`K^(+)` ions control

Options

A. opening and closing of stomata
B. guttation
C. formation of mitotic spindle
D. all of these

Answer

### Step-by-Step Solution:

1. **Understanding Potassium Ions (K+)**:
- Potassium ions are classified as macronutrients, which means they are required in large amounts by plants for their growth and development.

**Hint**: Remember that macronutrients are essential elements needed in larger quantities compared to micronutrients.

2. **Role of Potassium in Stomatal Function**:
- The primary function of potassium ions in plants is the regulation of stomatal movement. Stomata are small openings on the surface of leaves that allow for gas exchange.

**Hint**: Think about how stomata function in relation to plant respiration and photosynthesis.

3. **Mechanism of Stomatal Opening**:
- Stomata are surrounded by guard cells, which are responsible for their opening and closing. When potassium ions are actively absorbed into the guard cells, it causes the guard cells to become hypertonic.

**Hint**: Active transport is crucial here; consider how energy is used to move ions against their concentration gradient.

4. **Turgidity of Guard Cells**:
- As potassium ions accumulate in the guard cells, water follows by osmosis, causing the guard cells to swell and become turgid. This turgidity causes the guard cells to bow outward, which opens the stomatal pore.

**Hint**: Recall the relationship between turgor pressure and cell shape in plant cells.

5. **Stomatal Closure**:
- At night, potassium ions move out of the guard cells, leading to a decrease in turgor pressure as water exits the cells. This causes the guard cells to become flaccid, resulting in the closure of the stomatal pore.

**Hint**: Consider how environmental factors like light and darkness influence stomatal behavior.

6. **Evaluating Other Functions of Potassium**:
- The question also mentions other functions of potassium. Guttation, which is the loss of water in the form of droplets from leaf margins, is not controlled by potassium but occurs through structures called hydathodes. Additionally, the formation of spindle fibers during cell division is influenced by calcium, not potassium.

**Hint**: Differentiate between the roles of various nutrients in plant physiology to avoid confusion.

### Conclusion:
The correct answer to the question is that potassium ions control the opening and closing of stomata in plants.