Soil-Plant Atmosphere Continuum (SPAC)

This article provides notes on Soil-Plant Atmosphere Continuum (SPAC).

“Because water is generally free to move across the plant-soil, soil-atmosphere, and plant atmosphere interfaces it is necessary and desirable to view the water transfer system in the three domains of soil, plant and atmosphere as a whole…it must be pointed out that, as well as serving as a vehicle for water transfer, the SPAC is also a region of energy transfer”.

The concept of the soil, plant, atmosphere as a thermodynamic continuum (SPAC) for water transfer. A central, currently unresolved problem arises as a result of the SPAC concept. How can water move to the top of tall trees in a water continuum from soil to leaf surface without cavitation? A suction pump can lift water only to the height due to atmospheric pressure (1.0 atm = 10.33 m).

However, trees are taller than 10.33 m. To get water to the top of skyscrapers, standing tanks are used. In plants, there are no standing tanks, pumps (hearts), or valves that can move water up trees. At present, the cohesion theory is the theory generally accepted as the one that explains the way that water ascends in plants.

With the advent of the pressure probe, the necessary high tensions for the cohesion theory have not been found. Consequently, a new theory was put forward which postulates that solutes in the parenchyma cells of the tissue around the tracheary elements cause an imbibing of water, and this creates a pressure on them that prevents water cavitation.

However, the theory has been questioned both thermodynamically and anatomically. The challenging problem now is to use the SPAC concept on a fine scale and determine the water potential gradients in the xylem tissue to see how water can be at the top of tall trees.

Soil-atmosphere, and plant-atmosphere interfaces it is necessary and desirable to view the water transfer system in the three domains of soil, plant, and atmosphere as a whole.

Under some circumstances, and for some purposes, it may be possible to isolate certain parts of the total system and study only certain modes of water transfer; but a general appreciation of the plant water relations of the whole plant in nature must involve the soil-plant-atmosphere continuum (SPAC).

At present, the cohesion theory, or sap-tension theory, is the theory generally accepted as the one that explains most satisfactorily the way that water ascends in plants. The terms “sap” and “water in the xylem tissue” are used interchangeably. The fluid in the xylem tissue is not pure water, but a dilute aqueous solution.

Even in mangroves, which grow in salt water, the sap in the xylem tissue is very nearly salt free and lowers the temperature at which water freezes by < 0.1 °C. The cohesion theory assumes that diffusion of water from the non- collapsible xylem elements in contact with the leaf cells creates a state of tension within the water columns in the xylem vessels.

This tension is possible because of the cohesion of water molecules and their adhesion to the hydrophilic walls of the xylem elements. Tension in the water columns is assumed to lift water from the roots to the leaves, in addition to reducing the free energy of the water in the root xylem tissue until water diffuses from the soil into the root during absorption of the water.

The cohesion hypothesis assumes continuity of water columns, laterally and vertically, in the conducting elements of the xylem tissue. These water columns ultimately are placed under tensile strain most plant physiologist’s feel that the cohesion theory is probably the correct explanation for the rise of water in plants, it has limitations.

The main difficulty is that it postulates a system of potentially great instability and vulnerability. It is clear, however, that the water conducting system in plants must be both stable and invulnerable.

There are few following objections that have been raised by plant physiologists concerning the theory of cohesion:

1. The tensile strength of water is inadequate under the great tensions necessary to pull water to the top of plants, especially tall plants.

2. There is insufficient evidence for the existence of continuous water columns (that is, water columns under tension are not stable and cavitate).

3. It seems impossible to have tensive channels in the presence of free air bubbles, which can occur when trees in cold climates freeze and then thaw.

The challenging problem now is to use the SPAC theory on a fine scale and determine the water potential gradients in the xylem tissue to see the direction of movement of water.

Closure of the water budget for an ecosystem requires that precipitation and flows of water from neighbouring ecosystems be returned to the atmosphere through evapotranspiration, transferred to storage pools, or allowed to flow out of the system.

Transfer and storage of water creates capacitance in the liquid phase of the water cycle and delays the inevitable return of water vapour to the atmosphere; but, a globally-balanced water cycle requires that the molar equivalent of precipitated water be accounted for in the fractions stored in surface and subsurface reservoirs, plus that evaporated back to the atmosphere.

Recognizing that in terrestrial ecosystems a large fraction of precipitation is returned to the atmosphere through leaf transpiration, plants occur at an important interface between the liquid and vapour phases of the water cycle.

Water moves from soil into plants through viscous flow in the liquid phase, as it is ‘pulled’ by thermodynamic forces through roots, vascular tissues and leaf mesophyll cells, following negative pressure (tension) gradients.

Tension develops in the conduction tissues as water is evaporated faster from leaves than can be replaced by flow from the soil. Physical continuity within capillary ‘threads’ of the ascending water column is maintained by cohesive and adhesive forces that are facilitated by the electrostatic polarity of the water molecules. In the vicinity of stomata, water is evaporated to the atmosphere.

In the atmosphere, water is carried in the vapour phase to and from leaf and soil surfaces through diffusion near the surfaces and turbulent air motions in the well-mixed atmosphere.

Given the continuous nature of these water transfer paths, and their serial relation to one another, it was recognized early in the study of plant water relations that the ‘whole plant’ must be considered at the center of an integrated and articulated soil-plant-atmosphere continuum, or SPAC.