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，学界普遍认为根系所受的机械阻抗（mechanical impedance）会抑制根伸长，进而促进根系径向生长。我们在砂培实验中确实观察到了根系的径向扩张，但这一现象的出现早于伸长抑制的发生。根据与根系的相互作用位点，砂培环境中的机械阻抗可分为轴向（frontal）与侧向（lateral）两种类型。因此我们推测，根系的径向扩张主要受侧向而非轴向机械阻抗的诱导。为验证上述推测，我们分别对根系施加轴向与侧向机械阻抗：采用小型塑料帽对根尖施加纯轴向机械阻抗，同时使用圆柱形塑料容器提供纯侧向机械阻抗。实验结果显示，施加塑料帽轴向阻抗后，根系伸长受到显著抑制，且抑制程度高于砂培条件下的伸长抑制水平；但佩戴塑料帽的根系径向扩张程度远低于砂培根系。显微结构分析表明，砂培条件下根系的增粗主要依赖皮层扩张，而塑料帽处理组的根系增粗则主要源于中柱（stele）的增厚。在圆柱形塑料容器中，机械阻抗仅来自侧向，且与砂培条件一样，能够促进皮层扩张。综上，我们提出：砂培条件下，根系皮层扩张及由此产生的径向生长主要由侧向机械阻抗介导。