Botany - Photosynthesis in Higher Plants Question with Solution | TestHub

${\mathrm{CO}}_2$ 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.