This is an index of free energy content of water

Options

A. `Psi`
B. `pi`
C. `Omega`
D. `tau`

Answer

### Step-by-Step Text Solution:

1. **Understanding the Question**: The question asks for an index that represents the free energy content of water. In biological terms, this is often related to how water moves through plants and its potential energy.

2. **Identifying Key Terms**: The term "free energy content" is crucial. In the context of water in plants, this is often referred to as "water potential," which is denoted by the Greek letter psi (Ψ).

3. **Analyzing the Options**: The options given are:
- Psi (Ψ)
- Pi (π)
- Omega (Ω)
- Tau (τ)

4. **Recalling Definitions**:
- **Psi (Ψ)**: Represents water potential, which is a measure of the potential energy of water in a system.
- **Pi (π)**: Generally refers to the mathematical constant and does not relate to water potential.
- **Omega (Ω)**: Often used in different contexts, but not for water potential.
- **Tau (τ)**: Also does not relate to water potential.

5. **Selecting the Correct Answer**: Based on the definitions and the context of the question, the correct answer is **Psi (Ψ)**, as it directly refers to the free energy content of water.

### Final Answer:
The index of free energy content of water is **Psi (Ψ)**.

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