Nitrogen nutrition and the accumulation of free and sinapoyl-bound malic acid in Raphanus sativus cotyledons

Seedlings of red radish (Raphanus sativus L. var. sativus) accumulated high amounts of free malic acid and sinapoylmalate, when grown on nitrate as the sole N-source. In the presence of ammonium (NO ₃ ^– : NH ₄ ⁺ , 1:2) both metabolites failed to accumulate, and the levels of arginine, asparagine, glutamine, histidine, and serine were greatly increased. The extractable activity of 1-sinapoylglucose: l-malate sinapoyltransferase, an enzyme which plays a key role in channelling malic acid into the sinapic-acid metabolism of this plant, was positively correlated with the malic-acid level in cotyledons. The possibility is discussed that free malic acid might be the likely candidate for regulating the activity of 1-sinapoylglucose: l-malate sinapoyltransferase.Abbreviation SMT sinapoylglucose: L-malate sinapoyltransferase