Horticultural Responses to Water

Horticultural Responses to WaterBy C. Kohn, Waterford WI1How do they do it?Each day, huge sequoia trees transpire hundreds of gallons of moisture into the air.This moisture must travel nearly 400 feet into the air, equivalent to the height of a skyscraper.To pump water to the top of the Empire State Building, it takes a pressure of over 500 lbs per square inch!So how does a sequoia move hundreds of gallons of water each day to towering heights without a mechanical pump or electricity? 2

The Nature of WaterTwo forces affect the movement of water 1. Adhesion water is sticky and adheres to surfaces2. Cohesion water is attracted to water - water sticks to itself and forms long chains

These properties occur because water is a polar moleculeThis means that it has a positive end and a negative endLike opposite sides of a magnet, the positive end is attracted to the negative end.

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Capillary Action Capillary action is the tendency of a liquid to rise in narrow tubes or to be drawn into small openings.For example, water will be drawn into small tubes and rise against the force of gravityWater will move into the areas between grains of sand or soilA dry paper towel absorbs water because of the spaces between the fibers of the towelThe smaller the opening, the stronger the capillary action

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Capillary Action & SoilFor this reason, different kinds of soil have different abilities to hold onto waterSand has the lowest ability because the spaces between its pores are the largestClay has the strongest capillarity because the spaces between its pores are the smallest5

Types of Soil WaterBecause of adhesion/cohesion, 3 types of water exist in the soil. 1. Hygroscopic water tightly bound to the surface of each soil particle; this water is not accessible to the plant2. Capillary not as tightly bound into the soil; held in place by the cohesive bonds with the hygroscopic waterIt is held in place but can be removed by plants3. Gravity water that drains deeper into the soil at a rate determined by the soil texture.

6Soil WaterThe amount of water that remains in the soil after a rain is called the Field Capacity. A clay-soil will have a higher field capacity than a sandy soil Water is more available and more easily absorbed at field capacity than at the wilting pointThe Wilting Point is the degree of dryness at which water cannot be absorbed by the plants roots

7Water PotentialWater Potential is the tendency of water to move from an area of high pressure to an area of low pressureFor example, water flows from a tap in your bathroom or kitchen because electrical pumps are increasing the pressure of the water in the pipesBecause of this, water in your faucet has a positive potential. Once it is pumped into your pipes, it does not require additional energy to get it out. 8Water Potential of SoilWater in the soil has a negative potential it takes work to get the water out of the soil because of adhesion and cohesion. The energy required to remove that water from the soil decreases as the saturation of the soil increases.This should make sense its easier to wring water out of a soggy sponge than a dry spongeIt is easier to get water out of wet soil than out of dry soil. 9Hydro Tug of WarThis means that there is a constant tug of war between the soil and the plant cells of the roots. Whichever has the greater pull will get the waterThe force of the roots must exceed that of the soil in order for water to move into the roots. To accomplish this, plants use solutes (water always moves toward salt). The osmolarity inside the cell draws in water from the soil by osmosis. 10Double Sided Tug of WarJust as plants will draw water out of the soil, the surrounding air will draw water out of a plantWater always moves from wetter to drier. The air is almost always drier than the plant itself (which is mostly water).The drier the air, the more water that is pulled out of the plant, and in turn pulled out of the soil.11Relative HumidityWe use Relative Humidity to measure the dryness or wetness of the air. Relative humidity is the percentage of moisture in the air in proportion to the maximum possible. For example, 50% relative humidity would mean that the air is halfway to saturation. Relative humidity is misleading because it changes at each temperatureWarm air can hold more moisture than cold air. 12Calculating Relative HumidityTemperature (C/F)Absolute Humidity (g/m3 )0/324.85/416.810/509.415/5912.820/6817.325/7723.030/8630.4For example, if the air is at 80% Relative Humidity on a cool 50 degree spring day, there would be 7.5 g/m3 of humidity in the air (80% of 9.4 = 7.5)

80% humidity on a hot, 77 degree day would be .8 x 17.3 = 13.84 g/m3. If that wet air settled into your cool, 50 degree basement, you would have 13.84/9.4 = 147% humidityin other words, a puddle of moisture on your cool basement floor. 13Humidity and CondensationRelative Humidity can also be used to explain condensation.For example, on a hot day, your can of cola will become saturated; moisture will drip onto your napkin or even run off the table.This moisture collected on your can of soda because the air around the can was cooledCool air holds less moisture than warm air.The moisture in the warm air exceeded the saturation point for the cool air around the can, and water began to condense as a liquidThis occurs at night as the moisture in the formerly warm air creates a relative humidity above 100%, forcing it to condense as dew in the morning.

