| Abstract: | Allen, S. and Raven, J. A. 1987. Intracellular pH regulationin 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 studiedin Ricinus communis grown in water culture with 1–0 molm–3  or l-0 mol m –3 as sole N source. Seedling and 70-d-old plant parts were analysed for  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+ andbase were also measured. It was shown that in -N plants, H + produced and excreted directly bythe roots accounted for all net —COOH produced in theplant, but not for cation uptake by net H+ exchange. IntracellularpH perturbation in the shoot was regulated partly by  reduction and partly by the transportof OH- -generating dicarboxylates in the xylem. Phloem sap hadthe capacity to transport organic N and carboxylates excessto shoot requirements back to the root. In -N plants, 60% of total reduction occurred in the root, and 70% of all OH- produced by root and reduction was excreted directly as base. Very little —COO–from root reduction was stored in the root: most was transported to the shoot as xylem dicarboxylateand stored in the shoot. Of the OH- produced from shoot and reduction, 40% was stored as shoot carboxylate: the phloem saphad the capacity to transport the rest back to the roots whereit was excreted as base. Key words: Intracellular pH, ammonium nutrition, nitrate nutrition, xylem dicarboxylates, phloem dicarboxylates, charge balance |
