Question

Which pressure developed in the root, forces the water into the xylem vessel and upwards into the stem for a certain height?
A. Osmotic pressure
B. Root pressure
C. Atmospheric pressure
D. Turgor pressure

Answer 1

Plants lack both regular circulation systems as well as interstitial fluids. But they also have to transport various types of substances to long distances such as water from the roots to the top of the plant and food from the leaves to the tips of the roots for proper functioning. Root pressure is a positive pressure developed in the xylem sap of the roots of certain plants. It is a manifestation of active water absorption.

Complete answer:
Option A Osmotic pressure: Osmotic pressure is defined as the required pressure to completely stop the entry of water into the osmotically active solution through a semipermeable membrane. As it allows only selective materials to pass through, osmotic pressure can’t make the transport of water and other substances to the various parts of the plants.
Option A is not correct.
Option B Root pressure: It is the pressure with which water is pushed into the xylem tubes of the root. Due to root pressure the water moving upwards forms a column, which is maintained up to a certain height.
Option B is correct.
Option C Atmospheric pressure: Atmospheric pressure is the force exerted on the surface by air above it as gravity pulls it to Earth. It is measured with an instrument called a barometer. There is a column of mercury in a glass tube in the barometer and as it rises or falls, we come to know that the weight of the atmosphere changes.
Option C is not correct.
Option D Turgor pressure: It acts within a cell. It is a force that pushes the plasma membrane against the cell wall. It is defined as the pressure measured by a fluid at a certain point within itself at equilibrium. It is also called hydrostatic pressure.
Option D is not correct.

Therefore, Option B is the correct answer.

Note:
Root pressure helps the plant to transport water at night. Other techniques of absorption play a minor role in transporting water in tall trees as these are slow processes and can’t make up the water lost by the process of transpiration. As transpiration is rapid during the day, it creates a suction force that pulls water up through the xylem vessels. Only this transpiration pull serves as the main force for the transport of water through xylem.