When growing roots are placed in a horizontal position gravity induces a positive curvature. It is classically considered to be the consequence of a faster elongation rate by the upper side compared to the lower side. A critical examination indicates that the gravireaction is caused by differential cell extension depending on several processes. Some of the endogenous regulators which may control the growth and gravitropism of elongating roots are briefly presented. The growth inhibitors produced or released from the root cap move preferentially in a basipetal direction and accumulate in the lower side of the elongation zone of horizontally maintained roots. The identity of these compounds is far from clear, but one of these inhibitors could be abscisic acid (ABA). However, indol-3y1 acetic acid (IAA) is also important for root growth and gravitropism. ABA may interact with IAA. Two other aspects of root cell extension have also to be carefully considered. An elongation gradient measured from the tip to the base of the root was found to be important for the growth of both vertical and horizontal gravireactive roots. It was changed significantly during the gravipresentation and can be considered as the origin of the differential elongation. Sephadex beads have been used as both growth markers and as monitors of surface pH changes when they contain some pH indicator. This technique has shown that the distribution of cell extension along the main root axis is related to a pH gradient, the proton efflux being larger for faster growing parts of roots. A lateral movement of calcium is obtained when Ca2+ is applied across the tips of horizontally placed roots with a preferential transport towards the lower side. Endogenous calcium, which may accumulate inside the endoplasmic reticulum of some cap cells, may also act in the gravireception. These observations and several others strongly suggest that calcium may play an essential role in controlling root growth and several steps of the root gravireaction.

中文翻译：

---

重力反应玉米根的差异生长和激素再分配

当生长的根被放置在水平位置时，重力会引起正曲率。传统上认为这是由于上侧比下侧的伸长率更快的结果。严格的检查表明重力反应是由取决于几个过程的不同细胞延伸引起的。简要介绍了一些可能控制伸长根的生长和向地性的内源调节剂。从根冠产生或释放的生长抑制剂优先沿基叶方向移动，并在水平维持的根伸长区的下侧积累。这些化合物的身份尚不清楚，但这些抑制剂之一可能是脱落酸 (ABA)。然而，indol-3y1 乙酸 (IAA) 对根生长和向地性也很重要。ABA 可能与 IAA 相互作用。还必须仔细考虑根单元扩展的另外两个方面。发现从根尖到根部测量的伸长梯度对于垂直和水平重力反应根的生长都很重要。它在重力呈现过程中发生了显着变化，可以认为是差异伸长率的起源。当 Sephadex 珠子含有一些 pH 指示剂时，它们已被用作生长标记物和表面 pH 变化的监测器。该技术表明，细胞沿主根轴的分布与 pH 梯度有关，质子流出量越大，根部生长速度越快。当 Ca2+ 施加到水平放置的根尖并优先向下侧运输时，钙会发生横向移动。内源性钙可能在某些帽细胞的内质网内积累，也可能在重力感受中起作用。这些观察结果和其他一些观察结果强烈表明，钙可能在控制根系生长和根系重力反应的几个步骤中起重要作用。