The benefits which this paper addresses are those of maintaining the intracellular acid-base balance during growth, and of generating osmolarity related to regulation of turgor in environments of low water potential. These benefits may incur costs in terms of the quantity of potentially growth-limiting resources (photons, water, nitrogen) which are needed to produce unit quantity of ‘baseline’ plant biomass. The direction (excess H+ or excess OH) and magnitude of acid–base perturbation during growth depends on the nature of the N-source (NH4+, N2 or NO3), so that the costing of pH homoiostasis involves consideration of the costs of overall N-assimilation for comparison with the other costs of growth of a terrestrial C3 plant.

Photon costs for the various biochemical and transport processes involved in overall growth, N-assimilation, pH regulation and osmolarity generation are computed using known stoichiometries of coupled reactions. Water costs are deduced from the C-requirements for the various processes (including C lost in associated decarboxylations) by assuming a constant value of water lost in transpiration per unit net C fixed in an illuminated shoot. Nitrogen costs are deduced from the N-content of the plants or compounds under consideration. The computed costs for N-assimilation and the generation of osmolarity are referred to the costs of ‘baseline’ plant synthesis using the cheapest mechanisms (NH4+ as source for N-assimilation; inorganic ions as the basis for osmolarity generation) so that the increment of cost related to assimilation of N2or NO3, or of osmolarity generation using an organic compatible solute, can be presented.

Photon costs of growth with N2 fixation and the processes associated with regulation of pH are (granted the assumptions made as to stoichiometries and plant composition) 9 % higher than are those of growth with NH4+ as N˜ source. The predicted cost of growth with NO3 as N source depends on the location of NO3 reduction and the mechanism of OH disposal, and ranges from 5 to 12% more than that for growth with NH4+ as N source. H2O (transpiration) costs follow a similar pattern, with growth on N2 as N source costing 12% more, and growth on NO3 costing to 1–2 to 167 % more, than growth with NH4+ as N source.

The extra costs in photons of using compatible solutes (sorbitol, proline or glycine betaine) to generate an osmolarity of 500 osmol m−3 in all of the non-apoplastic water of the plant add 21·5 to 26·1 % to the total costs of growth, while use of compatible solutes to generate osmolarity in ‘N’ phases (i.e. cytosol, plastid stroma, mitochondrial matrix) alone would add 5·2 to 6·2% The costs of growth in terms of transpirational water are increased 7·9 to 98 % by the use of compatible solutes for osmolarity generation in the ‘N’ phases only. The increments for the N-containing solutes are higher when NO3 is the N-source rather than NH4+. The N-cost of growth with N-containing compatible solutes generating 500 osmol m−3 in ‘N’ phases increases the N cost of growth by 33%.

These predicted costs are under-estimates of ‘real’ costs which take into account the occurrence of alternate oxidase activity under some growth conditions and the production of additional organic acid anions with N2 as opposed to NH4+ as N source. Nevertheless, the predicted minimum costs of attaining the benefits of pH regulation and of turgor generation are of use in suggesting where selectively significant (i.e. low requirement for a scarce resource) alternative mechanisms may occur. Examples include a possible photon saving by using NH4+ rather than N2 or NO3 where all three are available; a possible water saving by use of photoreduction of NO3 in leaves in arid environments; and a possible N saving by use of non-N-containing compatible solutes (polyols) in environments of low water potential. Proof of these suggestions involves comparisons of inclusive fitness of genotypes possessing the trait under consideration with that of genotypes lacking the trait.