Lateral mechanical impedance rather than frontal promotes cortical expansion of roots

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.

长期以来，学界普遍认为根系受到的机械阻抗（mechanical impedance）会抑制根伸长（root elongation），进而促进径向生长（radial growth）。当幼苗在砂基环境中生长时，我们确实观察到了根系的径向扩张，但该现象早在伸长受抑制之前就已出现。根据与根系的相互作用位点，砂土的机械阻抗可分为正向（frontal）型与侧向（lateral）型两类。据此我们推测，根系的径向扩张主要受侧向而非正向机械阻抗的刺激。为验证这一推测，我们将根系分别暴露于正向与侧向机械阻抗环境中。实验采用小型塑料帽为根尖提供纯正向机械阻抗，使用圆柱形塑料容器提供纯侧向机械阻抗。结果显示，塑料帽施加的正向机械阻抗显著抑制了根系伸长，且抑制效果强于砂培环境。然而，佩戴塑料帽的根系其径向扩张程度远低于砂培组幼苗。显微结构分析表明，砂培环境下根系增粗主要依赖皮层（cortex）扩张，而塑料帽处理组则主要通过中柱（stele）增厚实现增粗。在圆柱形塑料容器中，机械阻抗仅来自侧向，且如同砂培环境一般促进了皮层扩张。综上，我们提出：当幼苗在砂基环境中生长时，根系皮层的扩张及其后续的径向生长主要由侧向机械阻抗介导。