Field Photosynthesis Measurement Systems
David G. Reta-Sánchez Alvaro Anchondo-Najera
Héctor M. Quiroga-Garza Jesús G. Arreola-Avila
Rafael Figueroa-Viramontes Concepción Lujan-Alvarez
Higher plants transform sunlight to chemical energy by means
of photosynthesis. During the process, plants fix carbon dioxide
and release oxygen while coping with the loss of water.
Measurements of photosynthesis are needed for comparing and
understanding productivity(biomass accumulation) of vegetal
systems at the leaf, plant or community level as well as their
response to environmental stresses.
Gas exchange -CO2 and H2O- by leaves form the basis for the
design of most photosynthesis meters. Since CO2 intake and H2O
release share the same pathway -the stomata-, photosynthesis
measurements commonly include the estimation of photosynthesis
itself (assimilation or CO2 uptake), stomatal conductance, and
transpiration. This document is concerned with the description of
the methods and instruments associated with photosynthesis
measurement in the field. In doing so, both open and closed
portable system designs are being considered.
System Designs for Photosynthesis Measurement
The term closed or open is used in the sense of whether or
not the atmosphere of the leaf-enclosing chamber is renewed
during the measurement.
In a closed system design, a given leaf, plant or canopy is
enclosed in a sealed chamber that is not resupplied with fresh
air. The CO2 concentration in the chamber is decreased by leaf
photosynthetic activity, while the H2O concentration increases.
The change in CO2 and H2O concentrations per unit of time are
correlated with net photosynthesis (assimilation) and
In an open system design, an air stream that has a known CO2
concentration is constantly passed through the leaf chamber. The
objective is to supply a steady state level of CO2. As a result
of leaf activity, the air exiting the chamber (the
"sample") will have a lower CO2 concentration as well
as a higher H2O concentration than the air entering the chamber
("the reference"). A variation of the open system is
the compensating system, where the CO2 removed by photosynthesis
is compensated by CO2 injection until reaching equilibrium
("null balance"). At that equilibrium point,
photosynthesis rate is equal to the CO2 injection.
Previous LI-COR photosynthesis meters (the models 6000 and
6200) were of the closed-mode type. The new LI-6400
is an open-mode system. The CI-500 and
CI-301PS from CID Inc., support both open and closed modes of
Main Components of Gas Exchange Systems
The basic components of a photosynthesis measurement system
are the gas exchange chamber, infrared gas analyzer, flow meters,
gas lines, CO2 and water vapor filters,power batteries and a
console with keyboard, display and memory.
Leaf chamber architecture, aerodynamics and properties of
building materials profoundly affect system performance. Precise
control of temperature, CO2 concentration, humidity and light has
to be achieved. Particularly in close-mode systems, tight sealing
of the chamber and use of materials with low adsorption of water
and CO2 are critical.
Modern systems measure the CO2 concentration with a
non-dispersive infrared gas analyzer. This device includes an
infrared source that is shined through a gas sampling chamber and
then focused on a detector. The energy received at the detector
is the total entering the system minus the energy absorbed by the
CO2 in the sampling chamber. A major problem in IRGA performance
is the discrimination between CO2 and water vapor, since both
gasses absorb energy at similar wavelengths. To solve this
problem, the gas sample is dried to a constant water content by
means of a dessiccant before reaching the IRGA.
The incorporation of advanced computation programs allows the
immediate access to data in the field and the possibility to
detect errors during the measurement.
Measurement procedure for LI-COR
Measuring leaf the photosynthetic rate is as follows:
- Turn on the analyzer pump and fans in the chamber.
- Place the CO2 and relative humidity channels in the
- Clamp the chamber onto the experimental leaf to be
Some precautions must be considered:
- Get a good seal
- Choose a leaf amount and disiccant flow rate that
keeps humidity constant
- Wait an appropriate interval until CO2 begins to reduce
into the chamber and press LOG.
- Enter the leaf area. There are two ways to enter leaf
area: 1) during gas exchange: if one can measure area as
the leaf site in the chamber, one can set up the console
to prompt you for leaf area before the program computes
the values. 2) After readings: one can always edit the
data files to enter correct area and recompute values
- To take a new measurement, clear the pad.
Formulas to calculate photosynthesis and related
The usefulness of photosynthesis (A) measurements are
enhanced by the simultaneous measurement of transpiration (E).
Both A and E share the stomata as a port of control. Thus, leaf
conductance to water vapor (glw) also determines the internal CO2
concentration (Ci). Ci represents an indicator of the
availability of the primary substrate (CO2) for A. Furthermore,
Ci is a meaningful parameter for balancing the biochemical and
diffusion limitations to photosynthesis. The determination of
conductance is thus an important aspect in both comparative and
predictive photosynthesis studies. See
Canopy gas exchange measurements
Canopy measurements have the advantage of minimizing the
inconsistencies commonly associated with individual leaf
measurements. These variations arise mainly from the heterogenous
distribution of leaves within the canopy caused by the
non-uniform distribution of radiation, humidity and heat.
Both LI-6400 and CI-500 portable photosynthesis meters can be
used for measurement of canopy gas exchange in the open mode
design. A special chamber has to be built, however. A brief
description of canopy measurements using the LI-6400 follows. see diagram
The canopy chamber structure can be covered with either
acrylic or propylene. The chamber houses a quantum sensor in its
top, a couple of mixing fans, and a thermocouple. The inlet air
stream is provided by a blower. The air flowed through the blower
is previously mixed by means of a buffer volume box and its
temperature and relative humidity measured as well. The infrared
analyzers of the LI-6400 measure CO2 and H2O concentrations as
air enters the chamber and as it leaves. A fraction of the inlet
air (the "reference") and from the outlet (the
"sample") are passed through the sensor head of the
Some limitations are inherently associated with large field
chambers. For example, leaf temperature, wind patterns and
evapotranspiration inside the sun-exposed chamber do not resemble
natural conditions. Control of the chamber temperature is
Field, C.B., J.T. Ball, and J.A. Berry. 1989. Photosynthesis:
principles and field techniques. In (Ed) Pearcy R.W., Ehleringer
J.R., Mooney H.A., and Rundel P.W. Plant Physiological Ecology.
Field methods and instrumentation. Chapman and Hall. NY. pp.
Anonymous. 1996. LI-6400 portable photosynthesis system.
LI-COR, Inc. 11p.
Anonymous. Measuring canopy gas exchange with the LI-6400
portable photosynthesis system. LI-COR, Inc. Application note 2.
Anonymous. CI-500 photosynthesis measurement system. CID, Inc.
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were reproduce with the approval of each company.