Biochemical Disposal of Excess H⁺in Growing Plants?

SUMMARY Many data support the view that, when NH 4 + [or N 2 , NH 3 , or CO(NH 2 ) 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 , H 2 PO 4 − , H 2 AsO 4 − , O 2 , SO 4 2 , SeO 4 2− and HVO 4 2 ; and for oxidative metabolism of Cr, Br − , I − , Fe 2+ and Mn 2+ . 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. H 2 PO 4 − reduction may be thermodynamically possible, but is quantitatively insignificant as an H + sink in plants. H 2 AsO 4 − reduction is probably a detoxification mechanism, and it is not a significant H + sink in plants for which quantitative data are available. O 2 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. SO 4 2− 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’ SO 4 2− 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. SeO 4 −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 CH 2 F.COO − with no net H + consumption. Oxidation of Fe 2+ and Mn 2+ is H + ‐consuming as long as oxides or hydroxides of Fe 3 and Mn 4+ 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’.