Transpiration  

Soil Water

Soil Types play an important role in water retention and availability. The two extremes are sand and clay.

Sand contains large particles (1mm) and wide spaces (channels) between them. There is a relatively low surface area, as well.

Water moves rapidly through sand channels such that only a thin film remains attached to the sand particles. The channel is largely filled with air.

Clay is composed of minute particles (2 microns) with very narrow spaces between them. It has a large proportion of surface area compared to other soil types. The presence of organic matter in clay soils leads to the formation of solid "crumbs". These improve soil aeration and water penetration. Water is retained by the narrow channels of clay particles and it is held more tightly than other soils.

Field Capacity expresses the amount of water left in the soil after it has been supersaturated and allowed to completely drain. Sandy soil has a low Field Capacity while clay soils have a much greater Field Capacity.

The presence of organic matter (humus) increases the field capacity.

Soil Water does not usually contain a lot of dissolved materials. Consequently it has a high Water Potential. Saline soils are an obvious exception to this rule.

The Hydrostatic Pressure in wet soil is near ZERO!

As soils dry their water potential decreases. This is largely due to the physical properties of water and their interactions with soil particles.

Water evaporates first from the center of any meniscus that forms in soil channels. As soil dries, water is replaced by air. The soil particles are lined with a thin coat of water that clings to their surface.This surface coat of water is held tenaciously by the adhesive properties of water. The smaller channels hold water better than larger ones.

As soil dries, water recedes into the smallest channels present. These can have extremely small diameters. Consequently, the adhesive properties of water greatly reduce the water potential and make it unavailable for plants.This may reach -2.0 MPa (MegaPascals). Remember that the water potential of Pure Water is 0! Consequently, water potentials in nature are usually negative.

Soil Water moves by Bulk Flow. Plants deplete the local levels of water that are immediately adjacent to absorbing roots. This lowers the proximal water potential. If more distal areas of soil have greater water potential, water will flow through soil channels towards the roots. This is possible because the soil channels form an interconnected system. This is an example of Bulk Flow because water and everything dissolved in it moves from an area of high water potential to one of low water potential.

Soil Hydraulic Conductivity represents the ability of water to move through soil.

In extremely dry soils the Permanent Wilting Point may occur. This is the water potential at which plant cells loose their turgor pressure and can't regain it even when transpiration ceases. Wilting is the visible symptom of PWP. This means that the water potential of soil water is less than that in the roots.

Roots and Water Absorption

Most plants produce Root Hairs which greatly increase the absorptive area of the root. They may account for as much as 60% of the root surface area! In most cases there is a clearly defined Root Hair Zone which occurs at a constant distance from the growing apex.

Remember that the Cell Wall is part of the Apoplast. Water can be absorbed via the Cell Wall and stay in the Apoplast until it gets to the Endodermis where it must cross the Plasmalemma and enter the Symplast.

Alternatively, water can enter the Symplast of the Root Hair and pass from one cell to the next via Plasmodesmata.

The relative importance of these two pathways has not been determined in most cases. However, the Apoplastic route was more important with corn roots.

It is possible to measure the Hydraulic Conductance of Root. This decreases at low temperatures or after exposure to inhibitors of respiration. This indicates that there are physiological mechanisms which help to regulate these processes.

Roots grown in waterlogged solid have poor Hydraulic Conductance. This is due to anaerobic conditions which inhibit respiration. Consequently, plants that grow in swamps and bogs may have Xeromorphic (Dry Form) leaf and stem traits.

Link to Leaf Adaptations 

Water can stay in the Symplast until it reaches the Xylem or it may pass from the Endodermis into the Apoplast of the Pericycle.

Link to Endodermis

The conducting cells (Tracheary Elements) of the Xylem are part of the Apoplast because they do not have intact Protoplasts. In most cases they are free of debris and can be treated like pipes. However, Pits make the lateral walls uneven and this affects the passage of water through Tracheary Elements.

There may be intact remnants of the Primary Wall & Middle Lamella between opposite Pits. This has been called the "Pit Membrane". This is an unfortunate name because it can be confused with a true biological membrane like the Plasmalemma or Tonoplast. The "Pit Membrane" contains cellulose & middle lamella and is generally very porous.

