Root pressure develops due to

Question

Accepted Answer

B. 1. **Root pressure** is a positive pressure that develops within the root xylem, pushing water upward, especially noticeable at night or in early morning when transpiration is low.

2. This pressure arises because roots **actively pump mineral ions** into the xylem, a process that requires the plant to spend **metabolic energy (ATP)** to move ions against their concentration gradient — this is called **active absorption**.

3. As ions accumulate inside the root xylem due to this active process, the water potential inside the xylem drops, causing water to move in from the surrounding soil by **osmosis**, building up pressure.

4. **Passive absorption** (Option A), by contrast, does not involve energy expenditure and simply follows existing water potential gradients — it does not generate root pressure on its own.

5. **Increase in transpiration** (Option C) actually **reduces** root pressure because the pulling force of transpiration dominates over the pushing force from the roots, and **low osmotic potential in soil** (Option D) would make water uptake harder, not easier.

6. Hence, since active absorption of ions is what generates the pressure gradient, the correct answer is **B**.