Making Measurements
Light Response Curves
4-24
Using the LI-6400
4
Light Response Curves
Starting from total darkness, in which there can be no photosynthesis, the first
few photons to be absorbed by the leaf will be used with greatest efficiency.
As light increases, the efficiency drops, and eventually subsequent increases
in light yield little or no increase in photosynthesis. Thus, a light response
curve can provide measures of dark respiration rate, the light compensation
point (absorbed quantum flux for which photosynthesis and respiration are
balanced), the quantum efficiency (initial slope), and the maximum photo-
synthetic rate. Shade adapted species tend to have lower dark respiration
rates, lower compensation points, and lower maximum photosynthetic rates
than sun adapted leaves. Quantum efficiency tends to be conservative, how-
ever.
Light Curve Strategies
Depending upon what you are trying to measure, there are a couple of ap-
proaches to light curves.
Rapid
Since the photosynthetic apparatus responds almost immediately to light, es-
pecially drops in light, the quickest method is to start with a leaf equilibrated
to high light, and fairly rapidly decrease the light, spending perhaps 1 or 2
minutes at each light value, and dropping in steps of 200 µmol mol
-1
or less.
When you do this, youÕll find that the stomata have not had time to adjust, and
tend to be more open at the low light values than they normally would. This
manifests itself as a steadily rising C
i
throughout the measurement. ThereÕs
nothing wrong with this, but be careful how you use the conductance mea-
surements from a rapid light curve, because they are not equilibrated values.
Slow
Another approach is to do a slow curve, giving the stomata time to equilibrate
at each light level. Going slowly, you can work from dark to light, or light to
dark. (If you are using a red only light source, however, beware; the stomatal
behavior will be artificial. Our comparisons of the red+blue LED source and
sunlight show them to have the same influence for opening stomata, howev-
er.) If you wait 15 or 20 minutes at each light level, you will find that C
i
will
be fairly constant throughout the measurement, indicating that the stomata are
fully adjusted. In fact, you could use C
i
as an indicator of when to log the next
record, at all but the darkest light levels.
Survey
A third approach is to generate a light curve using multiple leaves that are
equilibrated at a range of light values. Experiment #6 on page 4-18 uses this
Summary of Contents for LI-6400
Page 1: ...Using the LI 6400 Portable Photosynthesis System ...
Page 15: ...Part I The Basics ...
Page 16: ......
Page 174: ...Making Measurements Answers to Questions 4 56 Using the LI 6400 4 ...
Page 175: ...Part II Useful Details ...
Page 176: ......
Page 200: ...Standard Tools Power ON Hooks 5 24 Using the LI 6400 5 ...
Page 214: ...Real Time Data Real Time Graphics 6 14 Using the LI 6400 6 ...
Page 234: ...Environmental Control Light Control 7 20 Using the LI 6400 7 ...
Page 244: ...Light Sensor Considerations Gallium Arsenide Phosphide GaAsP Sensor 8 10 Using the LI 6400 8 ...
Page 288: ...Data Logging Making Your Own AutoPrograms 9 44 Using the LI 6400 9 ...
Page 289: ...Part III Working With Files ...
Page 290: ......
Page 312: ...The LPL File System Troubleshooting 10 22 Using the LI 6400 10 ...
Page 340: ...Downloading Files Using a Data Capture Program 11 28 Using the LI 6400 11 ...
Page 375: ...Part IV Configuration Issues ...
Page 376: ......
Page 420: ...Defining User Variables Old Style vs New Style 15 18 Using the LI 6400 15 ...
Page 454: ...Using an Energy Balance Further Reading 17 12 Using the LI 6400 17 ...
Page 455: ...Part V Maintenance Troubleshooting ...
Page 456: ......
Page 572: ...Troubleshooting Useful Information 20 46 Using the LI 6400 20 ...
Page 593: ...Part VI Programming ...
Page 594: ......
Page 622: ...Programming with LPL Compiler Directives 22 28 Using the LI 6400 22 ...
Page 846: ...Index I 16 Using the LI 6400 ...