Photosynthesis

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Photosynthesis
Determining the CO2-exchange rate of a leaf or
plant.
Determining the O2-exchange rate of a leaf or
plant.
Determining the rate of light reactions in a
tissue preparation, leaf, or plant.
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Photosynthesis Terminology
CER - CO2-Exchange Rate
Ci - Internal CO2 Concentration
Ca - Ambient CO2 Concentration
gco2 - stomatal conductance to CO2
Oftentimes, we determine net CO2-exchange rate or
net photosynthesis.
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Other Photosynthesis Measurements
Chlorophyll Fluorescence - Measurement of
Photosystem II Activity
Dry Matter / Growth - Weight Gain as a Function
of Time
Transpiration - Often directly correlated with CER
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CO2 Exchange in a Leaf
CO2
O2
H2O
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Ohms Law
Voltage Resistance Current
1 Resistance
Current Voltage
Photosynthesis Potential Conductance
The principle of Ohms Law is also applicable to
many physiological phenomena.
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Generalized CER Equation
CER gco2 (Ca - Ci)
However, the actual equations take many other
factors into account. These include leaf
temperature, air temperature, humidity, and
atmospheric pressure. For a given system and
measurement, chamber size, flow rates, and leaf
area are accounted for.
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Other Factors to Consider?
There are several places where air moving into
the chloroplast confronts resistance. Boundary
Layer Stomatal Mesophyll
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Other Factors to Consider?
Boundary Layer Resistance relates to the layer of
air around the leaf surface. It is often an
unstirred layer of air near the leaf surface and
has a lower CO2 concentration than the air away
from the leaf surface. Fans in a photosynthetic
chamber are able to minimize this resistance
during measurements of CER.
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Types of CER Measurement
Open System - Fresh gases of known
concentration are passed over the leaf surface,
exported to the analyzer, and then released.
Closed System - Gas enclosed in the chamber is
continually recycled over the leaf and through
the analyzer.
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What Measures CO2?
IRGA- Infrared Gas Analyzer
IRGA can be used for CO and CH4.
Fourier Transform Infrared, FT-IR is capable of
measuring many different gases.
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Alternative Methods for CER
Provide leaf or homogenate with 14CO2. When
14CO2 is fixed, it is converted into acid-stable
products. Once unfixed 14CO2 is removed, the
amount of fixation as a function of time is
determined.
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Other Factors To Be Aware Of
Corn and sorghum are C4 plants and respond
different to CO2 concentration than C3 plants
such as wheat and cotton. CAM plants do not open
stomata during the day so CER cannot be
determined easily.
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Measuring Chlorophyll
Leaf discs placed in a solvent.
Chlorophyll Meter
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Calculating Chlorophyll
In ethanol Chlorophyll 6.1 (A665) 20
(A649) units are mg per liter
If we have four leaf discs (0.5 cm2 each) in 10
mL of ethanol and absorbance is A6650.2 and
A6490.1. Chlorophyll 3.22 mg/L 3.22 mg/L
0.01 L / 2.0 cm2 16.1 µg/cm2
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Chlorophyll Meter
Measures absorbance of leaf at two wavelengths.
Correlation with extracted chlorophyll and/or
leaf nitrogen are high.