NITROGEN ASSIMILATION AND TRANSPORT IN VASCULAR LAND PLANTS IN RELATION TO INTRACELLULAR pH REGULATION

Abstract

SUMMARY The assimilationof ammonium ion in plant cell cytoplasm produces at least one H+ per NH+4; N2 fixation generates 0.1‐0.2 H+ per N assimilated; NO‐3 assimilation produces almost one OH‐ per NO‐3. H+ or OH‐ produced in excess of that required to maintain cytoplasmic pH for H+ or OH‐, the major process involved is H+ efflux (frequently by active transport) from the cell. IN higher land plants, much of assimilated N occurs as hoot protein; the shoot cells have no direct acess to the H+ and OH‐ sink of the soil solution. When ammonium ion is the N source it is assimilated into organic‐N in the roots. The shoot is supplied with a mixture of amino‐acids, amides and organic acids which an be incorporated (with neutral photosynthate) into cell material without damaging pH changes. Similar considerations apply to symbiotic N2 assimilation in root nodules. IN both cases the excess H+ generated in the root cell cytoplasm is exerted is excreted to the soil solution; there is no mechanism whereby photolithotrophic plant can, in the long term, counter intracellular acidity without resort to active H+ efflux to an extracellular sink. When nitrate is reduced in roots, the organic compounds involved in N transportged to the shoot are similar to those used when ammonium or N2 is the N source with similar implications for the regulation of shoot pH. The excess OH‐ generated in the roots is partly excreted to the soil solution, and partly neutralized by the ‘biochemical pH stat’ which produces strong organic acids from essentially neutral precursors. When nitrate is assimilated solely in shoots, the excess OH‐ is initially neutralized by the operation of the biochemical pH state. Storage of the inorganic cation‐organate in shoot cell vacuoles could lead to turgor and volume regulation problems in these cells. These are avoided when an insoluble salt (calcium oxalate) is the product of the pH stat, or when the cation organate is translocated to the roots where organate breakdown regenerates OH‐, whcih is lost to the soil solution. This mixture of biochemical, and long and short distance transport processes, enables cells remote from a large sink for H+ or OH‐ to produce protein without unfavourable pH changes. These processes related to pH regulation during N assimilation have important consequences for the carbon and energy economy of the plant.

Redox potential (Eh) and pH as drivers of soil/plant/microorganism systems: a transdisciplinary overview pointing to integrative opportunities for agronomy

“…This is explained by: (i) the necessary release by plant roots of an H + ion when they absorb a NH 4 + ion to counterbalance the corresponding excess of positive charges (Hinsinger et al 2003;Raven and Smith 1976); (ii) conversely, the release of OH -ions when plants absorb NO 3 -ions to counterbalance the corresponding excess of negative charges, the excess OH -being also partly neutralized by the 'biochemical pH-stat' (Raven 1986;Raven and Smith 1976). In addition to roots, many soil microbes, such as ectomycorrhizal and saprophytic or pathogenic fungi, can produce organic acids and acidify the rhizosphere ).…”

Kinetics of nitrate and ammonium absorption and accompanying H+fluxes in roots ofLolium perenneL. and N2‐fixingTrifolium repensL.

“…For L. perenne, rates of net uptake, influx and efflux of NH4"^ exceeded those of NO3", while the opposite was the case in T. repetis (Macduff & Jackson 1992), Uptake and assimilation of N are associated with a transport of H"^ between roots and root medium. These H' fluxes affect the pH of the rhizosphere with subsequent impacts on nutrient availability and microbial activity (Nye 1981), While NO^,' use corresponds to an alkalization (net H"^ influx) and NH4"^ use to an acidification of the growth medium, NT fixation will generate only small amounts of excess H^ (Raven & Smith 1976), However, N2 fixation is normally assoeiated with an excess cation uptake, eausing substantial acidification of the growth mediutn (Israel & Jaekson 1978). In addition, N and C metabolism are linked and regulated as they share organic carbon and energy supplied direetly frotn photosynthetic electron transport and CO2 fixation, or indirectly from respiration (Huppe & Turpin 1994).…”

The effects of root nitrogen supplies on the absorption of atmospheric NO2 by soybean leaves

“…Because assimilation of ammonium produces H"^ and assimilation of nitrate consumes H"^ (Raven & Smith, 1976), the H"^ concentration in leaves of the plants at 5 mM nitrate was half that for the plants at 5 mM ammonium (Table 4). This difference in H"*^ concentrations of leaves might refiect a difference in the H"^ concentrations of extracellular fluid, which in turn would decisively affect NOg absorption, even though there is also a small opposite effect due to the different nitrate concentrations in ammomium-grown over nitrategrown plants (Table 3).…”

“…In plants supplied witb 5 mM or 1 mM ammonium, tbe NO2 AR declined from 12 units at tbe beginning of exposure to 8-27 or 846 units on tbe day 7 of exposure (Table 2). Tbis migbt result from accumulation of H^ produced by NO2 absorption (Wellburn, 1990;Bambauer et al, 1994) and from ammonium assimilation (Raven & Smith, 1976) in tbe plants supplied witb ammonium as tbe only root N source. As assimilation of nitrate can consume H"p roduced from NO2 absorption, tbe NO2 AR of tbe plants grown witb nitrate declined more slowly tban in plants grown witb ammonium during exposure (Table 2; Fig 1).…”