Plant physiology: Water balance of plants

Water content and the rate of water movement depends on what?

• Soil type

  • Sand

  • Clay

• Soil structure

  • Sand = Low surface area = large spaces/channels between particles

  • Clay = great surface area = Smaller spaces/channels between particles

What happens to sandy soil and clay soil when its heavily watered?

• Sandy soil

  • because of the large space between sand particles, the water tends to drain.

  • only remain on the the surface of the particles

• Clay soil

  • Because of the small spaces between the particles, water cant drain easily

What is Field capacity?

The water content of a soil after its been saturated with water and excess water has drained away

Sand soil has a large field capacity.

False, Clay

Clay soil can retain 40% of water after being saturated

True, Sandy soil can retain 3%

How does water move through soil?

Predominantly by bulk flow

Driven by a pressure gradient

What does the rate of water flow depend on?

• The size of the pressure gradient through the soli

• Hydraulic conduction of the soil

What is Soil hydraulic conductivity?

A measure of the ease with which water moves through the soil

Sandy soil have a large hydraulic conductivity

True, because of the large spaces between particles

Water content of soil DECREASE = hydraulic conductivity DECREASE

True

What is the Permanent wilting point?

The point when there is no water available to the plant

Intimate contact between root surface and soil in essential for what

Effective water absorption by the roots

Contact provides the surface area needed for water uptake and is maximized by the growth of the root and of root hairs into the soil

True

What root hairs? What is their purpose?

Microscopic extension of the root epidermal calls

They increase the surface area to provide greater absorption capacity

Where does water enter the root most readily?

In the apical part of the root that includes the root hair zone

The contact between the soil and the root surface is not easily ruptured.

False, it is easily ruptured when the soil in disturbed

Explain the Apoplast pathway in which water moves in the plant.

• Water moves EXCLUSIVELY through the cell wall

  • WITHOUT crossing any membranes

• A continuous system of cell walls and intercellular air spaces in plant tissues

Explain the Transmembrane pathway in which water moves in the plant.

• Enters the cells from one side, and exits on the other side

• Water crosses at least 2 membranes

  • Plasma membrane on entering and on exiting

  • Transport across the tonoplast may be involved

Explain the Symplast pathway in which water moves in the plant.

• Water travels from one cell to the next through Plasmodesmata

  • At the Endodermis: Casparian strop obstructs the water moves through the apoplast pathway

• Water must cross the plasma membrane to enter the cytoplasm

• Apical region of the root is most permeable to water.

• 
  

Distance from root tip Decrease = Rate of water uptake per segment Increase

True

What is the Casparian strip?

Band of radial cell walls in the endodermis that is impregnated with the wax-like hydrophobic substance Suberin

What is the function of Suberin?

Acts like a barrier to water and solute movement

What happens to the water uptake if the plants roots are injured?

An INCREASE in water uptake

When does water uptake decrease in roots?

• When roots are subjected to

  • Low temperature

  • Anaerobic conditions

  • Treated with respiratory inhibitors (cyanide)

What is the Root pressure phenomenon?

A positive hydrostatic pressure generated in the roots that pushes water and dissolved minerals upward through the plant's xylem vessels

When is Root pressure more likely to occur? Why?

• Soil water potentials = HIGH

• Transpiration rates = LOW

Because when transpiration rates are high, water uptake is rapid. so now positive pressure is developed in the xylem

What is Guttation?

A phenomenon that occurs when a plant produces liquid droplets on the leaf edges due to a frequent development of root pressure.

The sap droplets are released through special pores called Hydathodes

Guttation affects to Absorption, Transpiration, and Humidity?

• Absorption = INCREASE

• Transpiration = DECREASE

  • when air is cool or dark = closed stomata

• Humidity = INCREASE

What is the difference between Guttation and Transpiration?

• Guttation:

  • At night

  • Humidity = HIGH

  • Through Hydathodes

  • As water droplets

• Transpiration:

  • At day

  • Humidity = LOW

  • Through Stomata

  • As water vapor

What are the major factors transpiration from leaves are dependent on?

1. Difference in water vapor concentration between the leaf air spaces and the external air

2. Diffusional resistance of this pathway

relation ship between the air space inside the leaf and the wet surface from which water evaporates

volume of air space inside the leaf SMALL = the wet surface from which water evaporates LARGE

What is the absolute concentration difference?

The driving force for water loss from leaves

It depends on leaf temperature

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