Alleviation of aluminium phytotoxicity on soybean growth by organic anions in nutrient solutions

Abstract

The effect of aluminium (Al) phytotoxicity on the growth of soybean (Glycine max (L.) Merr.) cv. ‘Forrest’ in nutrient solutions (pH 4.25) and its alleviation by organic anions has been investigated. Both 100 and 500 µM Al caused significant reduction in root and shoot lengths and root dry weights over 15 days. Citrate, oxalate, tartrate, malate, malonate and lactate at 1000 µM and humic acid at 1 g L^-1 were used to alleviate phytotoxicity caused by the presence of 500 µM Al. The effectiveness of the organic anions in alleviating Al phytotoxicity, measured by the change in root length, was in the order oxalate > citrate ≈ tartrate ≈ malate > malonate >> lactate. Humic acid at 1 g L^-1 proved almost as effective as 1000 µM citrate. When the remaining fraction of phytotoxic aluminium was measured by the aluminon method and the results combined with calculated values obtained from stability constant data, the order of effectiveness of complexation was citrate > oxalate > tartrate ≈ malate ≈ malonate >> lactate. It was concluded that benefits other than from simple complexation of phytotoxic aluminium were provided by the organic anions. Three of the anions were also used at different concentrations; these were citrate and oxalate (at 250, 500 and 1000 µM) and humic acid (at 0.01, 0.1 and 1.0 g L^-1 ). For citrate and oxalate, it was found that the concentration of the organic anion must be at least that of the Al to reduce phytotoxicity significantly, and twice as much oxalate as citrate was required to achieve the same reduction in phytotoxicity.