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Horizontally-placed segments of Avena sativa L. shoots show a negative geotropic response after a period of 30 min. This response is based on cell elongation on the lower side of the leaf-sheath base (pulvinus). Triticum aestivum L., Hordeum vulgare L. and Secale cereale L. also show geotropic responses that are similar to those in Avena shoots. The pulvinus is a highly specialized organ with radial symmetry and is made up of epidermal, vascular, parenchymatous and collenchymatous tissues. Statoliths, which are confined to parenchyma cells around the vascular bundles, sediment towards the gravitational field within 10-15 min of geotropic stimulation. Collenchymatous cells occur as prominent bundle caps, and in Avena, they occupy about 30% of the volume of the pulvinus. Geotropic stimulation causes a 3- to 5-fold increase in the length of the cells on the side nearest to the center of the gravitational field. Growth can also be initiated in vertically-held pulvini by the application of indole-3-acetic acid, 1-naphthaleneacetic acid or 2.4-dichlorophenoxyacetic acid. 2.3.5.-triiodobenzoic acid interferes with growth response produced by geotropic stimulation as well as with the response caused by auxin application. Gibberellic acid and kinetin have no visible effect on the growth of the pulvinus. Polarization microscopy shows a unique, non-uniform stretching of the elongating collenchymatous cells. Nonelongated collenchymatous cells appear uniformally anisotropic. After geotropic stimulation or auxin application, they appear alternately anisotropic and almost isotropic. Such a pattern of cell elongation is also observed in collenchyma cells of geotropically-stimulated shoots of Rumex acetosa L., a dicotyledon.

Cell elongation in the grass pulvinus in response to geotropic stimulation and auxin application