$\mathrm{CO}_2$ concentration of the atmosphere is $0.036 \%$ or $360 \mathrm{ppm}\left(360 \mu l . l^{-1}\right)$. It is a limiting factor for $\mathrm{C}_3$ plants as the available $\mathrm{CO}_2$ concentration is lower than the optimum for photosynthesis. Increase in its concentration upto $0.05 \%$ increases the rate of photosynthesis in most $\mathrm{C}_3$ plants. When $\mathrm{CO}_2$ concentration is reduced, there comes a point at which illuminated plant parts stop absorbing carbon dioxide from their environment. It is known as $\mathrm{CO}_2$ compensation point or threshold value. At this value $\mathrm{CO}_2$ fixed in photosynthesis is equal to $\mathrm{CO}_2$ evolved in respiration and photorespiration. The value is $25-100 \mathrm{ppm}$ in $\mathrm{C}_3$ plants and $0-10 \mathrm{ppm}$ in $\mathrm{C}_4$ plants. The reason for low compensation value for $\mathrm{C}_4$ plants is the greater efficiency of $\mathrm{CO}_2$ fixation through PEP-carboxylase.