Making Measurements
Light Response Curves
Using the LI-6400 4-25
approach. This has the advantage of being fairly quick, yet has equilibrated
values The potential for difficulty comes from using multiple leaves, thus
bringing age differences and other factors into the response curve. The survey
approach is better suited for some species than others. In deciduous trees, for
example, leaf age is not particularly related to position in the canopy. With
this approach you can achieve a range of light levels by selecting leaves that
are tilted with respect to the sun, and in varying degrees of shade. The orien-
tation of the sunlit leaves is a problem, however, unless you are using a light
source when you clamp onto them. With a clear chamber top, leaves that are
tilted with respect to the sun will be shaded by the chamber wall, and this is
to be avoided at all costs. If, however, you use a light source, you can set the
appropriate value first, or have it automatically track the ambient light as
measured by an eternal PAR sensor.
Sunfleck / Shade Method
The fourth approach offered here is to separate each new light level with the
starting light value, with time to equilibrate. That is, use a sequence such as:
1800, 1000, 1800, 500, 1800, 300, 1800 µmol m
-2
s
-1
. (The starting point
neednÕt be high; you could work the other direction with shade leaves.) Data
collected in this manner might be most appropriate for addressing questions
of light dynamics in canopies.
Operational Considerations
Once you decide on the strategy you wish to take, you then need to decide on
how the chamber controls should be set, and on how data is to be collected.
Light
The best light source for light response curves is the red+blue 6400-02B. The
red only 6400-02 source has the potential problem of allowing excessive (that
is, more than normal) stomatal closure as light decreases, or delaying sto-
matal opening as light increases.
Without the LED light source, a light curve cannot be automated, but is still
possible. Neutral density filters, for example, can provide means to reduce
sunlight or other sources by known amounts. The survey technique discussed
above could be done without a light source.
CO
2
It is important to maintain the chamber CO
2
concentrations as constant as
possible while measuring a light response curve. Otherwise, the effects of
CO
2
on photosynthesis will be confounded with the effects of light. If you
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