Rhizotoxicity of aluminate and polycationic aluminium at high pH

Although monomeric Al species are often toxic in acidic soils, the effects of the aluminate ion (Al(OH) ₄ ^? ) on roots grown in alkaline media are still unclear. Dilute, alkaline (pH 9.5) nutrient solutions were used to investigate the effects of Al(OH) ₄ ^? on root growth of mungbean (Vigna radiata L.). Root growth was reduced by 13% after 3 d growth in solutions with an Al(OH) ₄ ^? activity of 16 µM and no detectable polycationic Al (Al₁₃). This decrease in root growth was associated with the formation of lesions on the root tips (due to the rupturing of the epidermal and outer cortical cells) and a slight limitation to root hair growth (particularly on the lateral roots). When roots displaying these symptoms were transferred to fresh Al(OH) ₄ ^? solutions for a further 12 h, no root tip lesions were observed and root hair growth on the lateral roots improved. The symptoms were similar to those induced by Al₁₃ at concentrations as low as 0.50 µM Al which are below the detection limit of the ferron method. Thus, Al(OH) ₄ ^? is considered to be non-toxic, with the observed reduction in root growth in solutions containing Al(OH) ₄ ^? due to the gradual formation of toxic Al₁₃ in the bulk nutrient solution resulting from the acidification of the alkaline nutrient solution by the plant roots.