LI-COR LI-6400, Manual

Page: 536 / 846

Troubleshooting
Unreasonable Results
20-10 Using the LI-6400
20
Photosynthetic Failures
Photosynthetic rate (Equation (1-15) on page 1-10) is primarily based upon
a) the difference between sample and reference CO
2
readings and b) flow
rate, so look at those three variables ( CO2R_µml , CO2S_µml , and Flow_µml )
to determine which, if any, is causing problems. There is also a dilution cor-
rection, so wild values for
H2OR_mml and/or H2OS_mml can also have an
effect.
■ ■ ■ ■
Unstable Photosynthetic Rates
If photosynthesis seems to be jumping around, try these suggestions:
1 Are you just impatient?
Bear in mind that right after a change in input conditions (such as a substantial
change in the CO
2
mixer setting), there will be a short period (up to 1 or 2
minutes) where the photosynthetic rate may be nonsense, since both IRGAs
are coming to new equilibrium values.
2 WhatÕs the magnitude of the variation?
There will always be some variation in the displayed value of any measured
or computed quantity. Is the variation you see excessive? Is it due to the nor-
mal noise in the analyzers? Remember that at low rates, the noise in the CO
2
differential (typically 0.4 ppm) will become more and more significant. (So:
is the variation in
D
CO2 greater than 0.4 ppm?).
3 Watch those flow rates
For purposes of troubleshooting, operate in fixed flow mode, and set the flow
to about 500 µmol s
-1
. If you are operating in constant VPD mode, or constant
RH, you could be having problems by asking for a humidity value that cannot
be achieved given the flow limitations and transpiration rate of the leaf. For
example, if you have asked for 80% RH, and have a stressed leaf with nearly
closed stomata, and are using very dry input air, your flow rate will go to zero
(or about 30 µmol s
-1
with a CO
2
mixer installed) while the system waits in
vain for the leaf to raise the humidity to what you asked for. Meanwhile, the
computed photosynthetic rate will be quite unstable, being the product of a
growing CO
2
differential and a near-zero flow rate.
If the photosynthetic rate is low, try operating at a low fixed flow rate (such
as 100 µmol s
-1
); this will a) keep the flow rate stable, and b) make the CO
2
differential as large as possible. See Dealing With Low Rates on page 4-49
for other suggestions.