Boron-aluminum interactions affect organic acid metabolism more in leaves than in roots of Citrus grandis seedlings

Sour pummelo (Citrus grandis) seedlings were irrigated with nutrient solution containing four boron concentrations (i.e., 2.5, 10, 25 and 50 microM H3BO3) and two aluminum concentrations [i.e., 0 (-Al) and 1.2 mM AlCl3 * 6 H2O (+Al)]. B did not affect, but Al increased the Al content in the roots. The Al and citrate contents in the -Al leaves either did not change or slightly increased with increasing B concentration. On the other hand, the Al and citrate contents in the +Al leaves rapidly decreased as B concentration increased from 2.5 to 50 microM, then decreased at the highest B concentration. The Al and citrate contents were higher in the +Al than in the -Al leaves, except for at 25 microM B when they were similar. The leaf malate content did not change in response to B or Al, except for an increase in the +Al leaves and a decrease in the -Al leaves at 2.5 microM B. Similarly, the root malate and citrate contents did not change in response to B with or without Al, except for a decrease in the malate and citrate contents in the +Al roots at 50 microM B and an increase in the citrate content in the -Al roots at 50 microM B. The activities of acid-metabolizing enzymes were less affected by B-Al interactions in the roots than in the leaves.