Elevated root zone dissolved inorganic carbon can ameliorate aluminium toxicity in tomato seedlings

•  The potential amelioration of Al (Al) toxicity by elevated dissolved inorganic carbon (DIC = CO₂  + HCO₃ ^- ) in the root medium was investigated in both NH₄ ⁺ - and NO₃ ^- -fed Lycopersicon esculentum (tomato) plants. •  Hydroponically grown L. esculentum seedlings were intermittently supplied with 50 µM AlCl₃ and the nutrient solutions aerated with either 360 or 5000 ppm CO₂ . Relative growth rate (RGR), nitrogen uptake, root respiration and root incorporation of DI¹⁴ C and subsequent partitioning were measured. •  Al reduced the RGR of plants grown with 360 ppm root-zone CO₂ . At elevated root-zone CO₂ , Al had no significant effect on the RGR of NO₃ ^- -fed plants whereas the RGR of the NH₄ ⁺ -fed plants was increased by 21%. Al decreased the respiratory quotient (Rq) by 15% at 360 ppm CO₂ , but had no influence at 2000 ppm CO₂ . Exudation of organic ¹⁴ C, especially of [¹⁴ C]-organic acids derived from root incorporation of DI¹⁴ C, was increased by Al. •  It is concluded that elevated DIC partially ameliorated Al toxicity by providing anaplerotic carbon for organic acid synthesis.