Question

Root pressure theory of ascent of sap is unacceptable because
A) Water can ascend without root or root pressure
B) Root pressure cannot explain the ascent of sap beyond 10 meters.
C) Root pressure is more during early morning than afternoon.
D) Root pressure does not occur in spring.

Answer 1

Hint: Root pressure is known or defined as hydrostatic pressure, which is developed in the root for the accumulation of absorbed water. If the pressure is applied to the liquid contents of the cortical cells of the roots, under fully turgid conditions this root pressure pushes the water up the xylem vessels to the aerial parts. So the water absorption is an active process.

Complete answer:
The theory was put forward by Priestley (1916). Root pressure is a positive pressure that evolves in the xylem sap of the root of some plants. It is an expression of active water absorption. Water can ascend without root or root pressure -
 This theory is unacceptable because the ascent of sap was also noticed in plants in which roots were removed.
Root pressure is not common among trees of the temperate zone and occurs mainly in the springtime.

The correct option is (A) – Water can ascend without root or root pressure.

Additional information: At the time of root pressure, water passes by the root tissues freely, but the minerals do not (because the root acts as a semi-permeable barrier). As per the natural phenomenon or theory of osmosis, the water molecules naturally move from the area of low mineral concentration zone to the area of high mineral concentration zone, and this type of flow of water into the root pressurizes it. This process of osmosis is very regular in all other animal and plant cells.

Note:
It is maximum during the rainy season in tropical countries and during spring in temperate habitats. The amount of root pressure generally met in plants is 1-2 bars or atmospheres. Higher values (e.g., 5-10 atm) are also observed occasionally. Root pressure is retarded or becomes not present under starvation conditions, low temperature, drought, and reduced availability of oxygen.