In C4 plants, the CO2 compensation point is zero or nearly zero, reflecting their very low levels of photorespiration. In C4 plants, photosynthetic rates saturate at internal concentration values of about 15 Pa, reflecting the effective CO2-concentrating mechanisms operating in these plants.
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.
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.
The atmospheic concentration at which photosynthesis just compensated for respiration is referred to as the CO2 compensation point. in C4 plants is lower, as C4 plasnts have high CO2 levels in the bundle sheath of chloroplasts and high pools of CO2 in the mesophyll cells. The CO2, absorbed by C4 leaves is fixed into organic acids, which thus maintain high levels of CO2. The mesophyll of C3 plants has no such mechanism of fixing CO2.
In c4 plants, the high levels of CO2 inhibit photorespiration , because, at high concentrations, CO2 competes better than O2 for the Rubisco caboxylase site and is therefore fixed at a greater rate than O2.
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