CONTINUOUS SECRETION OF ORGANIC-ACIDS IS RELATED TO ALUMINUM RESISTANCE DURING RELATIVELY LONG-TERM EXPOSURE TO ALUMINUM STRESS

Secretion of organic acid has been suggested to be one of the mechanis ms for Al resistance in short-term experiments. In the present study, relatively long-term response of roots to Al stress was investigated i n terms of organic acid secretion. Eight plant cultivars belonging to 5 species that exhibited differential sensitivity to Al were used. Ten days of intermittent exposure to Al tone day in 0.5 mM CaCl2 containi ng 50 mu M AlCl3 at pH 4.5, alternating with (one day in nutrient solu tion without Al) inhibited root growth by 65% in an Al-sensitive culti var of wheat (Triticum aestivum L. Scout 66) and by 25-50% in two cult ivars of oilseed rape (Brassica napus L. 94008 and H166), two cultivar s of oat (Avena sativa L. Tochiyutaka and Heoats), and an Al-tolerant cultivar of wheat (Atlas 66). However, root growth was hardly affected by the same treatment in buckwheat (Fagopyrum esculentum Moench Jianx i) and radish (Raphanus sativus L. Guangxi). Organic acids were monito red during the first 6 h of each day of Al treatment, and both the kin d and amount of organic acids secreted were found to differ among diff erent species. Roots of buckwheat secreted oxalic acid, those of wheat exuded malic acid, while those of rapeseed, oats, and radish secreted both citric and malic acids. Three different patterns in response to relatively long-term treatment of Al were found in terms of total amou nt of organic acids secreted: (1) the amount secreted was very low dur ing the treatment (wheat cv. Scout 66, oat), (2) the amount gradually decreased with duration of treatment (wheat cv. Atlas 66, oilseed rape ), and (3) the amount maintained at a high level during the whole peri od of Al treatment (buckwheat and radish). Combined with the results o f growth inhibition, it is suggested that the continuous secretion of organic acids at a high level is related to high Al resistance.