Introduction to Water Flashcards
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…
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.
Standard Unit for Yw is the…
MPa (Megapascal which is a unit of pressure)
1 atmosphere is equal to…
1 bar
0.1 MPa
What are the three ways in which water moves through a plant?
- Diffusion
- Mass Flow
- Osmosis
What is Diffusion for?
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!
What is Mass Flow for?
Transport over large distances, it is driven by a pressure gradient
How does Osmosis work
Osmosis is driven by a water potential difference across a membrane – in other words, both pressure and concentration are important
It is known that solutes concentration inside the cell is ______ than
outside (i.e. cell walls and intercellular space).
Higher
______ is osmotically inert, most actively osmotic compounds are ______
- Starch
- Simple sugars
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
Turgor (P)
What is Osmosis?
This water concentration gradient causes water to move passively through aquaporins and across the menbrane into the cell and this movement is termed osmosis.
What is Osmotic Potential?
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).
In the absence of other forces, the movement of water during osmosis is always from a region of ______ to one of _______
- lower solute concentration (i.e. greater water potential)
- higher solute concentration (i.e. lesser water potential)
How does a pressure chamber measure the tension in the xylem?
Air pressure in the chamber “squeezes” on the leaf tissues, forcing water out of leaves, into xylem, and out of the cut stem
Cell wall “relaxation” is achieved through proteins known as _______ that are activated at low pH.
Expansin
Discuss water holding capacity of air at different temperatures (10, 22, 34)
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
Transpiration depends on two major factors, what are they?
- the difference in water potential between the leaf air spaces (ψ leaf) and the external air (ψair)
- the resistance of this pathway to diffusion (r).
What are the two types of resistance for the pathway to diffusion?
- Stomatal Resistance
- Boundary layer resistance
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
VDP (Vapor Pressure Deficit)
An increase in air temperature strongly increases the air’s water holding capacity and thus results in a greater VPD. This means that???
Temperature is a primary determinant of transpiration.
If LEAF TEMPERATURE is equal to AIR TEMPERATURE
Then the vapor pressure deficit of air is the same as the leaf-to-air difference in vapor pressure
What is the most relevant stomatal response (2)
- Light induced opening at sunrise
- Their closing at midnight
Why do the stomates close at midnight
It is a response to photosynthetically driven changes in the sub-stomatal CO2 concentration (ci)
Cavitation/embolism can result from…
both freezing and drought
For every green organ we have _______ and __________. With the leaf ______ is through the stomates, _______ is cuticular
- stomatal transpiration
- Cuticular transpiration
- 90%
- 5-10%
Discuss transpiration in berries
berries are exactly the opposite except that berries dont have stomates so they can not transpire via stomates.
What is transpiration?
Essentially the exhalation of water vapour through the stomata.
How can we calculate transpiration in its most basic equation?
Transpiration = VPD x Conductance
What is cavitation?
Cavitation is essentially an embolism in the plant and this can happen with water stresses.
What are three cooling tactics for the plant?
- Direct transfer (re-radiation)
- Convection
- Transpiration
What is the transpiration-cohesion-tension theory and what does it explain?
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.
What is Flux?
Flux = vapor pressure gradient x stomatal conductance
When soils are cold…
the viscosity of water increases.
