Even though a very few cells in a C₄ plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?
C₄ plants are highly productive because they lack photorespiration, which is a wasteful process found in C₃ plants. In C₄ plants, the C₄ acid from mesophyll cells is broken down in the bundle sheath cells to release CO₂, which increases the intracellular concentration of CO₂ at the enzyme site.
This high concentration of CO₂ ensures that RuBisCO functions primarily as a carboxylase and minimizes its oxygenase activity. Since photorespiration does not occur, there is no wastage of energy and resources, leading to better productivity and yields.
Additionally, C₄ plants are adapted to higher temperatures and show a response to high light intensities, which further contributes to their greater productivity of biomass.
Explanation
The textbook explains that C₄ plants have special features that make them highly productive despite the Calvin pathway occurring only in bundle sheath cells. The key reason is the absence of photorespiration. The C₄ mechanism pumps CO₂ from mesophyll cells to bundle sheath cells, creating a high CO₂ concentration that ensures RuBisCO acts as a carboxylase rather than an oxygenase. This prevents the wasteful photorespiration process. Combined with tolerance to high temperatures and high light intensities, C₄ plants achieve greater biomass productivity.