HIGH ALUMINUM RESISTANCE IN BUCKWHEAT - I - AL-INDUCED SPECIFIC SECRETION OF OXALIC-ACID FROM ROOT-TIPS

High Al resistance in buckwheat (Fagopyrum esculentum Moench. cv Jianx i) has been suggested to be associated with both internal and external detoxification mechanisms. In this study the characteristics of the e xternal detoxification mechanism, Al-induced secretion of oxalic acid, were investigated. Eleven days of P depletion failed to induce secret ion of oxalic acid. Exposure to 50 mu M LaCl3 also did not induce the secretion of oxalic acid, suggesting that this secretion is a specific response to Al stress. Secretion of oxalic acid was maintained for 8 h by a 3-h pulse treatment with 150 mu M Al. A nondestructive method w as developed to determine the site of the secretion along the root. Ox alic acid was found to be secreted in the region 0 to 10 mm from the r oot tip. Experiments using excised roots also showed that secretion wa s located on the root tip. Four kinds of anion-channel inhibitors show ed different effects on Al-induced secretion of oxalic acid: 10 mu M a nthracene-9-carboxylic acid and 4,4'-diisothiocyanatostilbene-2,2'-dis ulfonate had no effect, niflumic acid stimulated the secretion 4-fold, and phenylglyoxal inhibited the secretion by 50%. Root elongation in buckwheat was not inhibited by 25 mu M Al or 10 mu M phenylglyoxal alo ne but was inhibited by 40% in the presence of Al and phenylglyoxal, c onfirming that secretion of oxalic acid is associated with Al resistan ce.