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 DI14C 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 14C, especially of [14C]-organic acids derived from root incorporation of DI14C, was increased by Al. • It is concluded that elevated DIC partially ameliorated Al toxicity by providing anaplerotic carbon for organic acid synthesis.