Question

$C_4$ plants have better productivity because
A. $C_4$ plants absorb more light
B. $C_4$ plants absorbs more $C{O_2}$
C. $C_4$ plants lack photorespiration
D. All of the above

Answer 1

$C_4$ plants are plants which follow $C_4$ carbon fixation or the Hatch–Slack pathway which is one of the photosynthetic processes of carbon fixation. It occurs in two partially connected compartments – mesophyll cells and bundle-sheath cells.

Complete solution: To understand $C_4$ pathway better, let us learn how $C_3$ pathway works. In $C_3$ pathway, the whole process revolves around the enzyme Ribulose- $1,5$-bisphosphate carboxylase/oxygenase (rubisco) which catalyses carboxylation and oxygenation which results in photorespiration, an energy depriving activity.
But in $C_4$ photosynthesis photorespiration is reduced by concentrating rubisco with $C{O_2}$ to promote more carboxylation thereby reducing oxygenation. This is performed efficiently as the $C_4$ pathway happens in two partially connected compartments. The reduced/lack of photorespiration gives them an advantage over $C_3$ plants to survive in drought, hot climate and even in limited nitrogen and carbon dioxide availability.

Thus, $C_4$ plants have better productivity because (C) $C_4$ plants lack photorespiration.

Note: Some of the examples of $C_4$ plants are sorghum, millets, and sugar cane and $C_3$ plants are rice, oats, cotton and eucalyptus. Given the advantages of the $C_4$ pathway for plants, scientists are trying to produce rice crops which use the $C_4$ pathway as it may give increased yield of rice. Though photorespiration is reduced, the increased carboxylation increases the accumulation of carbon in the plants, which causes bio sequestration of carbon dioxide affecting the climate. The final pathway for carbon fixation is crassulacean acid metabolism (CAM) which is adapted by the plants and succulents that grow in extreme arid conditions. The stomata of these plants remain closed during the day but open at night to store $C{O_2}$ in order to minimize evapotranspiration. During the day, the stored $C{O_2}$ is used to perform photosynthesis. Pineapple uses this pathway for carbon fixation.