[A]: High pH in guard cells activate phosphorylase to convert starch into glucose 1-`PO_4` .
[R]: Stomata open when pH is low

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To analyze the given statements about guard cells and stomatal behavior, let's break down the assertion and reason step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "High pH in guard cells activates phosphorylase to convert starch into glucose 1-PO4."
- This means that when the pH in guard cells is high (alkaline), it triggers the enzyme phosphorylase to break down starch into glucose-1-phosphate.

2. **Understanding the Reason**:
- The reason states that "Stomata open when pH is low."
- This implies that stomata tend to open when the pH is acidic (low pH).

3. **Analyzing the Relationship**:
- During the daytime, when photosynthesis occurs, carbon dioxide is utilized, leading to a decrease in its concentration. This results in a higher pH (more alkaline) in the guard cells.
- The high pH activates phosphorylase, leading to the conversion of starch to glucose, which is essential for the guard cells to maintain turgor pressure and thus open the stomata.

4. **Contradiction in Reason**:
- The reason states that stomata open when pH is low, which contradicts the assertion. Stomata actually open when the pH is high (alkaline), not low.

5. **Conclusion**:
- Therefore, the assertion is true, but the reason is false. The correct relationship is that high pH leads to the activation of phosphorylase, allowing stomata to open, while low pH does not cause stomata to open.

### Final Answer:
- The correct option is: **If assertion is true, the reason is false.**