in the soil water. This process is called passive absorption, as the absorption can occur independently of any activity
of roots. The rate of water absorption is approximately equal to the rate of transpiration. Passive water absorption is
regulated by transpiration.
Root hairs function as tiny osmotic systems. Water potential is negative, due to presence of osmotically active cell
sap in central vacuole of root hair cell. Higher water potential in soil water (–0.1 to –0.3 bars) than root hairs (–3 to –
bars) results in movement of water into root hair cell. It passes into the apoplast and symplast or cortical, endodermal
and pericycle cells, and enters the xylem channels passively. a gradient of water potential exists between root hair cell,
cortical cell, endodermal, pericycle and xylem channels, so that flow of water is not interrupted.
When the water is absorbed due to the activity of root itself, particularly root hairs, it is termed active absorption.
The first step in the absorption of water is its imbibition on the cell wall surface. The osmotic potential (or osmotic pressure)
of the soil solution plays a very important role in the absorption of water.
The water molecules move from the side of higher water potential towards the side of lower water potential. The soil
solution, having less OP, has greater water potential. On the other hand, the cell sap with comparatively higher OP
has more negative water potential. Thus, the water enters into the cell sap due to water potential gradient. Water
continues to enter the root hair cells and to a lesser extent into other epidermal cells. Water moves from cell to cell, along
the concentration gradient (or water potential gradient), and finally reaches upto the endodermis and pericycle. finally,
water enters into the xylem elements and is translocated upwards. This theory involves symplast movement of water.
Root pressure is a manifestation of active water absorption.
Pathways of water movement
The responsibility of absorption of water and minerals is more specifically the function of the root hairs, that are present
in millions, at the tips of the roots. Once, water is absorbed by the root hairs, it can move deeper into root layers by two
distinct pathways:
● Apoplast pathway
● Symplast pathway
The apoplast is the system of adjacent cell walls, that is
continuous throughout the plant, except at the casparian
strips of the endodermis in the roots. The apoplastic
movement of water occurs exclusively through the
intercellular spaces and the wall of the cells.
Movement through the apoplast does not involve crossing the cell membrane. This movement is dependent on the gradient.
The apoplast does not provide any barrier to water movement and water movement is through mass flow.
as water evaporates into the intercellular spaces or the
atmosphere, tension develops in the continuous
stream of water in the apoplast; hence, the mass flow
of water occurs due to the adhesive and cohesive
properties of water. The symplastic system is the system
of interconnected protoplasts. Neighbouring cells are
connected through cytoplasmic strands that extend
through plasmodesmata.
During symplastic movement, the water travels through
the cells- through their cytoplasm; while intercellular
movement is through the plasmodesmata. Water has to
enter the cells through the cell membrane, hence, the
movement is relatively slower. Movement is again down
a potential gradient.