BIOCHEMICAL DISPOSAL OF EXCESS H⁺IN GROWING PLANTS?

Abstract

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 se...

The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

“…1986,19876,1988). Acid-base balance during growth demands an H+ efflux of 4/3 H+ per NH/ entering and assimilated (see Raven, 1985Raven, a, 1986Raven, , 19876, 1988. Consideration of SO42-and H2PO4" uptake and assimilation reduces the net H"^ efflux to 1-22 H^ per N assimilated, although the net negative charge on organic matter remains at 0-33 mol negative charge per mol N or 0-022 mol negative charge per mol C. Kirkby & Mengel (1967) report some 0-17 mol negative charge on uronate per mol N, or 0-011 mol negative charge on uronate per mol C. As mentioned in Section II, this negative charge arises by oxidation of reduced C derived from Rubisco-catalysed CO2 fixation, so does not involve non-Rubisco carboxylases.…”

Influence of malate and 2-oxoglutarate on the NIA transcript level and nitrate reductase activity in tobacco leaves

“…(iii) Plants were darkened for 16-24 h to reduce the NIA transcript level and NIA activity to low levels, and leaves were then detached and supplied with malate or 2-oxoglutarate for 4 h in the light to investigate their impact on the light-induction of NIA. The increase of the NIA transcript and NIA activity was antagonized by malate, and synthesized and exported via the phloem to the roots where it is decarboxylated (Raven 1986(Raven , 1988Martinoia & Rentsch 1994). During the day, nitrate assimilation and the accompanying synthesis of malate proceed more rapidly than malate export, and malate accumulates to high levels that are equivalent to 25-30% of the amount of assimilated nitrate (Touraine, Grignon & Grignon 1988;Deng, Moureaux & Lamaze 1989;Kaiser & Förster 1989;Scheible et al 1997b).…”

“…Geiger et al 1998Geiger et al , 1999, which is affected by a range of extraneous factors including the temperature and water availability. A third aspect is the low pH-buffering capacity of plant leaves (Raven 1986(Raven , 1988, which makes it important to coordinate nitrate assimilation with processes involved in pH regulation, including the synthesis and export of malate. As we show elsewhere (Scheible et al, 2000), phosphoenolpyruvate carboxylase expression is tightly coordinated with expression of NIA and nitrite reductase, whereas enzymes for the synthesis of 2-oxoglutarate are expressed out of phase with a maximum later in the diurnal cycle facilitating remobilization of nitrogen that has temporarily accumulated in intermediates of the GOGAT pathway and photorespiration.…”

“…Changes in organic acid metabolism are also important for pH regulation. Protons are consumed during the conversion of nitrate to ammonium (Raven 1986(Raven , 1988. In single-celled organisms, the resulting alkalinization can be countered by regulating proton exchange with the surrounding medium.…”

“…In single-celled organisms, the resulting alkalinization can be countered by regulating proton exchange with the surrounding medium. In leaves the surrounding cell wall has only a small buffer capacity (Raven 1986(Raven , 1988) and a counter-anion, usually malate, is…”