Pit Membranes from the tracheids of Gymnosperms like Pine have a thick, impermeable center which is called the Torus. This acts like a valve and can open or close lateral transport between Tracheids. Most Gymnosperms are adapted to grow in challenging environments with protracted dry periods. The ability to definitively block lateral transport could prevent the spread of embolisms. Embolisms occur when a large air bubble displaces water and occupies the Tracheary Element. This blocks transport and can spread to adjacent Tracheary Elements. This can be disastrous in terms of water movement in the Xylem. Mechanisms which prevent embolisms are very important. The Torus represents one of these.

If you were drinking a milkshake with a plastic straw and your buddy stuck a pin in the straw, you would not be able to drink the rest of the shake with that straw. This is the way an embolism works in the xylem.

Tracheids have overlapping end walls which are connected by Pits.

Vessel Members (Elements) are constructed like a series of tubes. These have larger openings (Perforation Plates) on their end-walls. These have less resistance than Pits and facilitate longitudinal transport. The most advanced Vessel Members have No Endwalls (Simple Perforation Plates). They are generally wider than Tracheids and are more specialized for water transport.

Root Pressure

Root pressure is a positive hydrostatic pressure that develops in roots. When a lawn is extremely well watered & the relative humidity is high, Guttation can occur. Guttation produces dew-like drops of water that emerge from the tips of some grasses. Modified Stomata called Hydathodes are the sites of water exudation. The driving force for this is Root Pressure. This may help to distribute important minerals when trnapiration rates are low.

The water that enters the root is usually dilute. The Plasmalemma of the Endodermis has specific carrier proteins which can preferentially accumulate and concentrate solute so that the water potential in the Xylem is lower than that in the Cortex. Water will move from the Cortex into the Xylem. This will cause a positive Pressure.

Root Pressure is responsible for the harvesting of Maple Syrup in the Spring. Sugars in the Root's Stele lower its water potential. Consequently, water from the soil is absorbed and creates a positive pressure in the Xylem. This makes the sap rise so that it can be tapped and collected.

Water movement through the Xylem has a peak velocity of approximately 30 meters/hour. Water movement through living cells requires a driving force which is 10,000,000,000 times greater than through Vessel Members. This emphasizes the adaptive significance of Tracheary Elements.

Tracheary Elements (Tracheids & Vessel Members) have anatomical traits that are necessary for the vertical translocation of water in plants. In the "Bad Old Days" straws were made of wax paper. As the paper got wet, it weakened and collapsed. Nothing came through despite extreme sucking. Presently, we use plastic straws that don't get wet and don't collapse. The thick lignified walls of Tracheary Elements prevent their collapse under the tension that develops during water translocation.

It has been calculated that the amount of pressure required to move water to the top of a 100 m tree is approximately 3 MegaPascals (MPs). In order to do this a Negative Pressure or Tension must develop in the Xylem.

Root Pressure is usually less than 0.1 MPs. This is clearly insufficient to move water to the top of a tall tree.

The pulling force for water translocation comes from Transpiration in the Leaves. The prevailing theory that is used to describe this phenomenon is called the "Cohesion-Tension theory of sap ascent".

Leaf Anatomy & Transpiration

Water is brought to leaves in the xylem that is present in the veins. Most cells are no more than 0.5 mm away from a minor vein. Water is transferred to the Protoplasts & Walls of Mesophyll Cells. Water evaporates from the cell walls until the atmosphere inside the leaf is saturated with water molecules. You should recall that soil water is held tenaciously within the capillaries between minute soil particles.

Plant Cell Walls are made of Cellulose Strands. The capillaries between these are microscopic. Consequently, they dramatically lower the water potential of water molecules associated with them. As a leaf dries, the strong tension that develops in the cell walls is sufficient to provide the 3 MPs that are needed to pull water to the top of the tree.

Water molecules diffuse rapidly in the atmosphere. The term Water Vapor is applied to water molecules in the gaseous phase. Leaves need to open their stomata to let CO2 diffuse inside because CO2 levels are higher in the outside atmosphere than inside the leaf. However, CO2 levels in the atmosphere are far lower than the concentration of water molecules. 

Water molecules are far more concentrated inside the leaf than outside. Consequently, when stomata are open, water molecules rapidly pass through the Stomatal Pore to the outer atmosphere. This creates a physiological dilemma for the leaves. If the stomata remain open too long, they will wilt and possibly die. Plants have developed physiological means to control stomatal opening and closing.

