Pressure in roots develops due to

The correct option is active absorption.

Root pressure is a positive hydrostatic pressure developed in the roots of plants that helps in pushing water and dissolved minerals upward into the stem. Here is a step-by-step explanation of how active absorption leads to the development of root pressure:

1. Active Transport of Mineral Ions:
The concentration of mineral ions in the soil is typically lower than the concentration of minerals inside the root cells. To absorb these minerals against a concentration gradient, the root cells expend metabolic energy in the form of ATP to actively pump ions from the soil across their cell membranes into the vascular cylinder (xylem). This process is a key component of active absorption.

2. Establishment of a Water Potential Gradient:
As mineral ions accumulate inside the xylem vessels of the root, the solute concentration inside the xylem rises. This accumulation lowers the water potential (osmotic potential) within the root xylem compared to the surrounding soil water.

3. Osmotic Water Inflow:
Because water moves naturally from an area of higher water potential to an area of lower water potential, water enters the root cells from the soil via osmosis. As water continuously flows into the confined xylem vessels of the root, it creates a positive hydrostatic pressure that pushes the column of water upward. This hydrostatic pressure is called root pressure.

Why the other options are incorrect:
- Passive absorption: This is driven by transpiration pull (tension) created by evaporation at the leaves, which pulls water up passively and does not create positive pressure in the roots.
- Low osmotic soil potential: If the soil had a low osmotic potential (high solute concentration), water would tend to move out of the roots into the soil, preventing root pressure from developing.
- Rise in transpiration: Higher transpiration rates increase the tension (negative pressure) in the xylem, rather than generating positive root pressure.