Gravitropic sign reversal—a fundamental feature of the gravitropic perception or response mechanisms in some plant organs

The direction of the gravitational response of some plant organs reverses during development. For instance, the apical hook of an etiolated hypocotyl is formed because the youngest part of the elongation zone is positively gravitropic while the more mature elongating region is negatively gravitropic. Likewise, the youngest nodes of some trailing plants such as Oplimenus hirtellus are negatively gravitropic, but the same nodes later in their developmental progression are positively gravitropic. The sign of the gravitropic response in some organs not only changes developmentally, but can also be controlled by light. The positive gravitropic phase of hypocotyls and nodes is absent in light-grown plants