Question

Root pressure is maximum,when
A. Transpiration is high and absorption is very low
B. Transpiration is very low and absorption is high
C. Absorption is very high and transpiration is also very high
D. Absorption is very low and transpiration is also very low

Answer 1

Hint: Transpiration is the process in which water vapour is evaporated through stomata. Absorption is the process in which water is absorbed through roots.

Complete answer:
In transpiration water will move from the roots to its aerial parts and escape through leaves, flowers and stems along with the diffusion of carbon dioxide through stomatal apertures. Transpiration also becomes the reason for the cooling down of plants and it brings about a change in osmotic pressure which will result in flow of nutrients and water from root to aerial parts of plants. This action depends upon pressure gradient. For water to move upwards the pressure at bottom should be high and top should be low.
Absorption is the process of absorbing water and nutrients from the soil through roots. This process is characterised by diffusion,imbibition,osmosis,active transport etc.The absorbed water and nutrients will create a pressure on the roots ,thus positive pressure is known as root pressure. When these water and nutrients are stored in the xylem sap the cell in the tissues will put pressure on the cell walls thereby increasing the pressure. When transpiration is high these water is lost into the atmosphere resulting in decrease of the pressure. But when the rate is low the water and nutrients will be stored in the roots itself thus the root pressure will remain high. Now let’s analyse the options to find the answer.

Option A)Root pressure is minimum when transpiration is high and absorption is low.That is when the output is low and input is high. Therefore this is an incorrect option
Option B) Root pressure is maximum when transpiration is low and absorption is high. Therefore this is the correct option
Option C) Root pressure is maximum when absorption is high but not when transpiration is high. So this is an incorrect option
Option D) Root pressure is maximum when transpiration is low but not when absorption is low. So this is an incorrect option

Hence, the correct answer is option (B)

Note: Primarily the root pressure is developed through osmotic pressure and it is demonstrated with the secretion of fluids by cutting off the stem just above the ground level. It helps the movement of spa against the gravitational force.
The absorption of minerals is active whereas the absorption of water is passive due to the drop of osmotic potential when the xylem sap has more nutrient concentration.