Introduction to Plant Gas Exchange Measurements

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Introduction to Plant Gas Exchange Measurements

• LI-COR Inc., Lincoln, NE, USA

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Who Measures Photosynthesis?

• Mainly scientists measure photosynthesis
• Crop producers (farmers, horticulturalists) do
  not usually measure photosynthesis
• In the paste, improvements in crop production
  were achieved by lengthening the growing period,
  by selecting higher grainfoliage ratio, by the
  application of fertilizer and irrigation -
  without understanding photosynthesis.

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Gas exchange versus agronomic measurements

• Gas exchange short term, high sensitivity
  e.g. reducing PAR reduces A
• Agronomic longer term, integrative (final
  yield, biomass production, LAI, etc.)
• Analogous to monitoring heart, blood pressure,
  sugar etc. versus monitoring weight, height of a
  child

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Practical applications of gas exchange
measurements examples (I)

• Cold tolerance of Maize genotypes
• Screening for fungicides, insecticides, with
  least harmful effect on crop
• Screening for selective herbicides

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Practical applications of gas exchange
measurements examples (II)

• Correct drought stress for growing sweet grapes
  by monitoring stomatal conductance
• Finding optimum light levels for growing
  medicinal herbs absence of active compounds
  under high light conditions
• Screening for reduced photorespiration

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Basic Research
Should we never study anything unless it has an
immediate practical application?
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Historical examples of basic research

• History of electricity - Michael Faradays
  experiments in electromagnetic induction
• Rutherfords comments on nuclear science in 1936
  of no practical value
• Mendels experiments on the genetics of sweet
  peas. He was told to go plant more flowers in
  the garden

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Basic research applications of gas exchange
measurements

• Basic research on understanding photosynthesis -
  a reaction on which all life depends
• Scientists want to study how plants grow, how
  ecosystems work.
• Global Change research how rising level of CO2
  and temperature could affect agriculture, as
  well as the ecology (C3C4 species balance).

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Applications of gas exchange

• When choosing a topic for research, it is
  important to pick something which interests you.

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Checking the LI-6400

• How do you know if the LI-6400 is working
  properly?
• Would you test it on a leaf to see if it reads
  photosynthesis correctly?

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LI-6400 system checklist
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Checking the LI-6400 Calibration

• User calibration - setting zero and span
• Does the LI-6400 IRGA needs factory calibration?
• New internal chemicals?
• How do you know?

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Examples of data with weaknesses
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Data Quality avoiding noisy measurements

• Measurement precision and IRGA noise

Typical IRGA noise of the LI-6400 is /- 0.2 ppm.
So the ?CO2 fluctuates by /- 0.2 ppm For a 5
measurement precision, DeltaCO2 should be 5 ppm
(because 0.2/5 5).
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Data Quality avoiding noisy measurements

• If DeltaCO2 is only 1 ppm, then noise in
  photosynthesis will be 1 /- 0.2 or 20
• If in above case flow is reduced to half, then
  DeltaCO2 will double to 2ppm, and noise in
  photosynthesis will be reduced to 2 /- 0.2 or
  10
• If in above case a 2 cm2 leaf area, is increased
  to 6 cm2 then deltaCO2 will increase to 6 ppm and
  reduce noise in photosynthesis to 6 /- 0.2 or
  3

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Equation Summary
Transpiration
Photosynthesis
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Intercellular Water Vapor
Water Vapor Mole Fraction
Water
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Equation Summary -continued
Stomatal Conductance - obtained by restating
transpiration in terms of Ohms law
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Calculating Ci
If assimilation is expressed in terms of Ohms law
(i.e. in terms of internal leaf to chamber air
CO2 concentration difference and stomatal
conductance)
Also it is known that gcs gws/1.6
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0
CO2 concentration in the mesophyll
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Energy Balance Leaf Temperature Measurement

• 0 Q L R
• R Net radiation, made up of solar (total leaf
  absorption) and thermal (black body radiation
  balance from Tleaf, Tair, ?, and ?)
• L Latent heat of vaporization transpiration
• Q Sensible heat flux, a function of
  (Tleaf-Tair), specific heat capacity of the air,
  and one-sided boundary layer conductance of the
  leaf
• Express R in terms of L Q, solve for
  (Tleaf-Tair) to determine Tleaf

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Configuring the LI-6400 for surveys

• RefCO2 - Ambient expected Delta
• Flow fixed, high, but still adequate Deltas
• Light consider leaf and sun relation
• Use prompts for data identification

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Configuring the LI-6400 for Light Curves

• Constant Sample CO2 - not Reference CO2
• Why?
• If choosing constant humidity, then start with
  high flow, and slow RESPNS
• Fixed temperature
• Going from high to low light levels is faster

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Configuring the LI-6400 for CO2 Response Curves

• Allow plenty of time for leaf to acclimate to the
  light level
• Matching IRGAs is very important
• Measurements can be very fast as there is no need
  to wait for acclimation to changes in light
• Diffusive leaks can be significant

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Photorespiration inhibition in a C3 leaf
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Effect of O2 concentration of a C4 leaf
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Diffusion Leaks
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Custom Chambers
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Stages of Photosynthesis
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Leaf Structure
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Chloroplast Structure
The Light Reactions occur in the grana and the
Dark Reactions take place in the stroma of the
chloroplasts.
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Light Reaction Stages
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Fate of Absorbed Light

• Typical for low light conditions
• 97 Photochemistry
• 2.5 Heat
• 0.5 Fluorescence
• Under high light conditions
• low Photochemistry
• 95 Heat
• 2.5-5 Fluorescence

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6400-40 Leaf Chamber Fluorometer

• Red (630nm)
• Blue (470nm)
• Far red (740nm)
• Fluorescence Detection at gt715nmlt1000nm

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Relative spectral outputs of the LCF
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Pulse Amplitude Modulation (PAM)
Measuring on, Actinic on
Fm, Fm
Light Intensity
Fm
Fm
After demodulation
F
Measuring on, Actinic on
Fs
Fs
Fo
Fo
Time
Measuring On, Actinic off
Fo, Fo
0
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Fluorescence Parameters
Light-energized chlorophyll molecules release
energy in one of three ways Fluorescence, Heat
or Photosynthetic photochemistry. F H P
1 As light intensity increases, P reaches a
maximal value, and any additional light incident
on the leaf is as Heat and Fluorescence. For any
light above the saturation intensity the above
equation thus becomes Fm Hm 0 1 So Hm
1- Fm where Fm and Hm are the fluorescence and
heat de-excitation energies from the modulated
signal when the leaf is given enough light to
saturate the photochemistry.
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Fluorescence Parameters - continued
Also if it is assumed that the ratio of
heatfluorescence de-excitation remains constant
(for a given state of the leaf), then
and
Also P 1 - F - H
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Fluorescence Parameters - continued
If the F is measured on a dark-adapted leaf, then
it is referred to as Fo and P becomes
Fv/Fm is the fraction of absorbed photons used
for photochemistry for a dark adapted leaf. For
most plants Fv/Fm is around 0.8
Under non-saturating steady-state photosynthesis
the above equation takes the form
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Other Fluorescence Parameters
Another relation similar to is
The photochemical quenching of fluorescence,
includes - photosnythesis and photorespiration
The non-photochemical quenching of fluorescence
heat, etc.
Another non-photochemical quenching parameter
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A Fluorescence Induction Curve