Factors Impacting Primary Production: Which of the following would most likely increase the rate of photosynthesis in a C3 plant under otherwise limiting conditions?

• A. Reducing available water so stomata close and $C_0{O}_2$ diffusion into leaves decreases
• B. Increasing light intensity (photon flux) up to the point of saturation
• C. Decreasing atmospheric $C_0{O}_2$ concentration
• D. Lowering temperature far below the optimum for photosynthetic enzymes

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the process of photosynthesis in C3 plants, which involves the fixation of carbon dioxide (CO\_2) by the enzyme Rubisco during the Calvin cycle. The rate of photosynthesis depends on factors such as light intensity, CO\_2 concentration, temperature, and water availability.
2. Step 2: Analyze how each factor affects photosynthesis: Reducing water availability causes stomata to close, which limits CO\_2 diffusion into the leaf, thereby decreasing photosynthesis. Decreasing atmospheric O\_2 concentration can reduce photorespiration, potentially increasing photosynthesis, but this effect is less direct than light intensity.
3. Step 3: Consider the role of light intensity (photon flux). Increasing light intensity provides more energy for the light-dependent reactions, producing ATP and NADPH needed for the Calvin cycle, thus increasing the rate of photosynthesis up to a saturation point where other factors become limiting.
4. Step 4: Evaluate temperature effects. Photosynthetic enzymes have an optimal temperature range; lowering temperature far below this optimum slows enzyme activity and reduces photosynthesis rate.
5. Step 5: Conclude that among the options, increasing light intensity up to the saturation point most directly and effectively increases the rate of photosynthesis in C3 plants under limiting conditions, because it enhances the energy supply for carbon fixation without the negative side effects associated with the other factors.