The transpiration pull is just one of the mechanisms that explain the movement or translocation of water in plants, particularly water ascent in tall trees.
There is no single exacting explanation as yet for the ascent of water but several theories have been proposed.
Of these, the one which has gained wide support is the cohesion-tension theory which recognizes the crucial role of transpiration pull as a driving force. These theories are briefly described below.
Transpiration Pull and Other Theories Explaining the Ascent of Water in Plants
1. Capillarity Theory
According to this theory, water is translocated because water molecules adhere to the surfaces of small, or capillary, tubes.
This adhesion causes water to somewhat “creep” upward along the sides of xylem elements.
In glass tubes, this upward movement is visible as the curved or crescent-shaped (concave) meniscus.
However, it was shown that capillarity (or capillary rise) alone in tubes of similar diameter as that of a xylem element raises water less than 1 meter (Moore et al. 2003).
Hopkins (1999) explained that the relationship between the rise of water in a capillary tube and the size of the tube is inversely proportional.
This means that the thinner is the tube, the higher will be the rise of water.
Thus in a large tracheid or small vessel having a diameter of 50 µm, water will rise about 0.6 m high.
But in a large vessel in which diameter is about 100 µm, water will rise to a height of only 0.08 m.
To reach the top of a 100-meter tree, the capillary tube must have a diameter of about 0.15 µm.
This renders capillarity as insignificant for the rise of water in tall trees because the smallest tracheids in existence are much bigger.
2. Atmospheric Pressure Theory
This is based on the observation that normal atmospheric pressure is able to push water in a tube upward up to about 10.4 meters.
This is demonstrated by first filling with water a long tube with one end closed.
This tube is then placed with its open end down in a tub of water.
The force of gravity will tend to pull the water in the tube downward, but atmospheric pressure exerted on the water surface in the tub will push it up.
These opposing pressures equilibrate when the height of the water column in the tube is 10.4 m (Moore et al. 2003).
However, as with capillarity, this cannot explain how water is able to reach the tops of tall trees.
The normal atmospheric pressure, or 1 atm, is equivalent to about 101 kilopascals (kPa) or 0.1 megapascals (MPa).
But Hopkins (1999) explained that 10 to 15 times of this pressure, or 1.0 to 1.5 MPa, is required to push water to the tops of trees 100 m to 150 m tall.
The author further enlightened that to overcome resistance (or friction) along with the xylem tissue due to structural irregularities and the like, a total pressure of 2.0 to 3.0 MPa would be needed.
3. Root Pressure Theory
This explains the exudation of sap from the stumps of decapitated or dropped plants including those of trees that were newly felled.
It also accounts for guttation under conditions that favor mineral and water absorption but are unfavorable to transpiration.
As the term implies, this mechanism of water ascent involves the participation of live roots.
In this regard, it is considered an active process because live cells are involved in the absorption of mineral salts.
However, the root pressure that is created is due to an osmotic gradient, considered passive.
The accumulation of salts (solutes) in the apoplast which surrounds the xylem elements decreases the water potential of the xylem and causes water from the surrounding cells to move into them (Devlin 1975; Hopkins 1999; Moore et al. 2003).
However, there are contrasting views against root pressure being the primary mechanism for the ascent of water in plants.
Devlin (1975) enumerated the following arguments:
(1) the magnitude of pressure developed is either very insignificant to be able to “push” water to the tops of tall trees or, in most conifers, absent;
(2) data supporting water ascent by root pressure were generated without considering friction which could affect the flow of water in the xylem ducts;
(3) exudation of xylem sap generally occurs at lower rates than transpiration;
and (4) under normal conditions, the xylem sap is under tension (pulled) rather than pressure (pushed).
4. Cohesion-Tension or Transpiration-Cohesion Theory
This explains that the upward movement of water is mainly due to the creation of a negative force or tension attributed to the continuous evaporation of water at the surfaces of leaves in the process of transpiration.
As molecule after molecule of water evaporates through the stomata, it creates a pulling action on the next molecules of water in the transpiration stream.
This pulling force, otherwise called transpiration pull, is strong enough to overcome the force of gravity which is responsible for the tendency of water to move downward.
The transpiration pull is similar to the suction force when drinking some fluid from a bottle or glass with a straw.
Water can also be sucked into a pipette with the use of an ordinary rubber aspirator or with a common medicine dropper.
However, the transpiration pull alone will not be sufficient to move water upward.
The cohesion or the attraction of one molecule to another molecule of water through hydrogen bonding ensures that water moves in an unbroken, continuous column.