Water Potential PDF

Whole Plant Water Relations =========================== **Water Potential** - **the free energy needed for the transport of a water molecule in a system** - **represented by psi, ψ** - **a relative measurement in the sense that it is relative to water of pure water at atmospheric p...

Whole Plant Water Relations =========================== **Water Potential** - **the free energy needed for the transport of a water molecule in a system** - **represented by psi, ψ** - **a relative measurement in the sense that it is relative to water of pure water at atmospheric pressure and temperature** - **expressed in units of pressure, megapascal (Mpa, where 10 Mpa= 1 bar= 1 atm, a useful parameter in evaluationg the water status of plants** - **0 in chemically pure water** - **0 is the maximum value and decreases in aqueous solutions where water is physically or chemically bound** - **In cell, its value is negative since solute molecules in all living cells lower the free energy of water.** - **Minus sign indicates that dissolved solutes reduce the ψ of a solution by reducing the concentration of water.** **Chemical Potential** - **the chemical activity of any subsatance in terms of free energy per mole of the substance** - **Plant cells must be sufficiently saturated with water for plant functions and processes to take place.** - **The hydration capacity of the cell is determined not by the absolute amount of water in the cell but by the chemical potential of water.** **Water Movement Between Cells** - **through osmosis which is a spontaneous process where movement from a region of high ψ to a region of low ψ involves no work, free energy is released** **Osmotic Pressure** - **produced during osmosis as water enters freely, diluting the cell contents and membrane obstructs passage of solute molecules** **Turgor Pressure** - **pressure exerted by the protoplast equally but opposite to the pressure by cell wall** - **pressure potential arises because plant cells are surrounded by a rigid cell wall that restricts swelling of the protoplast as water enters by osmosis** - **The cell is at incipient plasmolysis when cell volume is minimum and the protoplast barely touches the cell wall.** - **When cells are plasmolyzed, the protoplast shrinks away from the wall.** - **At maximum turgor, water cannot anymore enter the cell; this state can be only achieved if the cell is placed in pure water, ψ gradient or difference is 0 since ψ both outside and inside the cells are 0.** **Hypertonic Solution** - **a cell is surrounded by a solution of greater concentration than the cell sap** - **leads to plasmolysis** **Hypotonic Solution** - **a cell is placed in a solution of much lower concentration** - **protoplast returns to the original position, a phenomenon called deplasmolysis** **Isotonic Solution** - **cell sap is of the same concentration as that of the external solution** **Water Potential Gradient (** **ψ)** - **driving force for the movement of water** - **water always moves down gradients of ψ** **Water Movement from Soil to Root Xylem** - **Water is absorbed from the soil to the root system along a ψ gradient that exists between the absorptive surface of the root hairs (rhizines) and the thin layer of soil surrounding them** - **ψ cell is never higher than ψ soil** - **From rhizine cells, water is absorbed by neighbor cortical cells, radially crossing the root through apoplast or symplast** - **At the endoderms, the apoplast is blocked by the casparian strip, thus, water has to cross the plasma membrane of the endodermal cells to reach the vascular tissues** **The rate of water uptake by the root system depends on:** 1. **development of total absorptive surface or rhizines** 2. **rate of respiration of rhizines that increased rate lowers solute concentration, therebynegatively affecting water uptake** 3. **osmotic potential of root hairs, it should never be higher than of ψ soil** 4. **temperature such that 0-10⁰C usually slows down uptake** 5. **lack of O~2~ which can stop absorption as well as growth of whole plant (optimum content of O~2~ is 10-12%)** 6. **CO~2~ content such that too low or too high inhibits or even stops water uptake (CO~2~ soil content ranges from 5-15%)** 7. **Vegetative profile like the rooting layer** - **The build-up of solutes in the xylem sap leads to an increase in xylem**  **(osmotic pressure) resulting in decrease in xylem ψ** - **This lowering of ψ provides the driving force for water absorption leading to a + hydrostatic pressure, referred to as root pressure in the xylem in response to solute accumulation** **Guttation** **- exhalation of liquid along margins of leaves** **- exhibited by plants that develop root pressure** **- induced by high relative humidity especially at night or in well-hydrated plants** **Water Movement from Root Xylem to Leaf Xylem** **Transpiration Stream** - **movement up the xylem roots to shoots through dead lignified vessels and tracheids when plants are transpiring** - **Water flow is by mass flow or bulk flow because liquid and dissolved or suspended substances in it move together** **Transpiration-Cohesion-Adhesion Theory** - **Explains how negative pressure (tension) gradients arise in xylem of transpiring plants and pull water column to treetops** 1. **Evaporation of water water from cell walls into the air spaces of a leaf and from the leaf into the atmosphere (transpiration) sets up a gradient of ψ not only between living cells but also along cell walls.** 2. **The high surface tension of water means that the walls don't dry out but remain permanently wetted so that due to cohesion (mutual attraction between water molecules) and adhesion (attraction of water to a solid phase such as cell wall). Water flows along ultimately from root xylem into the leaf veins to replace that which is lost.** 3. **Water removal from leaf xylem creates negative pressure- a pulling force that drives the transpiration stream. Tension is relayed to root cells resulting to water uptake.