The mechanism of floral heliotropism in the snow buttercup, Ranunculus adoneus

We designed field experiments using solar-tracking Ranunculus adoneus flowe rs to determine where photoreception occurred, which organs responded, and how movement was achieved. Flower peduncles bend eastward in the morning an d gradually unbend over the course of the day. Peduncles were found to bend significantly more frequently in the middle region near the floral bracts, 1-3 cm below the flower, than elsewhere on the peduncle. Because the pedun cle tip continued to track the sun even after the flower itself was removed , our experiments concentrated on shielding (or conversely, exposing) vario us portions of peduncles from (or to) sunlight. Photoreception occurred pri marily in the portion of the stem just beneath the floral receptacle. By fo llowing the position of landmarks applied to the stem, we found that 40% mo re growth occurred on the shaded side of bent peduncles, compared to the su nlit side. In contrast, top-shielded peduncles did not solar track well and grew only 25% more on the shaded side than on the sunlit side. This growth differential corresponded to differences in cell length on the two sides o f bent peduncles, with significantly longer epidermal cells occurring on th e shaded side than on the sunlit side.