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.

A recent Journal article gives an excellent description of protocols used to parameterize the model. An Idealized A/Ci response. The rates of photosynthesis that would be achieved depending on whether Rubisco, (Wc), RuBP regeneration (Wj) or TPU triose phosphate utilization (Wp) are limiting

If there was no limitation to diffusion of CO2 by boundary layer and stomatal resistances then the assimilation rate would be A”…..however resistances do exist and when Ca =370 then Ci = about 240 in this figure and assimilation is lowered to A’

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 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%).

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 Transpiration Photosynthesis

Intercellular Water Vapor Water Vapor Mole Fraction Water

Equation Summary -continued Stomatal Conductance - obtained by restating transpiration in terms of Ohms law

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

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 R: Sigma is the thermal emissivity of the leaf (0.95 for most leaves: Cotton (0.96), Tobacco (0.97), Cactus (0.98), Bean (0.96), and poplar (0.98)). Gamma is the Stefan-Boltzmann constant (5.67x10-8 W/m2 K).

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

Effect of O2 concentration of a C4 leaf

Diffusion Leaks

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) 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’

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.

Fluorescence Parameters - continued Also if it is assumed that the ratio of heat:fluorescence de-excitation remains constant (for a given state of the leaf), then: and Also P = 1 - F - H

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:

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

A Fluorescence Induction Curve