Charge balance and intracellular pH regulation were studied in Phaseolus vulgaris grown in water culture on 1-0 mol m-3 NH4+ or 1-0 mol m-3 NO3- 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 1-0 mol m-3 NO3-, 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.
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Allen et al. (1988) studied this question.