To investigate the Al tolerance mechanism in young carrot, we selected an Al-tolerant carrot by growing Kawatabi soil (acidic non-allophanic andosol) and hydroponic culturing with different Al levels. We used this selected carrot variety for investigating the Al tolerance level of carrots by "the simple method of visual detection of Al tolerance level in plant root by decoloring filter paper stained by hematoxylin, "distribution of Al in the young carrot root and the secretion of organic acid from the carrot root. 1) We selected "Shoyo 5 sun" as the representative variety for Al tolerance and "Kuroda 5 sun" as that for Al-sensitive. 2) Using this decoloring method, the secretion of organic acid was observed not only at the root tip but also at the root base. The degree of decoloration was clearly higher in the Al-tolerant variety than in the Al-sensitive variety. 3) The adsorption of Al on the root was observed at the root base of the Al-sensitive variety, but neither at the root tip or root base of the Al-tolerant variety nor the root tip of Al-sensitive variety. 4) A scanning electron micrograph of the root base of the Al-tolerant variety showed thicker and denser root hair as compared to that of the Al-sensitive variety. 5) Citric acid, as a major component of the organic acids, was detected in the secretion of Al-tolerant carrots roots by HPLC and ESI-MS analysis.
