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.