Modelling ¹⁸O₂ and ¹⁶O₂ unidirectional fluxes in plants: I. regulation of pre-industrial atmosphere.

In closed systems, the O₂) compensation point (Γ₀) was previously defined as the upper limit of O₂ level, at a given CO₂ level, above which plants cannot have positive carbon balance and survive. Studies with ¹⁸O₂ measure the actual O₂ uptake by photorespiration due to the dual function of Rubisco, the enzyme that fixes CO₂ and takes O₂ as an alternative substrate. One-step modelling of CO₂ and O₂ uptakes allows calculating a plant specificity factor (Sp) as the sum of the biochemical specificity of Rubisco and a biophysical specificity, function of the resistance to CO₂ transfer from the atmosphere to Rubisco. The crossing points (Cx, Ox) are defined as CO₂ and O₂ concentrations for which O₂ and CO₂ uptakes are equal. It is observed that: (1) under the preindustrial atmosphere, photorespiration of C3 plants uses as much photochemical energy as photosynthesis, i.e. the Cx and Ox are equal or near the CO₂ and O₂ concentrations of that epoch; (2) contrarily to Γ(C), a Γ₀ does not practically limit the plant growth, i.e. the plant net CO₂ balance is positive up to very high O₂ levels; (3) however, in a closed biosystem, Γ₀ exists; it is not the limit of plant growth, but the equilibrium point between photosynthesis and the opposite respiratory processes; (4) a reciprocal relationship exists between Γ₀ and Γ(C), as unique functions of the respective CO₂ and O₂ concentrations and of Sp, this invalidates some results showing two different functions for Γ₀ and Γ(C), and, consequently, the associated analyses related to greenhouse effects in the past; (5) the pre-industrial atmosphere levels of O₂ and CO₂ are the Γ₀ and Γ(C) of the global bio-system. They are equal to or near the values of Cx and Ox of C3 plants, the majority of land plants in preindustrial period. We assume that the crossing points represent favourable feedback conditions for the biosphere-atmosphere equilibrium and could result from co-evolution of plants-atmosphere-climate. We suggest that the evolution of Rubisco and associated pathways is directed by an optimisation between photosynthesis and photorespiration.