Field Photosynthesis Measurement Systems

David G. Reta-Sánchez Alvaro Anchondo-Najera

dretasan@nmsu.edu janchon@nmsu.edu

Héctor M. Quiroga-Garza Jesús G. Arreola-Avila

hquiroga@nmsu.edu jarreola@nmsu.edu

Rafael Figueroa-Viramontes Concepción Lujan-Alvarez

rfiguero@nmsu.edu clujanal@nmsu.edu

Introduction

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.

LI-6400 System

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 transpiration, respectively.

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 operation.

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 display.
Clamp the chamber onto the experimental leaf to be measured.
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 parameters

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 formulas.

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 LI-6400.

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 especiallly difficult.

REFERENCES

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. 209-253.

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. 8 p.

Anonymous. CI-500 photosynthesis measurement system. CID, Inc. 4p.