Transcript KramerSpr15x

The Use of Portable Gas Exchange Systems to Measure Plant Leaf Photosynthesis:
Comparing Different Methods to Control Humidity
Objective
The objective of this study was to determine if changing the LI-6400’s method of controlling atmospheric water affects
leaf photosynthesis and stomatal conductance (gas exchange) rates, the time it takes for these rates to stabilize, and
whether these differ in plants with contrasting photosynthetic pathways (C3,C4). I hypothesized that there would be no
significant difference in the gas exchange rates when either FLOW, RH, H2OS were held constant on the LI-6400. I further
hypothesized that the C3 species gas exchange rates would stabilize faster under constant RH and H2OS conditions when
compared to FLOW, but that the method of controlling water would have no effect on the C4 species.
LI-6400
s - 1 )
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C o le u s
A m a r a n th
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Coleus
Amaranth
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H2OS
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Measurement Constant
M easu rem en t C o n stan t
Figure 4. Gas Exchange Rates. A.) net leaf photosynthesis (µmol CO2 m-2s-1) and B.) leaf stomatal conductance to water vapor (mol H2O m-2s-1) of coleus and amaranth for each water control
method (FLOW, H2OS_mml, RH). Each bar represents the mean +/- SE of 4 replicates. Means with differing letters differ significantly at P<0.05, Tukey HSD within species
• The technique used to control atmospheric water did not affect leaf net photosynthesis or stomatal conductance to
water vapor in the C3 or C4 species (ANOVA, P=0.9784, Fig. 4A; P=0.9273, Fig. 4B).
Figure 1. LI-6400 portable infrared gas analyzer.
B.
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• This study was performed on the C3 species, coleus (Solenostemon scutellarioides), and the C4 species, amaranth
(Amaranthus caudatus), growing in the greenhouse at the University of Wisconsin-Eau Claire.
• Two separate plants of each species underwent measurements, for a total of two leaves per plant (n=4), using a LICOR 6400 when water was controlled for in three ways (FLOW, RH, H2OS).
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Methods
Coleus
Amaranth
H2OS
Time to Stabalization (s)
A.
Time to Stabalization (s)
The LI-6400 (LI-COR, INC., Lincoln, NE) portable infrared gas
analyzer consists of a console and a leaf chamber/IRGA
(Infrared Gas Analyzer). Once an attached leaf is in the
chamber, LI-6400 takes measurements of the gases going into
and out of the chamber. Using these measurements the
machine calculates net photosynthesis and stomatal
conductance, based on the equations derived by Caemmerer
and Farquhar (1981). Among the variables processed by the LI6400 are: flow rate, pressure, temperature, humidity, and leaf
area (Long, et al., 1996). All of these variables are
interdependent and a change in one can cause the others to
fluctuate. This creates a system where it is only possible to hold
certain variables constant while the others fluctuate such as
FLOW, RH, and H2OS.
B.
Stomatal Conductance
(mol m-2s-1)
• It is not known if the environmental fluctuations that result from using these different measurement techniques
differentially affect leaf responses, which could impact the speed and accuracy of measurements.
m - 2
• The LI-6400 can control for the water within the chamber three different ways by setting constant: the rate of airflow
across the leaf (FLOW), the relative humidity (RH), or the absolute amount of moisture in the chamber (H2OS).
( µ m o l
• Today over 95% of photosynthetic CO2 uptake measurements listed in journals use commercially available portable
infrared gas analyzers like the LI-6400 (Long and Bernacchi, 2003, Fig. 1).
Results
A.
N e t
• Photosynthesis is often measured during field and laboratory experiments as an important indicator of plant health
and function.
L e a f
Introduction
P h o t o s y n t h e s is
Bailey Kramer v Dr. Tali Lee (Mentor) v Biology Department v University of Wisconsin-Eau Claire
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Coleus
Amaranth
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H2OS
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Measurement Constant
Measurement Constant
Figure 5. Time to Stabilization (s) of A.) photosynthesis and B.) stomatal conductance to water vapor of coleus and amaranth for each water control method (FLOW, H2OS_mml, RH). Each bar
represents the mean +/- SE of 4 replicates. Means with differing letters differ significantly at P<0.05, Tukey HSD within species.
• FLOW was set at a constant rate of 400 𝜇 mol s-1.
• The technique used to control atmospheric water did not significantly affect the time to stabilization of photosynthetic
or stomatal conductance rates in coleus (ANOVA, P=0.124, Fig. 5A; P=0.3192, Fig. 5B), however time to stabilization in
amaranth did depend on measurement technique used (ANOVA,P=0.0011, Fig. 5A; P=0.0186, Fig. 5B).
• The LI-6400 was switched to holding H2OS constant and measurements were auto-logged every 30s until gas exchange
stabilized. This step was repeated holding FLOW constant and then RH constant.
• Photosynthesis in amaranth stabilized 88% faster, on average, when FLOW compared to H2OS was held constant and
85% faster compared to when holding RH constant (Fig. 5A).
• The sequence was reversed until all the leaves were measured, always allowing the leaf to stabilize before the
measurements were taken.
• Similarly, stomatal conductance in amaranth stabilized 76% faster, on average, when FLOW compared to H2OS_mml
was held constant, however, there was not a significant effect on time to stabilization when holding FLOW vs. RH
constant (Fig. 5B).
Discussion
• The overall trend shows holding FLOW constant to be the fastest measurement technique for both species, which does not
support my hypothesis that FLOW would increase time to stabilization for C3 vs. no affect on C4.
• One similar study found that there were significant changes to leaf stomatal conductance shortly after humidity was
changed when measuring apple tree leaves (Fanjul and Jones, 1982). Using the LI-6400, when FLOW is held constant
humidity fluctuates. If the stomatal conductance in leaves is very responsive to these fluctuations then this could explain
why the rate of stabilization varied across species and technique used to control atmospheric water.
Figure 2. Amaranth (Amaranthus caudatus)
Resources:
Fanjul, L., Jones, H.G. 1982. Planta. 154: 135-138.
Long, S.P., Bernacchi. C.J. 2003. Journal of Experimental Botany. 54: 2393-2401.
Long. S.P., Farage. P.K., Garcia. R.L. 1996. Journal of Experimental Botany.47: 1629-1642.
Figure 3. Coleus (Solenostemon scutellarioides)
Acknowledgements:
Lynn Young-Janik for supplying the plants
&
Learning and Technology Services (LTS) for printing this poster.
• Another study assessed the ability of stomatal function to decrease the photosynthetic rate of a leaf (Long and Bernacchi,
2003). Due to the dependence of C3 photosynthesis on stomata that readily fluctuate in response to changes in
atmospheric water this could explain why coleus did not stabilize faster under any of the measurement methods. The C4
species have a CO2 concentrating mechanism which allows for greater independence from stomatal fluctuations and
might be the reason the C4 species was able to stabilize faster under the constant FLOW conditions.
• In conclusion, these results suggest that when using the LI-6400 to measure photosynthesis and stomatal conductance,
the method used to control atmospheric water will not impact measured rates, but for some species the method chosen
could reduce the time it takes for rates to stabilize.