Blackman demonstrated that increasing illumination increased the photosynthetic rate up to a point when `CO_2` becomes limiting. If the light was not limiting, temperature becomes limiting. Emerson found that maximum `CO_2` fixation could be achieved with brief flashes of light. Mark the correct statement in the following

Options

A. Only one factor can be limited in photosynthesis
B. Photosynthesis consists of a light and dark reaction
C. The trapping of light by chloroplast is temperature dependent
D. The trapping of light by chloroplast can occur only if `CO_2` is present

Explanation

To solve the question regarding the findings of Blackman and Emerson on photosynthesis, we can analyze the statements provided and determine which one is correct based on the information given in the video transcript.

### Step-by-Step Solution:

1. **Understanding Blackman's Experiment**:
- Blackman demonstrated that increasing light intensity increases the rate of photosynthesis up to a certain point. Beyond this point, the availability of carbon dioxide (CO2) becomes the limiting factor for photosynthesis. This means that if there is enough light but not enough CO2, the rate of photosynthesis will not increase further.

2. **Considering Temperature as a Limiting Factor**:
- The transcript mentions that if light is not limiting, then temperature can become a limiting factor. This indicates that temperature plays a significant role in the photosynthetic process and can affect the rate of photosynthesis if other factors (like light and CO2) are sufficient.

3. **Emerson's Findings**:
- Emerson found that maximum CO2 fixation could be achieved with brief flashes of light. This suggests that even short bursts of light can provide enough energy for the light-dependent reactions of photosynthesis, which produce ATP and NADPH necessary for the dark reactions.

4. **Analyzing the Statements**:
- **Statement 1**: "Only one factor can be limited in photosynthesis."
- This is incorrect because multiple factors (light, CO2, temperature) can limit photosynthesis at different times.
- **Statement 2**: "Photosynthesis consists of a light and dark reaction."
- This is correct as photosynthesis involves light-dependent reactions (which require light) and light-independent reactions (dark reactions) where ATP and NADPH are used to fix CO2.
- **Statement 3**: "Trapping of light by chloroplasts is temperature dependent."
- This is misleading; while temperature can affect the rate of photosynthesis, the trapping of light itself is not directly temperature dependent.
- **Statement 4**: "Trapping of light by chloroplasts can only occur if carbon dioxide is present."
- This is incorrect; light reactions can occur independently of CO2 presence.

5. **Conclusion**:
- The correct statement based on the findings of Blackman and Emerson is **Statement 2**: "Photosynthesis consists of a light and dark reaction."