Compensation Point

The compensation point is the amount of light intensity on the light curve where the rate of photosynthesis exactly matches the rate of respiration. At this point, the uptake of CO₂ through photosynthetic pathways is exactly matched to the respiratory release of carbon dioxide, and the uptake of O₂ by respiration is exactly matched to the photosynthetic release of oxygen.

In assimilation terms, at compensation point, the net carbon dioxide assimilation is zero. Leaves release CO₂ by photorespiration and day respiration, but CO₂ is also converted into carbohydrate by photosynthesis. Assimilation is therefore the difference in the rate of these processes. At a normal partial pressure of CO₂ (338mbar in 1980), there is an irradiation at which the net assimilation of CO₂ is zero. For instance, in the early morning and late evenings, the compensation point may be reached as photosynthetic activity decreases and respiration increases. Therefore, the partial pressure of CO₂ at the compensation point, also known as gamma, is a function of irradiation. The irradiation dependence of the compensation point is explained by the RuBP (ribulose-1,5-bisphosphate) concentration. When the acceptor RuBP is in saturated concentration, gamma is independent of irradiation. However at low irradiation, only a small fraction of the sites on RuBP carboxylase-oxygenase enzyme (Rubisco) have the electron acceptor RuBP. This decreases the photosynthetic activity and therefore affects gamma. The intracellular concentration of CO₂ affects the rates of photosynthesis and photo respiration. At higher carbon dioxide concentrations, the photosynthesis rate is higher, while at low CO₂ concentrations, photo respiration is higher.