BLUE-LIGHT CONTROLS SOLAR TRACKING BY FLOWERS OF AN ALPINE PLANT

In at least 18 plant families, leaves or flowers can maintain a specif ic orientation with respect to diurnal movements of the sun. Previous work on heliotropic leaves has demonstrated that blue light (400-500 n m) provides the cue for their tracking response. Floral heliotropism o ccurs in several families of arctic and alpine plants, but the spectra l sensitivity of the response has not been studied previously. Moreove r, no studies on the spectral sensitivity of any heliotropism have bee n conducted on wild plants growing in their natural habitat. Working u nder field conditions, we used coloured acrylic filters to determine w hether heliotropism by flowers of the snow buttercup (Ranunculus adone us) is responsive to broad-band blue or red light. Flowers were able t o orient towards the sun under boxes made entirely of blue-transmittin g filters and in red-transmitting boxes having a single blue side that faced the sun. In these treatments, solar tracking ability was not si gnificantly different from that observed in adjacent control flowers. In contrast, the precision of solar orientation was significantly redu ced in red-transmitting boxes and red boxes with a single blue side or iented away from the sun. In the early morning, flowers covered by red -transmitting boxes failed to orient in the direction of sunrise, sugg esting that this floral response, unlike that seen in some heliotropic leaves, lacks a residual 'memory' for previous solar movements.