Even though very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?

Yes, even though only a small subset of cells in a C4 plant carry out the Calvin-Benson cycle (C3 cycle), these plants are highly productive due to several unique adaptations associated with the C4 photosynthetic pathway. Here's why C4 plants can be highly productive:

1. Efficient CO2 Concentration: C4 plants have a specialized carbon fixation mechanism that involves the initial fixation of CO2 into a four-carbon compound, typically oxaloacetate or malate, in mesophyll cells. These four-carbon compounds are then shuttled to bundle sheath cells, where they release CO2 to be used in the Calvin-Benson cycle. This spatial separation of initial carbon fixation and the Calvin-Benson cycle minimizes photorespiration and enhances the efficiency of CO2 utilization, even under conditions of low atmospheric CO2 levels.

2. Minimized Photorespiration: The primary advantage of the C4 pathway is the suppression of photorespiration. Photorespiration, which occurs in C3 plants when RuBisCO fixes oxygen instead of CO2, leads to the wasteful consumption of energy and carbon compounds. By concentrating CO2 around RuBisCO in bundle sheath cells, C4 plants minimize the occurrence of oxygenation reactions, thereby reducing photorespiration and improving photosynthetic efficiency.

3. High Water Use Efficiency: C4 plants are often found in hot and arid environments where water availability is limited. The C4 pathway allows these plants to maintain high rates of photosynthesis while minimizing water loss through stomatal closure. The spatial separation of carbon fixation and the Calvin-Benson cycle in C4 plants enables them to operate efficiently at lower stomatal conductance, leading to higher water use efficiency compared to C3 plants.

4. Adaptation to High Light Intensity: C4 plants are well-adapted to high light intensities, which are common in their natural habitats. The C4 pathway allows for rapid CO2 fixation and efficient utilization of light energy, enabling C4 plants to maintain high rates of photosynthesis even under intense sunlight.

Overall, the unique anatomical, physiological, and biochemical adaptations associated with the C4 pathway enable C4 plants to achieve high productivity and thrive in diverse environmental conditions, making them important contributors to global primary productivity.