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

In closed systems, the O2 compensation point (ΓO) 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 ΓC, a ΓO 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, ΓO 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 ΓO and ΓC, as unique functions of the respective CO2 and O2 concentrations and of Sp, this invalidates some results showing two different functions for ΓO and ΓC, and, consequently, the associated analyses related to greenhouse effects in the past; (5) the pre-industrial atmosphere levels of O2 and CO2 are the ΓO and ΓC 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.