Lignin deposition induced by aluminum in wheat (Triticum aestivum) roots

We investigated the relation between the toxic effect of aluminum (Al) on root growth and the lignin deposition in wheat (Triticum aestivum L. cvs Atlas 66 and Scout 66). In the Al-tolerant cultivar Atlas 66, control treatment without AlCl₃ at pH 4.75, cell length increased dramatically in the portion of the root that was 0.6 to 3.2 mm from the root cap junction (approximately 1.0 to 3.6 mm from the root tip). However, treatment with 20 μM AlCl₃ for 24 and 48 h completely inhibited root elongation and markedly decreased the length and increased the diameter of the cells in the same portion of the root. Moreover, marked deposition of lignin was observed in the cells that corresponded to the portion 1.5 to 4.5 mm from the root tip in Atlas 66 roots treated with 20 μM AlCl₃, while no deposition of lignin was detected in control roots. Treatment with 5 μM AlCl₃ slightly inhibited root growth and there was no deposition of lignin in the root. On the other hand, in roots of the Al-sensitive cultivar Scout 66, treatment with 5 μM AlCl₃ completely inhibited root growth and markedly induced deposition of lignin. These results suggest that lignification in the elongating region coincided with the extent of inhibition of root growth by Al in two wheat cultivars that differed in their sensitivity to Al.