Intracellular pH Regulation inRicinus communisGrown with Ammonium or Nitrate as N Source: The Role of Long Distance Transport

Abstract

Allen, S. and Raven, J. A. 1987. Intracellular pH regulation in Ricinus communis grown with ammonium or nitrate as N source: the role of long distance transport.—J. exp. Bot. 38: 580-596. Charge balance and intracellular pH regulation were studied in Ricinus communis grown in water culture with 10 mol m-3 NH4 or 10 mol m~3 NO J as sole N source. Seedling and 70-d-old plant parts were analysed for K+, Na+, Ca2+, Mg2+, CP, NO J, SO J , total P, C, organic N and S, and ash alkalinity; xylem and phloem saps were analysed for mineral ion content, and amino acids, amides, and dicarboxylates. Excreted H + and base were also measured. It was shown that in NH? -N plants, H + produced and excreted directly by the roots accounted for all net —COOH produced in the plant, but not for cation uptake by net H+ exchange. Intracellular pH perturbation in the shoot was regulated partly by SOJ ~ reduction and partly by the transport of OH ~ -generating dicarboxylates in the xylem. Phloem sap had the capacity to transport organic N and carboxylates excess to shoot requirements back to the root. In NO3-N plants, 60% of total NO J reduction occurred in the root, and 70% of all OH produced by root NOj and reduction was excreted directly as base. Very little —COO from root NOJ reduction was stored in the root: most was transported to the shoot as xylem dicarboxylate and stored in the shoot. Of the OH produced from shoot NO J and SO4 reduction, 40% was stored as shoot carboxylate: the phloem sap had the capacity to transport the rest back to the roots where it was excreted as base.