WATER:Properties, Role in Plants, Watering Strategies
Water Evaporation and Transpiration Evaporation - Change of liquid into gaseous stateTranspiration - Evaporative loss of water from the plant.
Transpiration & EvaporationandTemperature Temperature- measure of the average velocity of molecules (how fast they are moving) a.k.a. HEAT Molecule that evaporates from a surface has enough velocity to overcome the attraction of its neighbor. When water molecules escape, the temperature of the remaining liquid decreases.
Relevance to PlantsWhen water evaporates (transpires) from a leaf, the leaf is cooled.Much the same as how the evaporation of perspiration cools us.When water evaporates from greenhouse cooling pads, the air is cooled which in turn cools the plants it moves over.
Condensation is the return of a molecule of water to its liquid form. There is an EQUILIBRIUM when the rate of condensation (return) equals the rate of escape (evaporation)
EquilibriumEvaporation = Condensation No equilibriumEvaporation < or > Condensation
At equilibrium the atmosphere is saturated Relative humidity (RH) =Actual amount of water vapor in the atmosphereAmount of water vapor the atmosphere when saturated RH depends on air temperature- warm air holds more water than cold aire.g. 70F air at 100% RH is holding more water than 50F air at 100% RH.
Relevance to Plants and GreenhousesThe higher the RH, the slower the rate of transpiration from leaf so there is less cooling of the leaf.The higher the RH, the less effective evaporative cooling systems are in the greenhouse.
Relative Humidity (RH) =
Can easily measure using a
and a chart Sling psychrometer
Reading a psychrometer chart
Constructing your own sling psychrometer:
Tie two thermometers together, wrap the end of one in a wet cloth. Sling around in the air for a minute or so.
Measure air temperature (dry bulb)
Measure cooling effect of evaporation of water (wet bulb)
Compare the readings on a chart to get RH.
Relative humidity is not always a good way to measure the potential for evaporation to occur, because RH is temperature dependent.A better way is to measure the difference in vapor pressures between the atmosphere and the evaporative surface (leaf or cooling pad).
Vapor pressure - the pressure exerted by a vapor; often understood to mean saturated vapor pressure (the vapor pressure of a vapor in contact with its liquid form). Expressed as kPaWhen the vapor pressure of air is less than the surface the air is touching, there is a deficit of air vapor pressure (VPD) relative to the surface. The greater the deficit, the greater the rate of evaporation from the surface.
VPD determines how fast plants use water and how efficiently wet pads cool greenhouses.Which in turn determine how often you have to irrigate.The more you understand VPD, the better you (or the environmental control computer you program) can decide when its time to irrigate.
Vapor Pressure Deficit (VPD)Relevance to Plants VPD - Good way to determine watering needs of plants The greater the VPD between the leaf and the air, the more likely the plant water use will increase.
Understanding VPD also helps you to find out if you are wasting your money (and water resources) using cooling pads to cool your greenhouse.
You could be better off using natural ventilation.What else makes it beneficial for you to understand VPD?
What else do you have to know to be able to irrigate at the right time?You have to understand plant water relations and how water moves into and through the plant.
Plant Water Relations
Starts with the concept ofWater Potential
Plant Water Relations
Water Potential () :The difference between the activity of water molecules in pure distilled water at 1 atm and 30C (standard conditions), and the activity of water molecules in any other system. The activity of water molecules in a system may be greater (positive) or less (negative) than the activity of the water molecules under standard conditions.
Plant Water RelationsWater Potential ()Defines how tightly water is held by a systemDetermines how easily water move from one system to another Determines which direction water flows
Plant Water RelationsWater Potential () summary
units -- atm (atmosphere) or bar or kPa is 1 for pure water at sea levelFor most systems, is negative Water moves from higher to lower
Think of flow of water from high to low as a waterfall - flowing high to low is greater at the top of a waterfall than at the bottom.
Plant Water Relations
Implications for plantsWater moves into the root only if in root is lower (more negative) than the soil.Water moves through the plant in the from higher to lower .
T = + P + M Where:T = total potential = osmotic potentialP = pressure potentialM = matric potential
Components of T = + P + MOsmotic Potential due to the effect of dissolved solutesthe greater the concentration of solutes, the lower (more negative) the water potentialwater moves from an area of low salt concentration to an area of greater concentration.
Components of T = + P + MImplications for plants
Generally causes the plant to have more negative than soil/media because of the salts in the plant. This helps water move into the root from the soil.
Applying liquid fertilizer (a.k.a. salt solution) to a dry soil/media lowers the osmotic potential of the media/soil. If of the soil becomes less than the root, water will leave the root, causing fertilizer burn.
Components of T = + P + M
Pressure Potential P due to the forces on water from high water concentration in cells positive value for the most part in turgid (not wilting) plantsearly stages of decreasing P = incipient plasmolysis, useful for controlling length of young shoots stems
- Cell wall- Cell membraneWhen young cells are filled with water, the membrane presses on the growing cell wall. The cell walls elongate and stay relatively thin as the cells grow and divide.When water is slightly withheld from young plants, the membrane does not press on the growing cell wall (incipient plasmolysis). The cell walls stay more square and thick as the cells grow and divide.
If these were stem cells, which would provide the strongest and shortest stems which usually produce the most durable and probably attractive plant?
If you know what you are doing, drying down is one of the most effective and cheapest ways to regulate plant height.Have to be careful, drying down is only a few minutes away from drying up.Drying up can cause irreparable damage to plants.You do not want the plant and its young cells to become desiccated.
Components of T = + P + MMatrix Potential M the adhesion of water to particles the stronger the adhesion of water to a particle, the lower the matrix potential
Components of 3. Matrix Potential M involves potential of solid components (including soil) the stronger the adhesion of water to a particle, the lower the matrix potential
Implications for plantsThe lower the M in the soil or growing media, the more tightly the water is held by the media When you irrigate you are raising the M of the media and in turn you are making it easier for water to enter the plant.
Total water potential T = + P + MT determines how much water enters, leaves, and stays inside the plant.That in turn determines how the plant grows. You can control much of a plants growth by controlling any of the T components.
Triphasic growth pattern: Typical for most greenhouse plants.Characterized by:
1. Slow initial growth2. Rapid vegetative growth and elongation3. Slow reproductive growth.Growth regulation is most effective between low and mid-portions of rapid growth phaseTiming when water is withheld, as with every growth regulation technique, is very important.
Cohesion-Tension TheoryMechanism of water movement in xylem is driven by changes in from soil through plant to airNote that even at near 100% RH, air still more negative than leafThus: water flows from leaf to airHowever, even at air RH 100%, the slightest air movement across the leaf lowers air to less than in leaf so water flows from leaf to air
Temp = 20C = 68F
During all this pulling, hydrogen bonds hold water molecules together in columns inside xylem tubes = cohesion
The very negative of the air tugs onthe water column,causing the H2Omolecules to move up throughthe plant.
Rhizoshere (rootzone)(Water molecules, not Disney symbols)Air
Cohesion/tension explains how water can travel upwards against gravity in a plant.
Transpiration at leavesWater molecules pulled up stem to replace molecules lost to air
Tension on water in xylem
Water pulled into roots
Water into the Root Roots have evolved to increase water absorption area by formation of root hairs.New root hairs have to be constantly produced to have water uptake.Damaged or diseased roots do not produce root hairs, severely limiting their ability to take up water.
Disease and Water MovementMany fungal or bacterial pathogens cause diseases with a characteristic symptom of wilt. The wiltin