Early wrong-way response occurs in orthogravitropism of maize roots treated with lithium

Millet, B. and Pickard, B. G. 1988. Early wrong-way response occurs in orthogravitropism of maize roots treated with lithium. - Physiol. Plant. 72: 555–559.
Application of lithium ions to tips of roots of Zea mays L. cv. Silver Queen shifts the direction of initial orthogravitropic curvature from downward to upward. The production of this putatively incidental perturbation of orthogravitropic bending kinetics by a pharmacological agent might provide insight into both ortho- and plagiogravitro-pism. Additionally, the protocol of the experiments bears on recent claims that mucilage external to the root cap plays an essential role in gravitropism. External mucilage was removed before roots were stimulated, yet they reached about 50 degrees gravitropic curvature in an hour.     

The gravitropic set-point angle (GSA): the identification of an important developmentally controlled variable governing plant architecture*

The angle at which an organ is maintained by gravit-ropism is characteristic of the organ, its developmental state and the prevailing environmental conditions. We propose that this angle be called the gravitropic set-point angle (GSA), defined as the angle with respect to the gravity vector (with a vertically downward orientation being 0°) at which an organ is maintained as a consequence of gravitropism. Studies of the gravitropic behaviour of organs from trailing plants show that the GSA is subject to developmental regulation. Depending on the developmental age and prevailing environmental conditions, the GSA of an organ can he set at any value between 0° and 180° The previously reported reversal of the sign of the gravitropic response in such organs, whether this is brought about developmentally or induced by light, represents the change from one common extreme (GSA = 180°, conventionally referred to as negative orthogravitropism) to another (GSA = 0°, or positive orthogravitropism). The concept of a variable gravitropic set-point offers a more unified view of all forms of gravitropic behaviour than has been advanced previously, and places a new constraint on models of gravitropism. Current models of gravitropism appear to be unable to explain either the ability of organs to change their orientation with respect to gravity as they develop, or the re-orientation that can be observed when some organs are exposed to new environmental conditions.