Localization of Aluminum in Root Tip Tissues of Wheat Varieties Differing in Aluminum Tolerance

The purpose of this study was to investigate the mechanisms of Al tolerance in wheat varieties by the direct observation of Al distribution in root cells using the hematoxylin stain method. Five-day-old seedlings of Al-tolerant varieties, Atlas 66 and Shirosanjyaku, and an Alsensitive variety, Chikushikomugi, were treated with 150 PM Al in 0.4 mM CaCl_2, pH 4.5 for 24 hours under aseptic conditions; afterwards the roots were stained with hematoxylin. Comparison of hematoxylin stain of root tips among the varieties showed that the root cap of Alsensitive seedlings was stained intensively, reflecting high accumulation of Al in this region. The roots developed symptoms of Al toxicity such as increased vacuolation, and swelling and rapture of the cells of the epidermis. On the other hand, in Al-tolerant varieties the central cap cells and the cortical region of the meristem and elongation zone were not stained. The site of the earliest Al accumulation in Al-tolerant varieties seemed to be the edge cells of the root cap. In the elongation zone, Al accumulated mainly in the cell wall and nuclei. Those differences in the staining pattern were probably due to immobilization of Al in the cytosol that could not react with hematoxylin and/or due to the ability of Al-tolerant varieties to better exclude Al from cytoplasm