Introduction

Plants have evolved different mechanisms to optimize their water use efficiency, particularly in hot and dry conditions. Among them, C4 plants are known to be more water-efficient than C3 plants, which are the most common photosynthetic pathway. In this article, we will explore the reasons behind the increased water use efficiency of C4 plants.

Photosynthetic Pathways

Plants use the process of photosynthesis to convert sunlight energy into chemical energy, which is stored in organic compounds. The two most common photosynthetic pathways are C3 and C4. C3 plants, such as wheat, rice, and soybeans, are the most widespread and dominant photosynthetic pathway. They directly fix carbon dioxide into a three-carbon compound called phosphoglyceric acid (PGA) through the Calvin cycle.

C4 plants, on the other hand, use a modified pathway that has a preliminary stage before the Calvin cycle. They first fix carbon dioxide into malate or oxaloacetate in mesophyll cells using an enzyme called phosphoenolpyruvate carboxylase (PEPc). Then, the malate or oxaloacetate is transported to bundle sheath cells, where they release CO2 and enter the Calvin cycle. This biochemical mechanism is called the C4 cycle, and it reduces photorespiration, which is the loss of carbon through a wasteful process.

Leaf Anatomy

The structure of C4 leaves is another factor that contributes to their water use efficiency. They have a unique arrangement of mesophyll and bundle sheath cells. In C3 plants, mesophyll and bundle sheath cells are morphologically similar and randomly arranged. However, in C4 plants, the mesophyll cells are arranged around the bundle sheath cells, forming a sheath around them. This arrangement of cells creates a diffusion gradient between mesophyll and bundle sheath cells, enabling CO2 to concentrate in the bundle sheath and reduce photorespiration.

First CO2 Concentration

C4 plants are capable of concentrating CO2 around the enzyme Rubisco in bundle sheath cells, which reduces the oxygenation reaction and photorespiration, hence enhancing water use efficiency. The first CO2- concentrating step occurs in mesophyll cells, where the PEPc enzyme catalyzes the initial fixation of CO2 to produce oxaloacetate, as we mentioned previously. The low oxygen concentration in mesophyll cells also helps Rubisco to fix carbon efficiently.

Second CO2 Concentration

The second step of CO2 concentration occurs in bundle sheath cells, where the malate or oxaloacetate produced in mesophyll cells is decarboxylated to release CO2. This CO2 is then fixed by Rubisco in the Calvin cycle. The proximity of mesophyll and bundle sheath cells, along with the narrow intercellular spaces, reduces photorespiration and water loss through transpiration.

Conclusion

C4 plants are more efficient in water use than C3 plants because of their unique anatomy and biochemical mechanisms. The mesophyll and bundle sheath cell arrangement and the two-step CO2 concentration process reduce photorespiration and conserve water while fixing carbon efficiently. Understanding the biology and physiology of C4 plants can help us develop more water-efficient crops and improve food security in arid and semiarid regions.