BIOCHEMICAL DISPOSAL OF EXCESS H+ IN GROWING PLANTS?

Authors

  • JOHN A. RAVEN

    1. Department of Biological Sciences, Florida International University, Tamiami Campus, Miami, Florida 33199, USA
    2. Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, Scotland, UK
    Search for more papers by this author
    • *

      Permanent address. Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, Scotland, UK


SUMMARY

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’.

Ancillary