The Role of Long-distance Transport in Intracellular pH Regulation inPhaseolus vulgarisGrown with Ammonium or Nitrate as Nitrogen Source, or Nodulated

Charge balance and intracellular pH regulation were studied in Phaseolus vulgaris grown in water culture on $1-0molm-3NH4+$ or $1-0molm-3NO3-$ as N source, or nodulated and in N-free medium. Seedlings and 36-d-old plants were analysed for $K+,Na+,Ca2+,Mg2+,NH4+,Cl-,NO3-,SO42-,$ total P, C, organic N and S and ash alkalinity; xylem sap was analysed for mineral ion content and amino acids, amides and dicarboxylates. Both water uptake and H⁺ or base excreted during water culture were measured. It was shown that in $NH4+$-grown plants, H⁺ excreted was related directly to the uptake and assimilation of $NH4+$; there was no cation uptake associated with net H⁺ exchange. All shoot N was supplied as organic N, mainly glutamine, so that shoot pH perturbation would be limited to uronate production in situ. This could be regulated by both shoot $SO42-$ reduction and the transport of OH⁻-generating carboxylates in the xylem. In plants on $1-0molm-3NO3-$, the great majority of $NO3-$ reduction occurred in the root and most of the OH⁻ produced was converted to -COO⁻, transported up the xylem and stored, so that the leaves in particular had a high ash alkalinity (20-fold that in the roots). Some OH⁻, however, was excreted. Nodulated plants had a low N content, compared with the others, indicating a low rate of N₂-fixation for the earlier period of water culture. At harvest, however, the xylem sap organic N content was comparable to that of $NO3--N$ plants. H⁺ excretion was in excess of that required for N assimilation. It was concluded that in these plants some net H⁺/cation exchange had occurred which may have been a response to low nutrient (i.e. nitrogen) status.