Organic acid metabolism in Citrus grandis leaves and roots is differently affected by nitric oxide and aluminum interactions

‘Sour pummelo’ (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 0 (−Al) and 1.2 mM (+Al) AlCl3·6H2O × 0, 10 and 500 μM sodium nitroprusside (SNP), a nitric oxide (NO) donor. Leaf malate content did not significantly change in response to SNP with or without aluminum (Al) except for an increase under 1.2 mM Al + 10 μM SNP, while leaf citrate content decreased with increasing SNP supply. Root malate content kept unchanging with or without Al except for an increase under 500 μM SNP in the absence of Al, while SNP-treated roots had a higher or similar citrate content. Al decreased or did not affect malate content in roots and leaves, and citrate content in roots, but increased leaf citrate content. Al-treated roots and leaves displayed lower or similar activities of acid-metabolizing enzymes [phosphoenolpyruvate carboxylase (PEPC), NAD-malate dehydrogenase (NAD-MDH), NADP-malic enzyme (NADP-ME), citrate synthase (CS), aconitase (ACO), NADP-isocitrate dehydrogenase (NADP-IDH), phosphoenolpyruvate phosphatase (PEPP) and pyruvate kinase (PK)] except that they had higher or similar activities of NADP-ME and PK and Al-treated leaves had a higher or similar activity of PEPP. In conclusion, the OA metabolism in leaves and roots is differently affected by NO and Al interactions.

► Al decreased or did not affect leaf malate content, but increased citrate content.
► Al lowered or did not affect root malate and citrate levels.
► Acid-related enzyme activities in roots and leaves were affected by SNP and/or Al.
► Leaf and root OA metabolism was differently affected by SNP and Al interactions.