** **Cavitation** - **filling up of xylem conduits with water vapor** - **water column under tension in xylem can be unstable such that if the pressure falls too low as when sudden jarring occurs, water will vaporize locally** - **there is formation of air bubble or embolism which blocks water movement** **Causes of Cavitation and Embolism** 1. **water stress associated with high rates of transpiration and low xylem pressure particularly in leaves and small branches** 2. **freezing of xylem leading to extensive formation of air bubbles** 3. **pathogens may relase compounds such as oxalic acid that lower surface tension facilitating air seeding to pit membranes** **Transpiration** - **the loss of water from the plant in the form of water vapor** - **the largest proportion of water vapor by far (more than 90%) escapes from leaves.** **Factors Affecting Transpiration** **1. Relative Humidity** **- the percentage of water vapour present in the air at a given time and temperature relative to the amount required to be present to make the air saturated at that temperature** **- the rate of transpiration is inversely proportional to the relative humidity, i.e., the rate of transpiration is higher when the relative humidity is lower and lower when the relative humidity is higher.** **2. Atmospheric Temperature** **- increased temperature opens stomata even in darkness** **- Besides producing a heating effect, it lowers the relative humidity of the air and increases vapour pressure inside transpiring organ.** **- For 10°C rise in atmospheric temperature vapour pressure inside leaves doubles while relative humidity decreases by 50%. Consequently, rate of transpiration increases.** **3. Light** **- In the majority of plants stomata open in the presence of light and close in darkness.** **- Because most of the transpiration occurs through stomata, the rate of transpiration is quite high in light. It falls down appreciably in the darkness.** **4. Air Movements (Wind)** **- Transpiration is lower in the still air because water vapours accumulate around the transpiring organs and reduce the DPD of the air.** **- The movement of the air increases the rate of transpiration by removing the saturated air around the leaves.** **- Up to 20-30 km/hr the rate of transpiration increases with the wind velocity. A wind velocity of 40-50 km/hr decreases transpiration by closing the stomata due to mechanical effect, drying and cooling of the transpiring organs.** **5. Atmospheric Pressure** **- Low atmospheric pressure enhances evaporation, produces air currents and increases the rate of transpiration.** **6. Availability of Water** **- The rate of transpiration depends upon the rate of absorption of soil water by roots.** **- This is further influenced by a number of soil factors like soil water, soil particles, soil temperature, soil air, etc.** **- A decrease in water uptake by the root causes partial dehydration of the leaf cells resulting in closure of stomata and wilting.** **Wilting** **- the loss of turgidity of leaves and other soft aerial parts of a plant causing their drooping, folding and rolling.** **- symptoms of wilting are not shown by thick-walled tissues** **Wilting is of three types:** **(i) Incipient Wilting** **There are no external symptoms of wilting but the mesophyll cells have lost sufficient water due to transpiration being higher than the availabil­ity of water. It occurs during midday for a brief period in almost all plants even when sufficient water is present in the soil.** **(ii) Temporary Wilting (Transient Wilting)** **It is the temporary drooping down of leaves and young shoots due to loss of turgidity during noon. At this time the rate of transpiration is maximum.** **The rate of water absorption is less due to shrinkage of roots and depletion of water around the root hairs. Lower leaves show wilting earlier than the upper ones. Temporary wilting is corrected only after the rate of transpiration decreases in the afternoon accompanied by replenishment of water around the root hairs.** **(iii) Permanent Wilting** **A permanent wilting is that state in the loss of turgidity of leaves when they do not regain their turgidity even on being placed in a saturated atmosphere. It occurs when the soil is unable to meet the requirement of plant for transpiration.** **Water is present in the soil largely in unavailable form (echard). At permanent wilting percentage (PWP) or coefficient (PWC) the soil contains 10-15% water depending upon its texture (about 10% in loam soil). After permanent wilting the plant dies.** **7. Leaf Area (Transpiring Area)** **- large leaf area increases transpiration** **- However, the rate of transpiration per unit leaf area decreases in a canopy due to density of foliage, shading effect and decrease of air movement inside the canopy.** **8. Leaf Structure** **(a) Thickness of Cuticle:** **Cuticular transpiration decreases with the thickness of cu­ticle and cutinisation of epidermal walls.** **(b) Number and Position of Stomata** **Because most of the transpiration takes place through the stomata, their number influences the rate of transpiration. Most dicots have stomata restricted to lower surface while the isobilateral monocot leaves possess equal number of stomata on both the surfaces.** **(c) Sunken Stomata** **The sunken stomata are device to reduce the rate of transpiration by providing an area where little air movement occurs.** **(d) Stationary Layer and Hair** **The hair insulates the surface of the leaf from air currents and air temperature. They hold a stationary layer of air (also called boundary layer). The thicker the boundary or stationary layer, the lower is the rate of transpiration. It is because the leaf will first lose water to stationary layer and from there it would travel to the outside.** **(e) Mesophyll** **Compact mesophyll (as having more of palisade tissue and fewer intercellular spaces) reduces transpiration while a loose mesophyll (having more of spongy tissue and larger intercellular spaces) increases transpiration.** **(f) Leaf Modifications** **Formation of prickles, leaf spines, scaly leaves, phyllodes, phylloclades (instead of leaves), are all modifications found in xerophytes to reduce transpi­ration. In xerophytes the leaves are also smaller (to reduce the effect of heating) and leathery (to prevent wilting).** **9. Root/Shoot Ratio** **- A low root/shoot ratio decreases the rate of transpiration while a high ratio increases the rate of transpiration.** **- The latter is due to the fact that an extensive root system is more efficient in water uptake from soil.** **- Increased availability of water also increases transpiration.** **10. Mucilage and Solutes** **- decrease the rate of transpiration by holding water tenaciously.**