ABSTRACT
The topics considered in this chapter have given rise to numerous reviews (Torrey, 1976; Audus, 1975, 1983; Pilet, 1977, 1996, 1998a; Jackson and Barlow, 1981; Feldman, 1984; Pilet and Barlow, 1984, 1987; Zeevaart and Creelman, 1988). For a long time after the publication in 1926 of the two papers by Cholodny and Went, auxin control of the growth and gravireaction of roots was accepted as being a reality (see Thimann, 1977; Pilet, 1994). Gradually, other endogenous regulators have been detected in growing roots (i.e., cytokinins, gibberellins, and ethylene). Combining the effects of such regulators with those of indole-3yl-acetic acid (IAA) and some new auxins and IAA conjugates had complicated the action of the implicated endoregulators. The discovery of growth inhibitors formed or released by the root cap gave rise to a new line of thought showing their possible action in the control of differential growth, which determines gravitropism (Pilet, 1977; Wilkins, 1979). The occurrence of abscisic acid (ABA) in growing roots indicated that ABA could be one of those growth inhibitors (Audus, 1983; Chapter 26 by Hose et al. in this volume). Proton secretion may also be an important characteristic of the growing roots and of their gravireacting zone (Mulkey et al., 1983; Pilet et al., 1983). Finally, transport and accumulation of calcium have induced new approaches into the research of root growth and gravitropism (Lee et al., 1984).
