introduction to plant gas exchange measurements
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Introduction to Plant Gas Exchange Measurements. LI-COR Inc., Lincoln, NE, USA. Who Measures Photosynthesis?. Mainly scientists measure photosynthesis Crop producers (farmers, horticulturalists) do not usually measure photosynthesis - PowerPoint PPT PresentationTRANSCRIPT
Introduction to Plant Gas Exchange Measurements
LI-COR Inc., Lincoln, NE, USA
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 grain:foliage ratio, by the application of fertilizer and irrigation - without understanding photosynthesis.
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
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
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
Basic Research
Should we never study anything unless it has an immediate practical application?
Historical examples of basic research
• History of electricity - Michael Faraday’s experiments in electromagnetic induction
• Rutherford’s comments on nuclear science in 1936 “of no practical value”
• Mendel’s experiments on the genetics of sweet peas. He was told to “go plant more flowers in the garden”
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 (C3:C4 species balance).
Applications of gas exchange
• When choosing a topic for research, it is important to pick something which interests you.
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?
LI-6400 system checklist
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?
Examples of data with weaknesses
Data Quality – avoiding noisy measurements
Measurement precision and IRGA noise
LeafArea
COFlowPhoto 2*
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%).
122
11.)(
Smmol
m
molmolSmol
S
ccuA SR
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%
Equation Summary
122
11.)(
Smmol
m
molmolSmol
S
wwuE Oi
122
11.)(
Smmol
m
molmolSmol
S
ccuA Oi
Transpiration
Photosynthesis
Intercellular Water Vapor
P
TeWand
P
eW Lsat
i
)(
Water Vapor Mole FractionWater
Equation Summary -continued
Stomatal Conductance - obtained by restating transpiration in terms of Ohms law
)( Sis wwgE
121
12
)(
Smmolmolmol
Smmol
ww
Eg
Sis
122
11.)(
Smmol
m
molmolSmol
S
wwuE SR
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):
)( iaCS CCgA
Also it is known that gcs = gw
s/1.6
wS
ai g
ACC
6.1
CO2 concentration in the mesophyll
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
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
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
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
Photorespiration inhibition in a C3 leaf
A-Ci curves for a bean leaf at 2% and 21% oxygen(PAR=1500 umol/m2/s)
At ambient CO2, difference is about {(14-10)/10} = 40%
-4.00
-2.00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00
Ci (ppm)
Ph
oto
(u
mo
l/m
2/s
)
21% Oxygen
2% Oxygen
Effect of O2 concentration of a C4 leaf
A-Ci curves for a Maize leaf at 2% and 21% Oxygen(PAR=2000 umol/m2/s)
-5
0
5
10
15
20
25
30
35
0 50 100 150 200
Ci (ppm)
Ph
oto
(u
mo
l/m2/
s)
21% Oxygen
2% Oxygen
Diffusion Leaks
Effect of CO2 diffusion - empty chamber measurements(Room [CO2] was approximately 500 ppm)
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0 100 200 300 400 500 600 700 800 900
Sample chamber CO2 (ppm)
Ap
par
ent
CO
2 ex
chan
ge
(um
ol/
m2/
s)
Custom Chambers
Stages of Photosynthesis
Leaf Structure
Chloroplast Structure
The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.
Light Reaction Stages
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
6400-40 Leaf Chamber Fluorometer
• Red (630nm)
• Blue (470nm)
• Far red (740nm)
• Fluorescence Detection at >715nm<1000nm
Relative spectral outputs of the LCF
Pulse Amplitude Modulation (PAM)
Fo, Fo’0
Fm, Fm’
Measuring On, Actinic off
Measuring on, Actinic on
Light Intensity
After demodulation
Fm
Fs
Fm’
Fo
Time
FMeasuring on, Actinic on Fs
Fo’
Fluorescence Parameters - continued
Fm
FmFH
)1(
Fm
FmFF
)1(1
Also if it is assumed that the ratio of heat:fluorescence de-excitation remains constant (for a given state of the leaf), then:
Fm
Hm
F
H
and
Also P = 1 - F - H
Fm
FFm
Fluorescence Parameters - continuedIf the F is measured on a dark-adapted leaf, then it is referred to as Fo and P becomes:
Fm
Fv
Fm
FoFmdarkP
)(
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:
PSIIFm
F
Fm
FsFmlightP
'
')(
Other Fluorescence Parameters
'
''
'
'
Fm
FsFm
Fm
Fv
'
'
FoFm
FmFmqN
Another relation similar to is:
''
'
FoFm
FsFmqP
PSII
'
'
Fm
FmFmNQP
The photochemical quenching of fluorescence, includes - photosnythesis and photorespiration
The non-photochemical quenching of fluorescence – heat, etc.
Another non-photochemical quenching parameter
A Fluorescence Induction Curve