Photosynthesis in Higher Plants — BIOLOGY STD 11 Science — Question

The C₃ and C₄ plants respond differently to CO₂ concentrations. At low light conditions neither group responds to high CO₂ conditions. At high light intensities, both C₃ and C₄ plants show increase in the rates of photosynthesis. What is important to note is that C₄ the plants show saturation at about $360\mu\text{IL}^{-1}$ while C₃ responds to increased CO₂ concentration and saturation is seen only beyond $450\mu\text{IL}^{-1}.$ Thus, current availability of CO₂ levels is limiting to the C₃ plants. The fact that C₃ plants respond to higher CO₂ concentration by showing increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes and bell pepper. They are allowed to grow in carbon dioxide enriched atmosphere that leads to higher yields.

Temperature: The dark reactions being enzymatic are temperature controlled Though the light reactions are also temperature sensitive they are affected to a much lesser extent. The plants respond to higher temperatures and show higher rate of photosynthesis while C₃ plants have a much lower temperature optimum.

The temperature optimum for photosynthesis of different plants also depends on the habitat that they are adapted to. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

Water: Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant, rather than directly on photosynthesis. Water stress causes the stomata to close hence reducing the CO₂ availability. Besides, water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well.