Aluminium tolerance is achieved by exudation of citric acid from roots of soybean (Glycine max)

Fourteen soybean (Glycine max [L.] Merr.) cultivars were analysed and found to differ considerably in aluminium (Al) resistance. The cultivars Suzunar i (Al-resistant) and Shishio (Al-sensitive) were selected for further analy sis of physiological mechanisms of Al resistance. The relative root growth of Shishio was 48% compared to 76% for Suzunari in response to 15 mu M Al ( 24 h). Aluminium accumulation and Al-induced callose formation in root apic es were 50 and 25% of that in Suzunari, respectively. Al inhibited both Suz unari and Shishio during the first 6 h of exposure. However, the root growt h inhibition was further increased in Shishio but not in Suzunari, suggesti ng an Al-induced Al-resistant mechanism operating in Suzunari. Organic acid analysis in root exudates of both cultivars revealed that they specificall y exuded citrate in response to Al. However, the titrate exudation rate was significantly higher in Suzunari during the 6 h/24 h Al treatment, which w as 52/330 compared to Shishio's 26/118 (nmol [g root fresh weight](-1) [6 h ](-1)), respectively. This Al-induced citric acid exudation was found to be specific for Al, as several other metals failed to induce citrate exudatio n in both cultivars. Fourteen days of P deficiency did not elicit citrate e xcretion in both cultivars, while application of Al to P-deficient plants r apidly induced citrate exudation in both cultivars, confirming the specific ity of the response of these soybean cultivars to Al. To our knowledge, thi s is the first report demonstrating an Al-exclusion mechanism in soybean cu ltivars, which is conferred by enhanced and specific Al-induced exudation o f citrate.