Many data support the view that, when NH4+ [or N2, NH3, or CO(NH2)2] is the N source for plant cell growth, the excess H+ generated in the synthesis of core metabolites is excreted to the bathing medium (biophysical pH-stat). This paper explores the possibility that a ‘biochemical’ disposal of these excess H+ could occur, thus allowing net NHJ assimilation to take place in the shoot of land plants.
A‘biochemical’ H+-neutralizing pH-stat requires that a non-toxic resource be taken into the plant in a form in which metabolism can convert into a non-toxic product with H+ consumption or OH− production. This possibility was explored for reductive metabolism of B(OH)3, Si (OH)4, H2PO4−, H2AsO4−, O2, SO42, SeO42− and HVO42; and for oxidative metabolism of Cr, Br−, I−, Fe2+ and Mn2+. For B(OH)3 and Si(OH)4, reductive metabolism (even if it were thermo-dynamically possible, granted the reductants available to plants) does not involve significant 11+ removal. H2PO4− reduction may be thermodynamically possible, but is quantitatively insignificant as an H+ sink in plants. H2AsO4− reduction is probably a detoxification mechanism, and it is not a significant H+ sink in plants for which quantitative data are available.
O2 assimilation (reduction) into -OH, and thence ≡O+, occurs in anthocyanin synthesis, but not to an extent which disposes of much of the excess H+ produced in growth with NHJ as N source.
SO42− reduction in excess of that required by primary, core metabolism (i.e. that leading to amino acids, thylakoid sulpholipid and cell wall esters) can generate OH−, but such ‘secondary’ SO42− metabolism is related to osmoregulation and to chemical defence rather than to H+ disposal per se. The quantity of S metabolism which is ‘negotiable’ is not, apparently, adequate to neutralize a substantial fraction of excess H+ formed during growth. SeO4−2 reduction and assimilation performs largely a detoxification and/or chemical defence role, and the quantities involved (even in Se-accumulators) do not generate enough OH− to offset much of the excess H+ formed during growth. Vanadate reduction is quantitatively insignificant as an H+ sink.
Oxidation of Cl−, Br− and I− in incorporation into C-halide groups is, in land plants, a quantitatively insignificant process. H+ disposal via HCl volatalization from an aqueous phase of low pH which contains Cl− does not seem to be an important sink for H+ in terrestrial plants. Oxidation of F− to form ≡C-F usually produces CH2F.COO− with no net H+ consumption. Oxidation of Fe2+ and Mn2+ is H+-consuming as long as oxides or hydroxides of Fe3 and Mn4+ are not formed. However, the oxides and hydroxides are often formed so that the oxidation processes consume OH− rather than H+.
These quantitative considerations, together with those of the availability of starting materials and the toxicity of end products suggest that the H+-consuming processes, even in combination, probably cannot dispose of all of the H+ generated in growth with NHJ as N source. In general, these H+-consuming reactions seem to be related to synthesis of osmoregulatory and chemical defence compounds, and to detoxification; the products appropriate to these functions generally have high molecular masses per mol H+ consumed in their synthesis, a feature which does not make them ideal parts of a ‘biochemical pH-stat’.