BIOCHEMICAL DISPOSAL OF EXCESS H+IN GROWING PLANTS?
Authors: RAVEN JOHN A. 
Published: May 2006

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)3and 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.

O2assimilation (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+disposalper se.The quantity of S metabolism which is ‘negotiable’ is not, apparently, adequate to neutralize a substantial fraction of excess H+formed during growth. SeO4−2reduction 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+disposalviaHCl 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 Fe3and 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’.