Who first of all indicated that water is electron donor in photosynthesis ?

Options

A. Arnon
B. Calvin
C. Blakeslee
D. Van Niel.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks who first indicated that water is the electron donor in the process of photosynthesis.

2. **Identifying Key Terms**:
- **Electron Donor**: In photosynthesis, an electron donor is a substance that provides electrons during the process.
- **Photosynthesis**: The process by which green plants use sunlight to synthesize foods with the help of chlorophyll.

3. **Reviewing Historical Context**:
- The role of water in photosynthesis was a significant discovery in understanding how plants convert light energy into chemical energy.
- Early studies on photosynthesis involved various scientists who contributed to our understanding of the process.

4. **Evaluating the Options**: The options given are:
- a) Anion
- b) Calvin
- c) Black-Soleil
- d) von Nehl

5. **Identifying the Correct Scientist**:
- According to the information provided, it was **von Nehl** who first indicated that water is the electron donor in photosynthesis in the year **1930**.
- He proposed that in green plants, water is used instead of hydrogen sulfide, which is used by some photosynthetic bacteria.

6. **Conclusion**: Therefore, the correct answer is **d) von Nehl**.