversotropism in greenhoods is thought to maximize light transmission through the window panels to manipulate the behaviour of fungus gnat pollinators. Conversotropism is so far documented in only seven species in five plant families, but is likely to be more widespread. The general characteristics of conversotropism are summarized and a new two stage model of window function in greenhoods is proposed. Window flowers in the Lady Slipper orchids (Cypripedium) and in Arums of the genus Arisaema are also predicted to be conversotropic. The evolution of conversotropism in cauline greenhoods highlights the importance of floral windows in these orchids. The highly unusual trap flowers may use light both as a pollinator attractant and repellent at different stages of the pollination process. Greenhoods appear to exploit the instinct of flies to escape confinement and the behaviours enabling it. This probably also applies to other trap flowers and adds insect escape behaviours to the many other drives, instincts, and behaviours exploited by plants for pollination.