Water Balance in Plants

http://www.botany.hawaii.edu/faculty/Webb/BOT470/

PlantWatMove/WaterBalancePlants.htm

Importance of Water to PlantsWater is the major abundant constituent of all the plant cells that are physiologically active.

The water present in the plants is not static; constitutes a part of the hydrodynamic system : process of transpiration and evaporation water absorption translocation across the plant

Soil TypesSoil plays an important role in water retention and availability : sand and claySand: large particles (1mm) ; wide spaces (channels) in between with a relatively low surface area 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 : composed of minute particles (2 microns) with very narrow spaces between large proportion of surface area 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 – the moisture-holding capacity of soilSandy soil has a low Field Capacity while clay soils have a much greater 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. 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.

The adhesive properties of water greatly reduce the water potential and can 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.

When soil is at Field Capacity , Roots Absorb water from the immediate water pervades all of the environment, creating air pockets. This channels between soil particles. Is replaced by water present in the nearest larger channels.

In extremely dry soils, water is tightly bound in the smallest channels of the soil particles. It can't replace water removed by the roots & large air pockets are formed.

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

Sandy soil would have high Hydraulic Conductivity (HC) while clay soil would have a low HC. This is largely due to the diameter of the soil channels. The amount of water in the soil also affects (HC). Wet soils have high HC while dry soils have low HC. Air replaces water in soil channels and blocks the flow of water.

• It is possible to measure the Hydraulic Conductance of Roots.

• 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.

• These environments have been called "Physiological Desserts".

In extremely dry soils the Permanent Wilting Point (PWP) 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 Root Hairs –increase the absorptive area of the roots ; 60% of the total surface area

Movement of Water in Roots:1. Apoplastic – via the cell until the endodermis2. Symplastic – across the cells through the cell membrane

Water can enter the Symplast of the Root Hair and pass from one cell to the next via Plasmodesmata

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

• The water 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.

Water Transport Through the XylemTypes of Tracheary Elements:1. Tracheids- found in Gymnosperms;have overlapping end walls which are connected by pits ( regions with thin and porous primary walls and with no secondary walls). 2. Vessel Members /Elements- form Vessels which are constructed like a series of tube; have larger openings (Perforation Plates) on their end-wall; 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.

TracheidsThere 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.

Vessel Elements

SEM images of Vessel Members (Elements): Note the uneven lateral walls which reflect the presence of Pits.

Vessel Members from Oak showing lateral Pits and Simple Perforation Plates (Open End Wall).

* 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.

Diagram of a typical dicot leaf: The Apoplastic route of water molecules from the Xylem to the Mesophyll Cell Walls is illustrated by the blue lines.

Physiological Dilemna• 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.

Plants have developed physiological means to control stomatal opening and closing. 1. Epidermal Trichomes which increase the Boundary Layer moderate Transpiration rates. 2. Stomatal aperture- Temporal regulation 3. Regulating leaf temperatue by the presence of wax, etc., 4. Guard cell anatomy - thickened inner radial walls which are not completely joined; Cellulose Fibrils (CF) in the Guard cell Walls have a Radial orientation as seen from above

Factors which affect closing /opening of stomata:

• CO2 concentration, light intensity and color, temperature & relative humidity.

Various Processes that Affect the Water Balance in Plants1. Transpiration and the mechanism of Stomata: Types of transpiration: a. cuticular transpiration b. stomatal transpiration- contributes approximately 90% of the foliar transpiration2. Osmosis or the Cellular water relations Cytoplasmic lining of cell wall – semi-permeable membrane3. Translocation of water : ascent of sap’ mechanism & Root pressure 4. Absorption of water - occurs in the root tip regions, zone of root hair : water potential in the root cells at the periphery is lower than that of the soil water

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 & other plants. 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.

• Guttation occurs when the soil and atmosphere are saturated with water. Water secretion occurs through modified Stoma called Hydathodes. Root Pressure provides the motive force for this process.

Soil Hydraulic Conductivity and Water Potential

Conductivity measures the ease with which water moves through the soil. As water content (and hence the water potential) decreases, the hydraulic conductivity decreases drastically.