L7. Evapotranspiration II

Question 1

What is the definition of transpiration?

Answer 1

The indirect transfer of water from the root-stomatal system to the atmosphere

Question 2

What needs to exist before transpiration can occur?

Answer 2

• a flow of energy to the transpiring surfaces
• a flow of liquid water to these surfaces
• a flow of vapour away from these surfaces

Question 3

What is the cohesion-tension theory?

Answer 3

energy differentials drive the water movement from the soil into the roots, up the stalk, into the leaves and out into the atmosphere. Water always moves from a less negative water pressure in the soil to a more negative pressure in the atmosphere

Question 4

What are stomata, and what do they do for plants?

Answer 4

They are air openings, mostly on leaf underside.
- they allow plants to acquire CO2 from the air
- allow plants to expel water
- open and close diurnally and in response to soil water and atmospheric water pressure

Question 5

What process drives transpiration?

Answer 5

• VPD between the internal spaces in the lead and the atmosphere around the leaf

Question 6

What causes the guard cells of stomata open or close

Answer 6

• Light causes the stomata to open. This is an essential adaptation that allows plants to take in CO2 for photosynthesis during the day when there is light.
• Water availability in soil. When a plant experiences water stress or dehydration, guard cells lose turgor pressure, causing them to become flaccid and leading to stomatal closure.

Question 7

Plant factors that affect transpiration

Answer 7

• plant type
• number of stomata (more means more transpiration)
• number of leaves (more means bigger surface area and more stomata for gas exchanges)
• leaf size (a leaf with a bigger surface area will transpire faster than a lead with a smaller surface area)

Question 8

Environmental factors that affect transpiration

Answer 8

• air temperature (higher temp, more transpiration)
• relative humidity (when RH around the plant increases, transpiration decreases)
• wind (more air movement around a plant means lower RH causing higher transpiration)
• soil moisture availability (premature aging of leaves, less transpiration)
• light (plants transpire more rapidly in the light than in the dark)

Question 9

What does SPAC it stand for?

Answer 9

The soil-plant-atmosphere-continuum

Question 10

What is the connection between SPAC and cohesion-tension theory

Answer 10

When water vapour is lost through open stomata a even more negative water pressure is created. Water then moves up through the stem and into the leaves to replace the water lost. Movement is a continuous chain of water molecules due to water cohesion. This creates tension at root-level due to the increased water pressure difference between roots and leaves, causing roots to pull water out of soil.

Question 11

Why is transpiration important?

Answer 11

• productivity is connected to transpiration (water limitations are more important than any other limitation to plant productivity)
• transpiration is primary leaf cooling mechanism
• provides a pathway for nutrient uptake, and a matrix for chemical reactions

Question 12

Methods to estimate transpiration (and pros/cons)

Answer 12

• Leaf-level gas exchange: measures the difference in water vapour concentration (humidity) between the air entering and exiting the chamber (direct measurements but interrupt the ambient environment, and needs a large amount of measurements to scale more)
• Tree-level xylem sap flow: Sap flow sensors measure sap flow rates which is assumed equivalent to transpiration rates (doesn’t interrupt ambient environment, large amount of measurements required, hard to scale in time and space)