ABSTRACT It has long been considered that mechanical impedance on root will restrict root elongation and consequently promote radial growth. When seedlings grew in sands, we did observe radial expansion of roots and it, however, arose before elongation restriction. Mechanical impedance of sands can be classified into frontal- and lateral-type based on the interaction site of root. Therefore, we suspected that radial expansion might be mainly stimulated by lateral- rather than frontal-impedance. To verify our speculation, roots were exposed to frontal- and lateral-impedance separately. Small plastic caps were used to provide pure frontal impedance on root tips and cylindrical plastic containers were used to provide pure lateral impedance. Root elongation was remarkably suppressed under the frontal impedance of plastic caps, and more than that in sand-condition. However, the radial expansion of the plastic-cap-fitted roots was far inferior to that of the sand-cultured roots. Microstructural analysis revealed that sand-condition thickened root largely dependents on cortical expansion, whereas plastic cap did it mainly by thickening stele. In cylindrical plastic containers, mechanical impedance came only from the lateral direction and promoted the expansion of cortex like sand-condition. Thus, we proposed that the expansion of cortex and the consequent radial growth of roots were mainly due to lateral impedance when seedlings grew in sands.

中文翻译：

---

横向机械阻抗而不是额叶促进牙根皮质扩张

摘要长期以来，人们一直认为根部的机械阻抗会限制根部的伸长，从而促进径向生长。当幼苗在沙子中生长时，我们确实观察到根的径向扩张，然而，它出现在伸长限制之前。砂土的机械阻抗根据根系相互作用的部位可分为正面型和侧向型。因此，我们怀疑径向扩张可能主要由横向而非正面阻抗刺激。为了验证我们的推测，根分别暴露于正面和横向阻抗。小塑料帽用于在根尖提供纯正面阻抗，圆柱形塑料容器用于提供纯横向阻抗。在塑料帽的正面阻抗下，牙根伸长得到显着抑制，并且在沙地条件下更是如此。然而，塑料帽根的径向扩张远不如沙培根。显微结构分析表明，沙条件下增厚的根主要依赖于皮质扩张，而塑料帽主要通过增厚石碑来实现。在圆柱形塑料容器中，机械阻抗仅来自横向，并促进了皮质的膨胀，如沙状。因此，我们提出，当幼苗在沙土中生长时，皮层的扩张和随之而来的根的径向生长主要是由于横向阻抗。显微结构分析表明，沙条件下增厚的根主要依赖于皮质扩张，而塑料帽主要通过增厚石碑来实现。在圆柱形塑料容器中，机械阻抗仅来自横向，并促进了皮质的膨胀，如沙状。因此，我们提出，当幼苗在沙土中生长时，皮层的扩张和随之而来的根的径向生长主要是由于横向阻抗。显微结构分析表明，沙条件下增厚的根主要依赖于皮质扩张，而塑料帽主要通过增厚石碑来实现。在圆柱形塑料容器中，机械阻抗仅来自横向，并促进了皮质的膨胀，如沙状。因此，我们提出，当幼苗在沙土中生长时，皮层的扩张和随之而来的根的径向生长主要是由于横向阻抗。