Flowers meet Newton: testing the role of gravitational pull in resupination of orchid flowers.

Resupination refers to the developmental orientation changes of flowers through ~180°, leaving them effectively upside-down. It is a widespread trait present in 14 angiosperm families, including the Orchidaceae, where it is a gravitropic phenomenon actively controlled by auxins. Here, we demonstrate that the passive gravitational pull on flower parts can have an additional influence on resupination. We studied a lady's slipper orchid in which some flowers naturally fail to resupinate. We conducted a manipulative experiment removing floral parts and showed that both the probability of complete resupination and the degree of flower vertical movement (from 0° to 180°) are related to the mass of floral organs. During flower development, the tip of the ovary slightly curves actively (14.75°) due to gravitropism. This promotes a lever arm effect so that the gravitational pull acting on flower mass creates a torque that bends the ovary, orienting the flower into a resupinate position that is accessible to pollinators. The role of the mass of floral organs in resupination provides new insights into flower development and its role in pollination mechanisms.