The typical N source for terrestial vascular plants is the soil in which they are rooted. N exchange between the aerial shoot and its environment, as gaseous, dissolved or particulate N, can involve either net loss or net gain of N by the plant. Net gain of N by the plant shoot at the expense of its aerial environment can be significant, under natural or agricultural conditions, for all three forms of aerial N (NH3,N2,NO3), Gaseous N gain can be substantial, as NH3, for plants growing on soils receiving large inputs of animal excreta and faeces. Soluble N uptake by shoots is the major N source for ‘atmospheric’ epiphytes. Particulate N supply to shoots is a major N source for carnivorous plants. Experimentally, several plants which normally grow with N supply mainly via their roots can be grown with a predominant, or sole, supply via their shoots.
Plant growth with CO2, as C source, SO42− as S source and H2PO4, as P source generates excess OH (∼ 0.78 OH−/N assimilated) in the synthesis of core metabolites when the N source is NO3− supplied to roots or shoots. All other N sources (except dicarboxylic amino-acids) yield excess H+during synthesis of core metabolites. NH4+ (in solution) supplied to roots or shoots yields some 1.22 H+/N assimilated, while NH3, N2 or NO2 supplied to shoots as gases, dissolved urea supplied to shoots or roots, and animal protein supplied to shoots of carnivorous plants, yields some 0.22 H+/N assimilated.
These biochemically caused acid-base perturbations may be exacerbated (NO2) or partially or, occasionally, completely offset (NH3) by effects related to solution and dissociation of the gas in growing shoots.
Assimilation of the shoot-acquired N in the shoots leads to somewhat different constraints on acid-base homoiostasis relative to those found with assimilation of root-acquired N. The shoot cannot generally excrete excess H+ or OH− to its environment (as is so for root-assimilation of N), nor can it dispose of H+ biochemically without co-operation of root metabolism and root–shoot and root-rhizosphere transport processes. Excess OH− from shoot-acquired and shoot-assimilated NO3− can, by contrast, be disposed of biochemically after the fashion of root-acquired, shoot-assimilated NO3−. Tentative conclusions as to the disposal of excess H+ generated in shoot acquisition and assimilation of N-sources other than NO3− and NO2− are different for rhizophytes and for haptophytes. Rhizophytes probably carry out a net synthesis of organic acid in the roots, with subsequent cation-H+ exchange at the root plasmalemma, transport of cation salt of the organic acid to the shoot in the xylem, and metabolism of the organic acid anion in the shoot to yield OH−. Haptophytes probably have an excess of liquid water supply over their needs for growth and transpiration due to the very low N concentration in this water; they thus have the potential for H+ (as strong acid) excretion to the excess surface water which is lost, at least in part, as liquid water.