14Relative Humidity and TranspirationRelative Humidity makes a big impact on the rate of transpiration.The higher the RH, the lower the transpiration.The higher the temp, the greater the transpiration (because warm air can hold more moisture)As such, 20% RH at 50 degrees is very different from 20% RH at 80 degrees.20% RH at 80 degrees will result in a higher rate of transpiration because the remaining 80% at 80 degrees is greater than the 80% at 50 degrees. 15WindWind is major player in transpiration because it prevents the building up of moisture around the plant.In other words, the air will be less saturated with moisture immediately around the plant because it is constantly being replaced by drier air due to the wind.The greater the wind, the greater the transpiration from the plant.16LightThe greater the intensity of light, the more transpiration that occursThis is because ofGreater temperature on the leafMore open stomata (stomata open in the light and close in the dark)More photosynthesis (which uses water and carbon dioxide to make sugar)17ReviewFactors that affect water uptake by plants:Soil TypeSaturation of Soil (Field Capacity to Wilting Point)TemperatureRelative HumidityWindLight

18Review Which will have the greater transpiration?Sandy, Clay, or Silt soil?Dry soil or wet soil?Soil at Field Capacity or Wilting Point?50% relative humidity or 80% RH?50% RH at 60 degrees or 50% RH at 80 degrees?A plant in your basement or in your attic?A plant in open air or a plant in a sheltered corner?A plant in the sun vs. a plant in the shade?Transpiration Animation: http://croptechnology.unl.edu/animationOut.cgi?anim_name=transpiration.swf

19Path of Water Through the PlantBy C. Kohn, Waterford WI20In the beginningWell start our story of the path of a water droplet in the sky.Water falls out of the sky when the relative humidity reaches 100%. Increasing evapotranspiration (evaporation and transpiration) can increase moisture percentages, as can falling temperatures.For whatever reason, a drop of water falls from the sky and lands on the ground.21Stage 2: the soilDepending on the soil conditions, the drop of water may immediately sink down or it may move slowly through the soil.In sandy soils, water immediately begins to sinkIn clay soils, water moves much more slowly.It is easier for roots to absorb water in sand than it is in clay, but they also have less chance to absorb water in sandA sandy loam is the best soil for plants to absorb water because it can easily obtain it from the soil, but the water also sticks around long enough for it to do so. Sandy Loam = 50% sand, 25% silt, 25% clay22Stage 3: the rootsMost of water absorption occurs in the youngest parts of the roots (zones of elongation and meristematic zone)Root hairs greatly increase the surface area available for water absorption. Again, the soil has a negative water potential, meaning it takes work to get the water outPlants over come the negative water potential of soil through osmosis. By having a higher osmolarity, they can pull water away from the soil particles.

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Stage 4: inside the rootsThe cells of plant roots pull in water because of their higher osmolarity. Once inside the root cell, water must be moved to the xylem so that it can be spread throughout the plant.Water can move either through cells (symplastic) or around cells (apoplastic)Water will move through cells because it goes from wetter to drier; cells on the inside will be comparatively drier than cells on the outside of the plant. The plasmodesmata connecteach cell, enabling water to pass from cell to cell. 24Cells in the roots have connections called plasmodesmata that enable them to move water symplastically. Symplastic movement of water is also slowerApoplastic movement (around cells) is fasterThe cells on the inside of the root are separate from the cells on the outside by the casparian strip.The Casparian strip prevents water from flowing back out of the cell once it gets into the xylem. The Casparian strip is an apoplast barrier cells must move into the xylem by symplastic movement inside cells. This reduces water loss from the plant.

Inside the roots (cont)25Stage 5: The XylemOsmolarity pulled water into the roots from the soil.Adhesion and cohesion pull water up the xylem against the pull of gravity.Water molecules will form long ropesThere will be a long, continuous pull of water up the plant as each molecule is pulled from the plant into the air. This rope phenomena is due to cohesion, or the ability of water molecules to stick to each other.

26Xylem (cont)Water will also pull itself up the plant (to a lesser extent) due to adhesion.Adhesion tendency of water to stick to other substancesBecause of adhesion, the column of water inside the xylem is somewhat self-supportingThe maximum height of a column of water in xylem at 1 atm is 30 feet (10 meters) The tendency of water to move up xylem because of cohesion from transpiration and adhesion to the xylem walls is called the cohesion-adhesion model.

27Adhesion Cohesion Model

28Stage 6: StomataThe last stage of water as it moves through the plant are the stomata.The stomata are the cells on leaves that open and close to regulate the loss of water. We want water to flow out of the plant (this is the only way to pull water against gravity) but we dont want the plant to lose all its water!

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Opening and Closing StomataThe stomata of plants are surrounded by two guard cells that change shape to regulate the opening of the stomata. To open the guard cells, potassium is pumped out to make them deflate by losing water.To close the guard cells, potassium (K+) is moved into the guard cells to make them swell with water (water follows salt)

30Adaptations to Reduce Water LossStomatal densityRecessed stomata (moved deeper into the leaf)Cuticle & cutin thicknessLeaf Orientation (position in the air)Leaf RollingPresence of trichomes (leaf hairs) which hold moist air around the plantC3, C4, and CAM plants next slide31C3, C4, CAM plantsC3 plants include peas, potatoes, and beansC4 plants include corn and sugar cane CAM plants include cacti and pineapple. C3 plants must open their stomata to get the CO2 they need to make sugar in photosynthesis.C4 plants can get their CO2 chemically, reducing their need to open stomata, reducing their water lossCAM plants open their stomata at night to get their CO2 and then store it for later. 32Effects of Water LossWhen plants are low on water, growth is most affectedThis is because plants need turgor pressure in order for cellular growth to occur.If a plant is low on water, turgor pressure will also be reducedTo a lesser extent, photosynthesis and respiration will also be slowed by low water levels. 33