LI-COR LI-6400, Manual

Page: 131 / 846

Making Measurements
Some Simple Experiments
Using the LI-6400 4-13
2 Change to constant sample cell CO
2
If you have a CO
2
mixer, switch over to controlling a constant sample cell
concentration. Target the present value of CO2S_µml . Wait for CO2S_uml to
stabilize.
3 Turn the desiccant knob to full scrub
Watch CO2R_µml and CO2S_µml . The latter is supposed to be held constant.
CO2S_µml will drift well off target, then come back to where it was as
CO2R_µml adjusts.
Question #9: Will CO2R_µml increase or decrease? (Answer on page 4-54.)
Note: Keep in mind that this test of abruptly changing the incoming humidity
while trying to control both chamber humidity and CO
2
is an artificial worst
case. More typically, the flow control system is balancing stomatal changes,
which happen much less rapidly, so the sample cell CO
2
control option isnÕt
faced with large swings in flow rate. The next two steps will illustrate a more
typical sequence of events.
4 Return the desiccant knob to mid-range
Watch the sequence reverse itself. Also note the order in which things hap-
pen: Once
Flow_µml stabilizes, then CO2S_µml can stabilize.
5 Shade the leaf
If you are using a light source, cut the light value in half. If you are not, just
shade the leaf with your hand. Before you do, however, hereÕs another learn-
ing opportunity:
Question #10: What do you expect to happen to photosynthesis ( Photo ), sto-
matal conductance ( Cond ), and intercellular CO
2
( Ci ) when you cut the light
in half? How do you expect the control systems to compensate: specifically,
how will flow rate change, and how will reference CO
2
change? (Answer on
page 4-54.)
6 Watch the response
Photosynthesis will immediately start to drop, and (if you wait 10 or 15 min-
utes), conductance will eventually decrease as well.