Citrate Secretion Induced by Aluminum Stress May Not be a Key Mechanism Responsible for Differential Aluminum Tolerance of Some Soybean Genotypes.

Eighteen soybean genotypes differing in aluminum (Al) tolerance were used to investigate genotypic differences in Al-induced citrate exudation and its role in Al tolerance. Aluminum accumulation and localization in soybean roots were examined by analysis of total Al and hematoxylin staining. Soybean genotypes exhibited a wide range of Al tolerance. Based on relative root elongation, several Al-tolerant genotypes from Brazil such as B 1, B 10, and B 15 were more tolerant than the Al-tolerant PT 416937 (PT) and Perry. All soybeans exuded citrate in response to Al stress, and some Al-sensitive genotypes secreted more citrate than tolerant ones, showing no correlation between the Al tolerance and Al- induced citrate exudation. Further study found that both copper (Cu) and cadmium (Cd) stimulated citrate and malate exudation in soybean, indicating that organic acid secretion is not specifically induced by Al. Aluminum concentrations were significantly higher in 2-3 and 3-4 cm of segments than that in 0-1 and 1-2 cm segments under 15 μM AlCl3. Both the root mature zone and apex were heavily stained by hematoxylin after exposure to 10, 15, or 20 μM AlCl3 (24 h), whereas root elongation zone was not stained. After exposure to 50 μM AlCl3 for 20 min, the Al-tolerant PI was less stained by hematoxylin than the Al-sensitive Young, suggesting that Al accumulation in root apices seem to be an immediate response to Al stress, and related to differential Al sensitivity. Present results suggest that citrate secretion induced by Al stress may not be a key mechanism responsible for the differential Al tolerance of some soybean genotypes and other mechanism(s) conferring Al exclusion should exist and operate immediately after exposure to Al stress. [ABSTRACT FROM AUTHOR]