Question

Root pressure may be caused by
A. Osmotic flow of water into xylem of absorbing part of root
B. Loss of water from xylem of the plant due to transpiration
C. Low water potential of leaves
D. High water potential of leaves

Answer 1

If transpiration does not occur, water doesn't go up. Water is absorbed from the ground by osmosis and accumulates in the xylem area of the root. Osmosis absorption induces root pressure and water flows upwards.

Complete answer:
Root pressure is the transverse osmotic pressure in the root system cells which causes the sap to increase by a plant stem to the leaves. Root pressure takes place in the xylem of certain vascular plants, either in the night or when transpiration is low in daytime when soil moisture levels are high. In higher transpiration, xylem sap is normally tense, not under pressure, due to transpirational pull. In certain plants, root pressure causes drops of xylem sap from the tips or edges of the leaves to be gutted or exuded at night. Root pressure is measured by extracting the plant spring at the level of the soil. Due to root strain, xylem sap exudes hours or days from the cut stem. The root pressure can be measured if a pressure scale is attached to the cutting stem.
The active spread of mineral nutrient ions to root xylem is responsible for root pressure. They collect in the root xylem and decrease the water potential without transpiration to move the ions up the stem. Then, because of osmosis, waters spread from soil to root xylem. This water accumulation in the xylem, which pushes rigid cells, leads to the root pressure.

So, the correct answer is option A. i.e., Osmotic flow of water into the xylem of absorbing part of root.

Note: The root pressure provides the power to drive water up the stalk, but the movement of water to the tops of the highest trees is not enough. The root pressure can raise water only 6.87 meters.