Assertion : Due to pollution atmospheric concentration of $\mathrm{CO}_2$ is increasing which will be…
Assertion : Due to pollution atmospheric concentration of $\mathrm{CO}_2$ is increasing which will be harmful for $\mathrm{C}_4$ plants whereas productivity is going to increase for $\mathrm{C}_3$ plants.
Reason : $\mathrm{C}_4$ plants have greater efficiency for $\mathrm{CO}_2$ as $\mathrm{CO}_2$ is fixed by PEP oxygenase.
If both assertion and reason are true and reason is the correct explanation of assertion
If both assertion and reason are true but reason is not the correct explanation of assertion
If assertion is true but reason is false
If both assertion and reason are false.
Solution
$\mathrm{CO}_2$ 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.