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An estimate of photorespiration is obtained from the relationship between the net exchange of CO2 of the leaf and the internal CO2 concentration, i.e. within the mesophyll intercellular spaces. The latter is obtained by calculation, taking into account the combined epidermal and boundary-layer resistances between the bulk atmosphere and the mesophyll intercellular spaces. The linear part of this relationship (at low CO2 concentrations) is extrapolated to zero internal concentration, at which none of the intercellular photorespired CO2 is available for reassimilation. The calculated output of CO2 under such conditions provides an estimate of photorespiration, but, by failing to take into account intracellular reassimilation of photorespired CO2 underestimates actual photorespiration. As the slope of this linear relationship represents the mesophyll (intracellular) resistance to CO2 uptake, this procedure was used to recalculate published data on effects of light intensity and of oxygen concentration on net photosynthesis. The analysis showed that increased oxygen concentration and decreased light intensity reduced photosynthesis largely by increasing mesophyll resistance to CO2 uptake. It is suggested that the CO2 compensation point (Г) is a function of both photorespiration (L) and mesophyll resistance (rm): Г = L. rm.
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Samish et al. (1968) studied this question.