Introduction to Water

Question 1

Water potential is a measure of…

The potential of a particular sample of water is defined…

Water potential is the work that would be required to…

Answer 1

The free energy content of water.

relative to energy status of pure free water (which by definition has zero potential).

move water from where it is to the pure free state.

Question 2

Standard Unit for Yw is the…

Answer 2

MPa (Megapascal which is a unit of pressure)

Question 3

1 atmosphere is equal to…

Answer 3

1 bar
0.1 MPa

Question 4

What are the three ways in which water moves through a plant?

Answer 4

1. Diffusion
2. Mass Flow
3. Osmosis

Question 5

What is Diffusion for?

Answer 5

Diffusion is extremely slow over large distances.
It would take about 32 years for a sugar molecule to diffuse through a stem 1 meter long!

Question 6

What is Mass Flow for?

Answer 6

Transport over large distances, it is driven by a pressure gradient

Question 7

How does Osmosis work

Answer 7

Osmosis is driven by a water potential difference across a membrane – in other words, both pressure and concentration are important

Question 8

It is known that solutes concentration inside the cell is ______ than
outside (i.e. cell walls and intercellular space).

Answer 8

Higher

Question 9

______ is osmotically inert, most actively osmotic compounds are ______

Answer 9

1. Starch
2. Simple sugars

Question 10

When solutes attract water into a cell, the cell swells and causes the cell membrane to exert a force on the cell wall which is termed

Answer 10

Turgor (P)

Question 11

What is Osmosis?

Answer 11

This water concentration gradient causes water to move passively through aquaporins and across the menbrane into the cell and this movement is termed osmosis.

Question 12

What is Osmotic Potential?

Answer 12

The presence of solutes in a cell exerts a “pull” on the water molecules surrounding the cell and this tension is termed osmotic potential (π).

Question 13

In the absence of other forces, the movement of water during osmosis is always from a region of ______ to one of _______

Answer 13

1. lower solute concentration (i.e. greater water potential)
2. higher solute concentration (i.e. lesser water potential)

Question 14

How does a pressure chamber measure the tension in the xylem?

Answer 14

Air pressure in the chamber “squeezes” on the leaf tissues, forcing water out of leaves, into xylem, and out of the cut stem

Question 15

Cell wall “relaxation” is achieved through proteins known as _______ that are activated at low pH.

Answer 15

Expansin

Question 16

Discuss water holding capacity of air at different temperatures (10, 22, 34)

Answer 16

The water holding capacity of air is affected by temperature, we know that at 22 the air can hold twice as much water as at 10. Air at 34 can hold four times as much

Question 17

Transpiration depends on two major factors, what are they?

Answer 17

1. the difference in water potential between the leaf air spaces (ψ leaf) and the external air (ψair)
2. the resistance of this pathway to diffusion (r).

Question 18

What are the two types of resistance for the pathway to diffusion?

Answer 18

1. Stomatal Resistance
2. Boundary layer resistance

Question 19

The difference in water vapor pressure between the inside and the outside of the leaf (i.e. the water potential gradient) is often called the

Answer 19

VDP (Vapor Pressure Deficit)

Question 20

An increase in air temperature strongly increases the air’s water holding capacity and thus results in a greater VPD. This means that???

Answer 20

Temperature is a primary determinant of transpiration.

Question 21

If LEAF TEMPERATURE is equal to AIR TEMPERATURE

Answer 21

Then the vapor pressure deficit of air is the same as the leaf-to-air difference in vapor pressure

Question 22

What is the most relevant stomatal response (2)

Answer 22

1. Light induced opening at sunrise
2. Their closing at midnight

Question 23

Why do the stomates close at midnight

Answer 23

It is a response to photosynthetically driven changes in the sub-stomatal CO2 concentration (ci)

Question 24

Cavitation/embolism can result from…

Answer 24

both freezing and drought

Question 25

For every green organ we have _______ and __________. With the leaf ______ is through the stomates, _______ is cuticular

Answer 25

1. stomatal transpiration
2. Cuticular transpiration
3. 90%
4. 5-10%

Question 26

Discuss transpiration in berries

Answer 26

berries are exactly the opposite except that berries dont have stomates so they can not transpire via stomates.

Question 27

What is transpiration?

Answer 27

Essentially the exhalation of water vapour through the stomata.

Question 28

How can we calculate transpiration in its most basic equation?

Answer 28

Transpiration = VPD x Conductance

Question 29

What is cavitation?

Answer 29

Cavitation is essentially an embolism in the plant and this can happen with water stresses.

Question 30

What are three cooling tactics for the plant?

Answer 30

1. Direct transfer (re-radiation)
2. Convection
3. Transpiration

Question 31

What is the transpiration-cohesion-tension theory and what does it explain?

Answer 31

It says that the water, due to this huge gradient of water potential among soil, plant, and atmosphere, is able to move passively from the soil into the atmosphere with no energy required.

Question 32

What is Flux?

Answer 32

Flux = vapor pressure gradient x stomatal conductance

Question 33

When soils are cold…

Answer 33

the viscosity of water increases.