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3. Field Capacity
1. Field capacity is the moisture content of a soil after it has been saturated with water and the excess water has drained away.
2. The water in a soil at field capacity is held by capillary forces. The amount of capillary water in a soil is a function of the surface-to-volume ratio of the soil particles (See Web Topic 4.1).
3. Capillary water is the water in the soil that is potentially available to the plant.
4. Clay soils and soils with high humus content have a high field capacity. Sandy soils have a low field capacity.

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5. A NEGATIVE HYDROSTATIC PRESSURE IN SOIL WATER LOWERS THE SOIL w
As the soil dries out, air spaces form between soil particles.
2. Water clings to soil particles due to adhesion, creating a large surface area between soil water and soil air.
3. Water develops a surface tension at the air-water interface. This creates a negative hydrostatic pressure in the soil capillary water.
4. As water recedes into soil interstices during drying, microscopic curved air-water interfaces (menisci) develop.
5. The smaller the radius of curvature (r) of the menisci, the more negative will be the soil hydrostatic pressure, according to the equation:
p = -2T/r (T = water surface tension)

6. WATER MOVES THROUGH THE SOIL BY BULK FLOW
Depletion of water around root lowers p of soil water.
2. Soil water moves toward root down a pressure gradient by bulk flow.
3. The rate of water flow depends on two factors:
the size of the pressure gradient
b. soil hydraulic conductivity.
4. Clay soils with small particles have low hydraulic conductivities, while sandy soils with large particles have high hydraulic conductivities.

7. PERMANENT WILTING POINT
Plants begin to wilt when their water loss from transpiration exceeds their water uptake.
As long as the water loss remains above a threshold amount, the plant can recover from wilting if transpiration ceases, as occurs during the night when the stomata close.
In very dry soils, the plant may lose so much water that it cannot recover during the night.
The soil water potential (w) at which irreversible wilting occurs is called the permanent wilting point.

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10. SOLUTE ACCUMULATION IN THE XYLEM AT NIGHT CAN GENERATE ROOT PRESSURE
Low w p
H20
SOLUTES IN XYLEM
H20
H20
H20
HIGH Soil w

11. ROOT PRESSURE CAN RESULT IN GUTTATION.
GUTTATION IS THE EXUDATION OF XYLEM FLUID THROUGH SPECIALIZED PORES AT THE MARGINS OF LEAVES CALLED HYDATHODES.

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15. WATER MOVEMENT THROUGH THE XYLEM REQUIRES LESS PRESSURE THAN MOVEMENT THROUGH LIVING CELLS
TO DRIVE WATER THROUGH AN IDEAL VESSEL ELEMENT WITH A 40 m RADIUS AT A VELOCITY OF 4 mm s-1 REQUIRES 0.02 MPa m-1.
BECAUSE OF PERFORATION PLATES AND ROUGH INNER WALLS, ACTUAL RESISTANCE IS DOUBLE THIS ABOUT AMOUNT (~0.04 MPa m-1).
RESISTANCE INCREASES BY TEN ORDERS OF MAGNITUDE IF WATER HAS TO CROSS MEMBRANES BETWEEN CELLS.

16. WHAT PRESSURE GRADIENT IS NEEDED TO LIFT WATER TO THE TOP OF A 100 METER TREE?
IF WE ASSUME A RESISTANCE OF 0.02 MPa m-1, THE TOTAL RESISTANCE IS 0.02 MPa m-1 x 100 m = 2 MPa.
MUST ALSO CONSIDER GRAVITY POTENTIAL (100 m x 0.01 MPa m-1 = 1 MPa.
THUS A PRESSURE GRADIENT OF AT LEAST 3 MPa FROM THE BASE TO THE TOP OF THE TREE IS REQUIRED.

17. THE COHESION-TENSION THEORY EXPLAINS WATER TRANSPORT IN THE XYLEM
Root pressure is typically less than 0.1 MPa, and is insufficient to lift water to top of tree.
Root pressure only occurs in absence of transpiration (at night or at 100% relative humidity).
Instead, water is pulled up the xylem by evaporation at the leaf surface (transpiration).
High tensile strength of water allows high tensions to build up in the xylem.
Can demonstrate tensions indirectly by dyes, and pressure chambers, and directly by pressure probes (see Web Topic 3.6).
Cavitation (embolism) occurs at very high tensions.

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23. Transpiration from the leaf depends on two major factors:
Difference in water vapor concentration
2. Diffusional resistance

24. Estimating Water Vapor Concentration (cwv)
Leaf air cwv is in equilibrium with leaf water potential (w).
2. The water vapor concentration decreases along the transpiration pathway from leaf to atmosphere.

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