Lec 7: ET II Flashcards
Define transpiration
Indirect transfer of water from the root-stomatal system to the atmosphere. plant-mediated evaporation. loss of plant water at leaf level
What needs to happen before transpiration can happen?
- A flow of energy to the transpiring surfaces;
- A flow of liquid water to these surfaces; and
- A flow of vapor away from these surfaces
Why is transpiration important?
- important for productivity
- primary leaf cooling mechanism under high radiation
- provides a pathway for nutrient uptake, and a matrix for chemical reactions
‒ Water pulled from the ground also has nutrients that are moved to plant
tissues
‒ For the pulling of water and nutrients to continue, “old” water must be lost by
the plants so that “new” water can be pulled from the ground
What drives the movement of water from soil to plant to atmosphere?
vapor pressure differences
Water always moves from a less
negative water pressure in the soil to
a more negative pressure in the
atmosphere
In what part of the soil does most plant water uptake occur?
Most plant water uptake occurs in upper half of the soil root zone
What do the stomata do?
- acquire CO2
- expel water
- open and close entrance
– Allow plants to acquire
CO2 from the air
(essential for
photosynthesis)
– Allow plants to “expel”
water (transpiration)
– Open and close diurnally,
and in response to soil
water and atmospheric
water pressure
What is the main function of stomata?
How many are there typically on a leaf?
When do stomata close and why?
maximizing CO2 uptake while minimizing H2O loss
50-200 stomata per mm2 of leaf surface
closing triggered by darkness or by the release of ABA (abscisic acid) when
soil water supply cannot keep up with transpiration
What cells open and close the stomata? How do they change when the stoma opening is open vs closed?
guard cells
open stoma: swollen guard cells
closed stoma: shrunken guard cells
Which plant factors affect transpiration?
- Plant type
- Number of stomata
‒ More stomata = more pores for transpiration - Number of leaves
‒ More leaves = Bigger surface area and more stomata for gas exchanges - Leaf size
‒ A leaf with a bigger surface area will transpire faster than a leaf with a smaller
surface area
Define stomatal conductance
Which plant types have the highest and lowest stomatal conductance?
Stomatal conductance = Rate of CO2 (H2O)
exchange with air (mmol m-2 s-1)
highest:
- herbaceous and cereal crops
lowest:
- tundra, deciduous shrub
Which environmental factors affect transpiration?
- Air temperature
‒ Higher temperatures = higher transpiration rates - Relative humidity
‒ When RH around the plant increases, transpiration decreases - Wind
‒ More air movement around a plant = lower RH = higher transpiration rates - Soil moisture availability
‒ In dry conditions, some plants undergo premature aging = leaf loss = less
transpiration - Light
‒ Plants transpire more rapidly in the light than in the dark
At what time of the day does transpiration mostly occur?
Loss of water mainly in daytime, with the highest usually around noon
What are the 3 transpiration steps of the soil-plant-atmosphere continuum
- water absorbed by roots
- water moves up plant
- water vapor lost from leaf pores
Explain the cohesion-tension theory
- water leaves leaf through open stomata, negative water pressure is created
- water moves from the stem to the leaves to replace the lost water. It moves as a continuous chain of water molecules due to water cohesion
- due to the increased water pressure difference between the leaves and the roots, soil water moves into roots
Name two ways of measuring transpiration and their pros and cons
leaf level gas exchanges: see how much CO2 is taken up by a leaf
* Advantage:
‒ Direct measurements
* Disadvantages:
‒ Measurements interrupt the
ambient environment
‒ A large number of
measurements are needed to
scale up
tree-level xylem sapflow: measure sapflow rates with sapflow sensors. Sapflow rates are assumed to be equal to transpiration rates.
* Advantage:
‒ Measurements do not interrupt ambient
environment
* Disadvantages:
‒ large number of measurements required
‒ Appropriate scaling in time and space required
