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GAS EXCHANGE MEASUREMENTS BY IRGA

Dr. Javier Gulías

University of the Balearic Islands (Spain)

Chania (Greece) June 2008

EFIMED Summer School on Do Mediterranean forests compete for water resources?. A scale approach from leaf to landscape.

1. PRINCIPLES AND FUNDAMENTALS

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1) Light-dependent reactions. PHOTOCHEMISTRY. Light energy is used for the synthesis of ATP and NADPH from ADP and NADP+.

2) Light-independent reactions. BIOCHEMISTRY. ATP and NADPH are used to reduce CO2 and produce sugars through the Calvin cycle.

THE PHOTOSYNTHETIC PROCESSPlants use solar energy to synthesize organic molecules from carbon dioxide (CO2) and water (H2O).

The process can be divided in two parts:

Spectroscopy

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Gas flow (pump)

CLOSED CIRCUIT

OPEN CIRCUIT

Open circuits avoid some problems (i.e., water condensation in the circuit), but requires the use of two IRGAs

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IRGAs position in the system head (close to cuvette), as in the Li-6400, improves accuracy

GAS EXCHANGE MEASUREMENTS BY INFRARED GAS ANALYZER (IRGA): Li-Cor 6400

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Measuring gas exchange

•Measured:

Net photosynthesis (AN)

Transpiration (E)

Air and leaf T, RH, PAR

•Estimated:

Stomatal conductance (gs),

Substomatal CO2 concentration (Ci)

1. E = gH2O x ([H2Oi]- [H2Oa])

2. AN = gCO2 ([Ca]-[Ci])

In (1) E and [H2Oa] are directly measured. Assuming 100% RH in the substomatal cavity and measuring leaf T, [H2Oi] can be

estimated and then solve for gH2O. In (2) AN and [Ca] are directlymeasured. Since gCO2 = gH2O / 1.6, then one can solve for [Ci].

IRGA determines the rates of photosynthesis (AN) and transpiration(E), as:

AN = (Air flux x ΔCO2) / Leaf areaE = (Air flux x ΔH2O) / Leaf area

From Fick’s First Law of diffusion, other parameters are then calculated:

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CO2 O2

H2O

CO2

O2

In the light, photosynthesis and respiration co-occur

FOTOSÍNTESIS

RESPIRACIÓN

RubiscoRubisco

COCO22 OO22

Carboxylation Oxygenation

COCO22OO22

Then, IRGA determines the NET photosynthesis rate:

AN = Gross photosynthesis - Photorespiration –Respiration

And PHOTORESPIRATION

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2. USEFULLNESS AND APPLICATIONS

What can we measure with an IRGA?

•Instantaneous gas fluxe measurements

•Daily courses of gas exchange

•Photosynthesis light response curves

•A/Ci curves

•Feed the models

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Annual Evolution of net photosynthesis and stomatal conductance of Quercus ilexJune 1998-October 2001

Binifaldó (Mallorca)P= 1000 mm

Puigpunyent (Mallorca)P= 600 mm

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How and When to do Instantaneous measurements: Diurnalevolution of Gas Exchange rates

High water availability

Mild water stress

Severe water stress

Mid-morningmeasurements

Sun leaf

Shade leaf

Light response curve

Net

pho

tosy

nthe

sis

Irradiance

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1. Rubisco-limitedphotosynthesis

2. RuBPregeneration capacity

3. Triose phosphate utilisation

Ci (μmol mol-1)0 200 400 600 800 1000

AN ( μ

mol

CO

2 m

-2 s

-1)

0

10

20

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CAREFUL: effect of mesophyll conductance to CO2(Ci NOT EQUAL TO Cc)

With adequate parameterization and a biochemical model(Farquhar et al. 1980), CO2-response curves allow estimates of

metabolic parameters

3. OVERVIEW OF Li-6400: SOME PRACTICAL

ASPECTS

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Li-6400

Li-6400

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Observations:

Date:

Slope:

Orientation:

Altitude:

Longitude:

Latitude:

Tree species:

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1

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ObservationsE (mmolH2O m2s-

1)Ci (mmol CO2

mol air-1)gs (mmol m2s-1)A (μmolCO2 m2s-

1)Tree

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