The Boundary Layer is the zone of unstirred air that lies immediately outside the Epidermis. This is a significant factor which affects the rate of water loss from the leaf. Factors like Epidermal Trichomes which increase the Boundary Layer moderate Transpiration rates.

The difference in water potential between the outside atmosphere and the intercellular spaces inside the leaf are always great enough to cause the movement of water molecules from the leaf into the air. This is true even at high relative humidities. There is a 100 fold difference in external and internal water potential at 25 C.

The general solution to the conflict between CO2 uptake and Water Loss via Transpiration is by the Temporal Regulation of Stomatal Apertures (SA). The most simple SA is composed of two Guard Cells. However, Subsidiary Cells may also be part of the SA. The term "Stoma" is currently used for the entire SA. The opening that is created between the Guard Cells is called the Stomatal Pore.

Temporal Regulation of Stomatal Activity

Stomata open during the early morning when the water potential of the leaf is high, the temperature is low and the water holding capacity of the air is Low.

Stomata close during midday when leaf water potential becomes low, temperatures rise and the water holding capacity of the air increases.

Stomata reopen in the late afternoon when the water potential of the leaf has recovered and lower temperatures prevail. The water deficit of the leaves is restored during the night when the stomata are closed.

Guard Cell Anatomy

Guard Cells have anatomical features which are related to their function. We will consider the Guard Cells that are typical for Dicots like Beans or Peas. Grasses have a different Guard Cell structure which is a little harder to grasp.

The individual Guard Cells resemble a Kidney Bean in Shape. They have thickened inner radial walls which are not completely joined. This part of the cell wall can have Ledges which project from the top or bottom. These create microenvironments which can reduce the rate of water loss during Transpiration. Despite the thickness of the Cell Wall, this part of the wall can retract so that a Stomatal Pore appears.

The outer Radial walls of the Guard Cells are usually thinner. These have an interface with Subsidiary Cells or Epidermal Cells.

The Guard Cells have Chloroplasts but other Epidermal Cells have small, translucent Leucoplasts.

The Cellulose Fibrils (CF) in the Guard cell Walls have a Radial orientation as seen from above. This has an important bearing on their function. The spaces between the CF are small near the inner radial walls but are wide near the outer radial walls.

A cell with parallel CFs would enlarge evenly. If the CF were close together there would be little enlargement. Cell enlargement would be greater if the CF were more widely spaced. The CFs in Guard Cells have an asymmetric organization because they are closely spaced at the inner radial wall but widely spaced at the outer radial wall.

If a Guard Cell enlarges, the outer radial walls can expand but the inner radial walls can not. As the outer radial walls enlarge, the inner radial walls are pulled apart. This opens the Stomatal Pore. If the Guard Cells shrink, the Stomatal Pore would close.

Turgor Pressure regulates the opening and closing of the Stomatal Pore. There are many factors which regulate this process. These include CO2 concentration, light intensity and color, temperature & relative humidity.

Following a dark period, stomata open in response to light. Light induces an influx of Potassium ions from adjacent cells to the Guard Cells. It also stimulates production of an organic acid (malate) and accumulation of sucrose. This lowers the Water Potential of the Guard Cells & this causes an influx of Water. This increases the Turgor Pressure of the Guard Cells. The Inner Radial Walls can expand due to the loose organization of their Cellulose Fibrils.

The Inner Radial Walls can not expand due to the tight organization of their Cellulose Fibrils and the overall thickness of these walls. They are deformed by the volume increase of the Guard cells, and they pull apart to form the Stomatal Pore. Guard Cells can increase their volume by 40 - 100%!

Whole Plant Overview

The uptake and translocation of water by plants is complex and involves bulk flow, osmosis and diffusion. However, it becomes very simple if we look at the main compartments that are involved in terms of their Water Potential.

Water moves down a gradient from high to low water potential. Water Potential is expressed in negative numbers because pure water has a WP of 0.0. The unit of measurement is MegaPascals MPa.

The illustration above depicts the WP of the Soil and the main compartments of a large Plant. You can see that there is a gradient of decreasing water potential from the soil, through the plant and into the atmosphere.