Test: Translocation, Symplastic & Apoplastic Pathways (Old NCERT) - ACT MCQ

Water moves from the xylem over the leaf to the air spaces by the apoplast and symplast and after that vanishes through the stomata (transpiration). It is the transpiration of water from leaves which is the primary main thrust for the development of water in xylem.

A common vital force theory about the ascent of sap was put forward by J.C. Bose (1923). It is called the pulsation theory. The theory believes that the innermost cortical cells of the root absorb water from the outer side and pump the same into xylem channels. So the correct option is 'Pulsation theory'.

The apoplast pathway comprises of non living components of a plant body i.e., intercellular spaces and cell wall. The movement through apoplast is unregulated and fast. Symplast comprises of living components i.e., protoplasm, cell membrane and plasmodesmata. Water potential or free energy of water depends on the number of water molecules in the system. More the number of water molecules greater will be water potential. When solutes are dissolved in water, there is a decrease in relative number of water molecules and this causes corresponding decrease in water potential. Membrane permeability depends on the nature of biological membrane and the nature of solutes. The membrane is composed of phospholipids and proteins. Consequently lipophilic substances can freely pass through the membrane, while hydrophilic substances cannot freely pass through the membrane and require special carrier or transport proteins for transport across the membrane.

Munch explained the translocation of the organic food material through the phloem tissue. This flow occurs along the gradient of the turgor pressure from a region of higher solute concentration to a region of lower solute concentration.

Apoplast is the route the water moves through the cell walls and intercellular space of the root cortex. In the symplastic route, the water moves through the protoplasts of the root cortex. The apoplast route is the fully permeable route in which the water movement occurs in passive diffusion.

Dixon and Jolly gave transpiration pull theory for ascent of sap. According to this theory, ascent of sap in plants is a passive physical process, which occurs under the influence of transpiration pull generated due to loss of water from aerial parts. Other than the pull, a continuous column of water is also essential for rise of water in xylem elements. The continuous column of water is maintained by cohesion forces between water molecules and adhesive forces between water molecules and vascular elements of the plant.

Pinus seeds cannot germinate without mycorrhizae due to lack of water as because the mycorrhizal symbiotic association
helps the plant in formation of root.