| Aluminum (Al) toxicity is one of the major factors limiting crop poduction in acid soils. In recent years, Al-induced secretion of organic acids and the role of cell wall have received strong support in Al resistance mechanisms. In the present study, the cell wall pectin content, CEC of root apices and Al-induced secretion of organic acid anions from roots of 10 triticale cultivars differing in Al resistance were examined. Besides, the effects of Al on root elongation, plasma membrane lipid peroxidation, and changes of cell-wall compostition in two triticale cultivars with the largest difference in Al resistance were investigated to elucidate the primary target sites of Al toxicity. In order to explore the role of cell wall and its interaction with Al-induced organic acids secretion, root cell walls were extracted from Al-resistant wheat cultivar Atlas 66 and from different root segments (0-1, 1-2, 2-3 cm) of a pair of near-isogenic wheat lines, ET8 (Al-resistant) and ES8 (Al-sensitive), to investigate their kinetics of Al adsorption and desorption, and the role of malate in protecting cell walls from binding Al. The results were summarized as followings:1. The mechanism of aluminum resistance in triticaleTriticale (X Triticosecale Wittmack) shows a high degree of aluminum resistance, but variation in Al resistance between cultivars does exist. There was no significant correlation among root apical pectin content, CEC and Al resistance, indicating the electrification of cell wall was not the primary factor determining Al resistance in triticale. Triticale roots secreted both malate and citrate in response to Al stress. The amount of organic acid anions secreted was correlated positively to the relative root elongation and negatively to the Al content in root apices under Al stress. The comparative studies with an Al-resistant cultivar (ZC 237) and an Al-sensitive cultivar (OH 1621) showed that a marked lag phase (3 h) in the secretion of organic acid...
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