Radial expansion is a classic response of roots to mechanical impedance that has generally been assumed to aid penetration. We analysed the response of maize nodal roots to impedance to test the hypothesis that radial expansion is not related to the ability of roots to cross a compacted soil layer. Genotypes varied in their ability to cross the compacted layer, and those with a steeper approach to the compacted layer or less radial expansion in the compacted layer were more likely to cross the layer and achieve greater depth. Root radial expansion was due to cortical cell size expansion, while cortical cell file number remained constant. Genotypes and nodal root classes that exhibited radial expansion upon encountering the compacted soil layer also thickened in response to exogenous ethylene in hydroponic culture, i.e. radial expansion in response to ethylene was correlated with the thickening response to impedance in soil. We propose that ethylene insensitive roots, i.e. those that do not thicken and are able to overcome impedance, have a competitive advantage under mechanically impeded conditions as they can maintain their elongation rates. We suggest that prolonged exposure to ethylene could function as a stop signal for axial root growth.

中文翻译：

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玉米根系土壤渗透的基因型变化与乙烯诱导的增厚负相关

径向膨胀是根部对机械阻抗的经典响应，通常认为其有助于穿透。我们分析了玉米节根对阻抗的响应，以检验以下假设：径向膨胀与根穿过压实土壤层的能力无关。基因型穿越致密层的能力各不相同，而对致密层采用更陡峭方法或致密层中径向扩展较小的基因型更可能跨过该层并获得更大的深度。根的径向扩张是由于皮质细胞的大小扩张，而皮质细胞的文件数量保持不变。在水耕栽培中，遇到外源乙烯后，遇到紧实的土壤层时表现出径向膨胀的基因型和节根类型也增加了 乙烯反应的径向膨胀与土壤中对阻抗的增稠反应相关。我们提出，对乙烯不敏感的根，即不增稠且能够克服阻抗的根，在机械障碍条件下具有竞争优势，因为它们可以保持其伸长率。我们建议长时间暴露于乙烯可能充当轴向根生长的停止信号。