Mechanisms of aluminum-tolerance in two species of citrus: secretion of organic acid anions and immobilization of aluminum by phosphorus in roots.

'Sour pummelo' (Citrus grandis) and 'Xuegan' (C. sinensis) seedlings were irrigated for 18 weeks with nutrient solution containing 0 (-Al) and 1.2 mM AlCl₃ · 6H₂O (+Al) x 0, 50 and 200 μM KH₂PO₄. C. sinensis was more tolerant to aluminum (Al) than C. grandis. Phosphorus (P) alleviated the toxic effects of Al on seedlings. Under Al stress, P increased root Al, but decreased shoot Al; C. sinensis accumulated more Al in roots and less Al in shoots than C. grandis. Al decreased root and shoot P. P content was higher in C. sinensis than in C. grandis seedlings. C. sinensis roots secreted more malate and citrate than C. grandis ones in response to Al. Al-induced-secretion of malate and citrate from +Al excised roots was higher than from -Al ones, while Al-preculture did not increase root malate and citrate. Al-induced-secretion of malate and citrate from +Al excised roots decreased with increasing P supply. To conclude, P can alleviate Al-toxicity through increasing immobilization of Al in roots and P level in seedlings rather than through increasing organic acid (OA) anion secretion. The higher Al-tolerance of C. sinensis may involve secretion of OA anions and precipitation of Al by P in roots.