LI-COR LI-6400, Manual

Page: 133 / 846

Making Measurements
Some Simple Experiments
Using the LI-6400 4-15
centration: 400, 300, 200, 100, and 30 (C
3
) or 0 (C
4
). The last point is de-
signed to be below the compensation point.
Question #11: Notice when you change the CO
2
concentration (whether you
did it with the mixer, or the soda lime tube scrub knob) the indicated photo-
synthetic rate (
Photo ) becomes quite erratic. Why? (Answer on page 4-54.)
Question #12: If you are controlling CO
2
by varying the soda lime scrub
knob, under what circumstances might you expect changes in this knob set-
ting to affect the flow rate through the chamber? (Hint: itÕs a humidity control
question. Answer on page 4-54.)
5 Wait for stability, then match and log
Wait for a minute or so for the photosynthetic rate to stabilize, match the IR-
GAs, then log another record (
f1
level 1).
6 Repeat until done
Repeat Steps 4 and 5 until you are done. Try to get about 4 or 5 points between
your starting value and ending points. Go down to 30 µmol mol
-1
or so for C
3
plants, and 0 for C
4
Õs. (Hint: If you are using the CO
2
mixer, you get 0
µmol mol
-1
by turning the mixer off.)
At any point along the way, you can view a graph of your logged data by do-
ing Step 8.
7 Finish the curve back at the starting point
Repeat the starting point. See how long it takes for the photosynthetic rate to
return to normal. (Hint: donÕt spend too long at lowest CO
2
value.)
If you have a CO
2
controller, do some points above ambient, such as 600,
800, and 1000 µmol mol
-1
. (If global warming scenarios are keeping you
funded, go right on up to 2000.)
8 View the Graph
You can view your curve with GraphIt (press
View File
(
f2
level 1 in New
Measurements mode). If the axes are not defined for an A-Ci, press
QuikPik Config
(
f1
) and select ÒA_Ci CurveÓ. Press
REPLOT GRAPH
(
f2
) and
draw it. Hint: If your low CO
2
point had negative photosynthesis (respira-
tion), you may want to change the default A-Ci plotting to automatically scale
the axis minimum for photosynthesis. Otherwise, it wonÕt show that point.