Root pressure is measured by:
(a) Photometer
(b)Barometer
(c) Manometer
(d)Auxanometer

Hint: This instrument contains a mercury-filled glass column .it is used by placing the instrument on a cut at the stem of the plant to measure the root pressure.

Complete answer:
The root pressure is measured by an instrument called a manometer. The osmotic pressure within the cells of a root system that causes sap or fluid to rise through a plant stem to the leaves is known as root pressure which occurs in the xylem of vascular plants when the moisture level of the soil is high at night or day time when transpiration is low.

Additional Information: -Root pressure is developed by osmotic pressure in the roots cell.
-When the stem is cut off exudation of fluid occurs. Then the manometer is placed there to measure the root pressure. The cut in the stem is made just above the ground.
-A Photometer is used to measure the rate of uptake of water by the leaf from the shoot that is equal to the amount of water loss through transpiration.
-A barometer is used to measure atmospheric pressure.
-An auxanometer is used to measure the rate of growth of plants.
So the correct answer is,’ Manometer’.

Note: -The root pressure theory was first given by Priestley in the year 1916.
-Otto Von Guericke discovered the manometer in the year 1661.
-The root pressure is caused by the active distribution of different mineral nutrient ions into the root xylem.