TARGAS-1 Operation Manual V. 1.02
26
Leaf Gas Exchange Measurements –
Recommended Set-up
When performing leaf gas exchange measurements in the lab or field, calculations are based on the
changes in CO
2
and H
2
O gas concentrations between the reference and analysis (sample) air streams.
The reference air source is normally outside air (ambient) which contains both CO
2
and H
2
O and is the air
that is supplied to the leaf cuvette. The analysis air is the air containing the sample CO
2
and H
2
O gas
coming from the leaf cuvette.
For healthy plants you would expect that the analysis CO
2
concentration (CO2a) will be less than the
reference CO
2
concentration (CO2r) as the plant will be taking up CO
2
. Initially the CO
2
analysis
concentration (CO2a) may increase when the cuvette head is open to enclose the leaf (normally due to
local breathing by the user) but it will fairly rapidly begin to decrease and slowly drop after closing the
head on the leaf. On the H
2
O side, the analysis H
2
O concentration (H2Oa) should be higher than the
reference H
2
O concentration (H2Or) as the leaf will be adding moisture to the air and commonly referred
to as “transpiration”. For best results you want to make sure that your reference air is as stable as
possible especially when working on plants that have low rates of assimilation. Therefore we strongly
recommend that your source of reference air is drawn from a stable source away from any CO
2
influences
or disturbances such as people breathing, ventilation systems, parking lots or highways, automobiles, etc.
This can be achieved by using a homemade buffer volume (i.e. 20 liter bucket, large water container, etc.)
or using the air supply intake unit provided by PP Systems. The air supply intake unit draws ambient air
from about 2.3 meters above the ground and should help to smooth the reference air allowing for rapid
and accurate measurement of leaf gas exchange. On windy days additional smoothing may be required
but for most conditions the air supply intake unit should work well.
When should I record a measurement?
This is a very good question. Generally speaking it really depends on the biological state of the plant and
local environmental conditions. For many healthy plants under sunny conditions you could expect the
plant to reach equilibrium in approximately 60 seconds. For plants that are not as healthy and
experiencing stress conditions (i.e. drought, heat, salinity, etc.) it may take longer up to several minutes.
After placement of the leaf inside the chamber we recommend monitoring either the CO2d value
(Measure Screen 1) or A value (Measure Screen 2) and when it stabilizes then it is time to record the
measurement. Measurements can be recorded by pressing the
RECORD
button on the console display
or by pressing the record switch located on the PLC handle.
Important Note.
It is not unusual to see fluctuating or unrealistic “calculated” parameters (i.e. A, gs, Ci,
etc.) with an empty chamber and the head closed. Do not be concerned by this. What is important is that
your measured data is stable and CO2d and H2Od values close to 0 as described on the previous page.
The calculated parameters become relevant only after enclosing a leaf in the chamber and during actual
leaf gas exchange measurements.
For more information related to photosynthesis measurements and equations see Appendix 1.
Photosynthesis Equations Used in TARGAS-1 on page 150.
