Transpiration: The Driving Force Behind Nutrient and Water Absorption

Transpiration is the word for describing the evaporative process of water from plant leaf surfaces. This process provides the driving force for the absorption of water and element ions into plant roots and then into the xylem vessels for upward distribution throughout the plant.WAb

How Transpiration Works

The rate and extent of evaporation of water from the plant leaf surfaces are determined by four external factors:

1. The vapor pressure deficient, which is the difference in water vapor content at the leaf surface and in the surrounding atmosphere
2. Air temperature
3. Wind movement over the leaf surface
4. Incident sunlight

Transpiration can be essentially stopped when plant leaves are surrounded by a water-saturated atmosphere (high relative humidity).

As for the plant, the rate of water loss from its leaf surfaces is correlated with:

• The water content in the leaves, as well as the whole plant
• The physical characteristics of the leaf surface (type of cuticle)
• The number of leaf stomata and whether they are open (which increases loss) or closed (which decreases loss). The amount of water evaporated from leaf surfaces is particularly and significantly affected by this.

The rate and extent of water absorption by plant roots will depend on these five physical conditions:

1. The root surface area in contact with the rooting medium
2. The quantity of water around the roots
3. The extent of aeration (oxygen level) around the roots
4. The salt content (EC level) of the water surrounding the root temperature
5. The physiochemical properties of the rooting medium (i.e. organic matter content, cation exchange capacity, texture and structure, etc.)

Also, the rate and extent of water absorption will be determined by the pull of water up the xylem vessel generated by the transpiration rate. The degree of this force can be measured by observing changes in the diameter of plant stems and tree trunks with changing atmospheric conditions and availability of water around the plant roots.

Only live, actively respiring, roots can absorb water, meaning that the roots must be in an aerobic (containing oxygen) environment. The plants will wilt when oxygen is insufficient. Plants will also wilt when the temperature of the rooting medium is less than the atmospheric temperature, particularly when the atmospheric demand is high. The optimum temperature range for optimum root function is between 68 to 86^oF.

Water moves up the plant in the xylem vessels, the rate and extent of movement dependent on the quantity of water being drawn into the roots and the rate and extent of transpiration occurring at the leaf surfaces. When the atmospheric demand is high, a plant will wilt when there is insufficient water being drawn into the plant through its roots.

With the absorption of water through the roots, the elements in solution are also carried into the root. In order for elemental absorption to occur, the elements must be in an ionic form. There exists a natural barrier (cell membranes) at the root interface that the element ions must traverse in order to be taken into the root cells to be then deposited into the xylem vessels. It is not well-understood how element ions transverse this barrier.

There are two theories, one being that there exists a carrier system that complexes with the element ions, and then the complex is carried through the barrier. The other theory proposes the existence of a biophysical-mechanical system, known as ion pumps, that provides the means for transference. Both of these proposed methods of ion transport require energy that is derived from root respiration.

Therefore, the roots must be alive and functioning in an aerobic atmosphere in order for elemental ion absorption to occur. There also exist both synergistic and competitive processes that will determine the extent of ion absorption based on electrical charge (anions versus cations) and level of charge (mono-, di- or trivalent).

Most molecules are not able to traverse cell membranes, although some have suggested that small molecules might be able to enter the root, possibly into the so-called free space that exists in plant root cells.

Water and nutrient movement within the plant is also related to fruit and plant disorders. Blossom end rot in tomato, for example, is triggered by water stress within the tomato plant; in particular, when insufficient quantities of calcium-carrying water are being delivered to the developing tomato fruit.

Water Stress

Magnesium deficiency is another disorder that is triggered by water stress. This deficiency occurs when the water level around the plant root is low over extended periods, and when this is coupled with cation competition favoring the absorption of potassium and calcium.

In summary, water and elemental ion absorption into plant roots and then their movement within the plant is driven by transpiration, which creates a vapor pressure gradient within the xylem vessels.

When faced with a possible plant nutrient element deficiency, its cause could be due to periods of water stress. Keeping the rooting medium supplied with sufficient water, as well as maintaining aerobic conditions within the rooting medium, will ensure plant nutrient element sufficiency.

That in turn results in sustained optimum plant growth. For the greenhouse grower, controlling the relative humidity in the greenhouse atmosphere is essential to ensure plant nutrient element sufficiency, as periods of high relative humidity will slow transpiration, which reduces the absorption of the essential plant nutrient elements than can lead to an insufficiency, thus resulting in poor plant growth and product yield.

There are two theories, one being that there exists a carrier system that complexes with the element ions, and then the complex is carried through the barrier. The other theory proposes the existence of a biophysical-mechanical system, known as ion pumps, that provides the means for transference. Both of these proposed methods of ion transport require energy that is derived from root respiration.

Therefore, the roots must be alive and functioning in an aerobic atmosphere in order for elemental ion absorption to occur. There also exist both synergistic and competitive processes that will determine the extent of ion absorption based on electrical charge (anions versus cations) and level of charge (mono-, di- or trivalent).

Most molecules are not able to traverse cell membranes, although some have suggested that small molecules might be able to enter the root, possibly into the so-called free space that exists in plant root cells.

Water and nutrient movement within the plant is also related to fruit and plant disorders. Blossom end rot in tomato, for example, is triggered by water stress within the tomato plant; in particular, when insufficient quantities of calcium-carrying water are being delivered to the developing tomato fruit. Magnesium deficiency is another disorder that is triggered by water stress.

This deficiency occurs when the water level around the plant root is low over extended periods, and when this is coupled with cation competition favoring the absorption of potassium and calcium. In summary, water and elemental ion absorption into plant roots and then their movement within the plant is driven by transpiration, which creates a vapor pressure gradient within the xylem vessels.

When faced with a possible plant nutrient element deficiency, its cause could be due to periods of water stress. Keeping the rooting medium supplied with sufficient water, as well as maintaining aerobic conditions within the rooting medium, will ensure plant nutrient element sufficiency.

That in turn results in sustained optimum plant growth. For the greenhouse grower, controlling the relative humidity in the greenhouse atmosphere is essential to ensure plant nutrient element sufficiency, as periods of high relative humidity will slow transpiration, which reduces the absorption of the essential plant nutrient elements than can lead to an insufficiency, thus resulting in poor plant growth and product yield.