1. Horticultural Responses to Water
By C. Kohn, Waterford WI

2. How 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?

3. The Nature of Water Two forces affect the movement of water –
1. Adhesion – water is ‘sticky’ and adheres to surfaces
2. Cohesion – water is attracted to water - water ‘sticks’ to itself and forms long chains
These properties occur because water is a polar molecule
This means that it has a positive end and a negative end
Like opposite sides of a magnet, the positive end is attracted to the negative end.

4. 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 gravity
Water will move into the areas between grains of sand or soil
A dry paper towel absorbs water because of the spaces between the fibers of the towel
The smaller the opening, the stronger the capillary action

5. Capillary Action & Soil
For this reason, different kinds of soil have different abilities to hold onto water
Sand has the lowest ability because the spaces between its pores are the largest
Clay has the strongest capillarity because the spaces between its pores are the smallest

6. Types of Soil Water
Because 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 plant
2. Capillary – not as tightly bound into the soil; held in place by the cohesive bonds with the hygroscopic water
It is held in place but can be removed by plants
3. Gravity – water that drains deeper into the soil at a rate determined by the soil texture.

7. Soil Water
The 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 point
The Wilting Point is the degree of dryness at which water cannot be absorbed by the plant’s roots

8. Water Potential
Water Potential is the tendency of water to move from an area of high pressure to an area of low pressure
For example, water flows from a tap in your bathroom or kitchen because electrical pumps are increasing the pressure of the water in the pipes
Because 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.

9. Water Potential of Soil
Water 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 – it’s easier to wring water out of a soggy sponge than a dry sponge
It is easier to get water out of wet soil than out of dry soil.

10. Hydro Tug of War
This 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 water
The 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.

11. Double Sided Tug of War
Just as plants will draw water out of the soil, the surrounding air will draw water out of a plant
Water 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.

12. Relative Humidity
We 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 temperature
Warm air can hold more moisture than cold air.

13. Calculating Relative Humidity
Temperature (C/F)
Absolute Humidity (g/m3 )
0/32
4.8
5/41
6.8
10/50
9.4
15/59
12.8
20/68
17.3
25/77
23.0
30/86
30.4
For 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 = g/m3. If that wet air settled into your cool, 50 degree basement, you would have 13.84/9.4 = 147% humidity…in other words, a puddle of moisture on your cool basement floor.

14. Humidity and Condensation
Relative 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 cooled
Cool 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 liquid
This 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.

15. Relative Humidity and Transpiration
Relative 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.

16. Wind
Wind 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.

17. Light
The greater the intensity of light, the more transpiration that occurs
This is because of
Greater temperature on the leaf
More open stomata (stomata open in the light and close in the dark)
More photosynthesis (which uses water and carbon dioxide to make sugar)

18. Review Factors that affect water uptake by plants: Soil Type
Saturation of Soil (Field Capacity to Wilting Point)
Temperature
Relative Humidity
Wind
Light

19. Review 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:

20. Path of Water Through the Plant
By C. Kohn, Waterford WI

21. In the beginning…
We’ll 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.

22. Stage 2: the soil
Depending 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 sink
In 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 sand
A 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% clay

23. Stage 3: the roots
Most 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 out
Plants over come the negative water potential of soil through osmosis. By having a higher osmolarity, they can ‘pull’ water away from the soil particles.

24. Stage 4: inside the roots
The 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 connect each cell, enabling water to pass from cell to cell.

25. Inside the roots (cont)
Cells in the roots have connections called plasmodesmata that enable them to move water symplastically.
Symplastic movement of water is also slower
Apoplastic movement (around cells) is faster
The 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.

26. Stage 5: The Xylem
Osmolarity 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 “ropes”
There 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.

27. Xylem (cont)
Water will also pull itself up the plant (to a lesser extent) due to adhesion.
Adhesion – tendency of water to stick to other substances
Because of adhesion, the column of water inside the xylem is somewhat self-supporting
The 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.

28. Adhesion Cohesion Model

29. Stage 6: Stomata
The 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 don’t want the plant to lose all its water!

30. Opening and Closing Stomata
The 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)

31. Adaptations to Reduce Water Loss
Stomatal density
Recessed stomata (moved deeper into the leaf)
Cuticle & cutin thickness
Leaf Orientation (position in the air)
Leaf Rolling
Presence of trichomes (leaf hairs) which hold moist air around the plant
C3, C4, and CAM plants – next slide

32. C3, C4, CAM plants C3 plants include peas, potatoes, and beans
C4 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 loss
CAM plants open their stomata at night to get their CO2 and then store it for later.

33. Effects of Water Loss
When plants are low on water, growth is most affected
This 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 reduced
To a lesser extent, photosynthesis and respiration will also be slowed by low water levels.