LI-COR LI-6400, Manual

Page: 126 / 846

Making Measurements
Some Simple Experiments
4-8 Using the LI-6400
4
vey measurements (page 4-21), going from leaf to leaf and only taking a
minute or so to get each measurement.
If you are new to gas exchange measurements on plants, continue on with the
next section (
Some Simple Experiments ). It will take you through some
things to look for to make sure you are making a valid measurement.
Some Simple Experiments
If you have not had much experience making gas exchange measurements be-
fore, you may wish to work through some of the experiments in this section.
To do any of them, however, first get a leaf established in the chamber:
■ ■ ■ ■
Do this first
1 Select a plant and leaf to measure
The easiest measurement will be on an adequately watered plant that is grow-
ing in full or partial sun. By contrast, a difficult measurement will be on a dry,
neglected house plant that has only seen dim (for the plant) fluorescent lights
its whole life. Try to find something more like the former than the latter.
2 Do Steps 1 through 6 in the previous section
This will take you through getting the leaf into the chamber.
3 Observe the CO
2
concentrations
Note CO2S_µml . Is it below CO2R_µml ? If so, thatÕs good, because it means
photosynthesis is occurring. (Photosynthetic rate will be on display line c , un-
der Photo .) Is CO2S_µml stable (within 0.2 or 0.3 µmol mol
-1
)? It should sta-
bilize after 30 seconds or so of clamping onto the leaf. If itÕs not stable, check
CO2R_µml for stability. Perhaps the mixer isnÕt stabilized yet, or you need a
buffer volume. Consult Unstable Photosynthetic Rates on page 20-10, if
you need help fixing this instability. If CO2S_µml is not below CO2R_µml ,
then perhaps you need to match (page 4-34).
4 Observe the humidity values
The RH_S_% value on display line b is the relative humidity in the sample
IRGA. ItÕs calculated from the water IRGA signal, which is H2OS_mml . Oth-
er things being equal, you want to make gas exchange measurements in as
high a humidity as possible, without letting the flow rate (which is determin-
ing that humidity) go too low. 200 or 300 µmol s
-1
is a reasonably low flow
range; but you can drop to 100 if you need to. (Leaks are a bigger problem at
low flow rates - see
Leaks on page 4-42.)
You are now ready to do some or all of the following elementary experiments.