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Intracellular pH Regulation in Ricinus communis Grown with Ammonium or Nitrate as N Source: The Role of Long Distance Transport

SUSAN ALLEN and J. A. RAVEN
Journal of Experimental Botany
Vol. 38, No. 189 (April 1987), pp. 580-596
Published by: Oxford University Press
Stable URL: http://www.jstor.org/stable/23691729
Page Count: 17
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Intracellular pH Regulation in Ricinus communis Grown with Ammonium or Nitrate as N Source: The Role of Long Distance Transport
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Abstract

Charge balance and intracellular pH regulation were studied in Ricinus communis grown in water culture with 1.0 mol m-3 $\mathrm{N}{\mathrm{H}}_{4}^{+}$ or 1.0 mol m-3 $\mathrm{N}{\mathrm{O}}_{3}^{-}$ as sole N source. Seedling and 70-d-old plant parts were analysed for K+, Na+, Ca2+, Mg2+, Cl-, $\mathrm{N}{\mathrm{O}}_{3}^{-}$, $\mathrm{S}{\mathrm{O}}_{4}^{2-}$, 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 $\mathrm{N}{\mathrm{H}}_{4}^{+}$-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 $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ 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 $\mathrm{N}{\mathrm{O}}_{3}^{-}$-N plants, 60% of total $\mathrm{N}{\mathrm{O}}_{3}^{-}$ reduction occurred in the root, and 70% of all OH- produced by root $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ reduction was excreted directly as base. Very little—COO- from root $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ 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 $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ 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.

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