Making Measurements
Answers to Questions
Using the LI-6400
4-53
Answers to Questions
Relative humidity is also a function of temperature, so using the
coolers to bring down the chamber temperature will increase the
RH_S_%
value (or warming the chamber will decrease it) even though
H2OS_mml
re-
mains unchanged. Conversely, raising the chamber temperature will lower
the
RH_S_%
.
D
CO
2
nor
D
H
2
O changed by a factor of 4.
The
D
CO
2
value wonÕt because we are holding the reference CO
2
constant,
so decreasing the flow lowers the ambient CO
2
for the leaf, and the photosyn-
thetic rate drops (unless you are on the flat part of a CO
2
response curve, such
as C
4
plants at high CO
2
). If, however, we had been maintaining a constant
sample cell CO
2
, we would have seen a 4-fold increase in
D
CO
2
, at least until
some stomatal changes occurred. The
D
H
2
O value, on the other hand, canÕt
increase by 4 because the transpiration rate has to fall as the humidity increas-
es in the chamber.
If the chamber walls were equilibrated at the higher humidity, a
drop in humidity will cause water to come off the walls and be added to the
air stream.This would make the
D
H
2
O too big, and the conductance high.
Both leaf conductance and water sorption affect the water vapor
in the chamber, but they have different time scales. Water sorption effects
will be most pronounced in the first minute or two after a change in chamber
humidity. Stomata usually takes many minutes to change. Therefore, appar-
ent conductance changes in the first minute after a large humidity change are
mostly due to water sorption; after that, itÕs probably stomatal change.
It either means the desiccant isnÕt very good, or the IRGA is not
properly zeroed.
Decrease. The incoming air is drier, so the flow rate drops (weÕre
maintaining a constant chamber humidity), so the leaf has more time to re-
move CO
2
from the air as it passes through the chamber.
Cooling will cause relative humidity to increase, so flow rate will
have to increase to compensate.
Warming the air will not directly change the vapor pressure of the
air, but it has a profound effect on the
saturation
vapor pressure of the air.
Therefore, warming will increase the vapor pressure deficit and lower the rel-
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 ...