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 1–0 mol m⁻³$NH4+$ or l-0 mol m ⁻³$NO3-$ as sole N source. Seedling and 70-d-old plant parts were analysed for $K+,Na+,Ca2+,Mg2+,Cl-,NO3-,SO42-,$ 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 $NH4+$ -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 $SO42-$ 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 $NO3-$ reduction occurred in the root, and 70% of all OH^- produced by root $NO3-$ and $SO42-$ reduction was excreted directly as base. Very little —COO⁻ from root $NO3-$ 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 $NO3-$ and $SO42-$ 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.