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

Abstract: ‘Sour pummelo’ (Citrus grandis) and ‘Xuegan’ (C. sinensis) seedlings were irrigated for 18 weeks with nutrient solution containing 0 (−Al) and 1.2mM AlCl3·6H2O (+Al)×0, 50 and 200μM 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. [ABSTRACT FROM AUTHOR]