EFFECT OF ENVIRONMENTAL DENSITY AND BUOYANCY ON GROWTH AND GRAVITROPIC RESPONSES IN MAIZE ROOTS

Jessy L Robbins, Timothy J. Mulkey

Abstract


The ability of roots to perceive and respond to gravity
has been investigated by botanists for many years (Moore & Evans, 1986). Charles and Francis Darwin (1881) were the first scientists to extensively examine the gravitropic response of primary roots. They observed that the removal of the root cap eliminated the gravitropic response of primary roots. This observation was confirmed by many researchers since the Darwins. The widely accepted theory for the perception of gravity by plant roots is based on the sedimentation of statoliths. This theory currently states that sedimentation of amyloplasts in columella cells of the root cap, which is accompanied by calcium redistribution on the lower side of the root cap, initiates the gravitropic response. However, there are many graviresponding plants which do not have statoliths. In these plants, the plasma membrane or other cell organelles may act as statoliths (Wayne et al, 1990; Staves et al, 1997 a, b, c; Staves, 1997). Edwart (1903) observed the polarity of the rotational cytoplasmic streaming in Characean cells during the gravitropic response. The influence of gravity on cytoplasmic streaming was confirmed by Hayashi (1957). The hydrostatic gravisensing model has been proposed by Wayne et al. This hypothesis suggests that plants sense the gravitational vector via a pressure differential exerted on the cell membrane. This pressure differential results from the action of gravity on the entire contents of the cell instead of specific organelles within the cell. Thus, the entire cell instead of organelles within a cell would function as statoliths. This study investigates the effects of solutions of various densities which alters the environmental buoyancy of cells on growth and graviresponse of primary roots of maize.

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