Amelioration of aluminum toxicity to sorghum seedlings by chelating agents

At the pH levels found in acid soils (4.5 to 5.5), theoretical equilibrium models predict that Al will be complexed on a nearly one to one molar basis by NTA, EGTA. oxalate (OX) and citrate (CIT). Growth chamber experiments were initiated using solutions containing Al (0, 2, or 10 μM), Ca (400 μ.M). and a chelate (0 or 10 μM) growing sorghum [Sorghum bicolor (L.) Moench cv. AT×399 × RT×430] for four days following germination to test the equilibrium models. The pH and concentration of Al in the solutions were measured before and after each experiment. Plant root length and weight, and shoot weights were used as a bioassay for the uncomplexed, toxic Al. Root length showed the greatest response to aluminum and chelate treatments, although root weight and shoot weight gave the same general results. Chelate effectiveness in reducing Al toxicity was NTA > OX = CIT > EGTA. The pH values were altered very little by NTA or EGTA and averaged 5.2 to 5.3; however, the pH was raised 0.2 to 0.9 units by OX and CIT. Thus, some detoxifying effect from the latter two could be a pH effect. No chelate effect was evident at pH values near 6 for CIT, but the chelate was effective in reducing Al toxicity at pH 5.6, indicating the importance of pH in Al toxicity. NTA alone did not affect root length, but the other chelates all decreased root length to a small degree at 0 μM Al indicating that the chelate itself was detrimental to growth. It was concluded that NTA was an effective chelate to detoxify Al and EGTA was not. Also it was found that OX and CIT behave quite differently from NTA and EGTA in that they affect pH and lower solution Al concentration. The method did not confirm the equilibrium models for EGTA, OX, or CIT because of complicating factors such as pH variation and damage to the roots by the chelates. The equilibrium model for NTA, though, was confirmed.