Download AGRON 183: Photosynthesis

AGRON 183: Photosynthesis
Dr. Brian Hornbuckle
December 1, 2016
Introduction
I think I can safely say that every agronomist is aware of the process of photosynthesis, as described
by the following chemical equation.
CO2 + H2 O + light → carbohydrate + O2
(1)
Photosynthesis is what makes plants so special. As Dr. Lamkey, the chair of our department, likes
to say,
I got hungry and went outside and stood in the sun for a while, but didn’t get full. So
I went and found some plants to eat!
Plants are special because they can make their own food from sunlight! This is really an amazing
feat, and when you think about it, it is perhaps the primary reason why life exists on our planet.
Agronomists have the important task of managing these amazing plants that use sunlight, carbon
dioxide, and water to provide us with food, feed for livestock, fiber for clothing and building
materials, and fuel for vehicles and other machinery.
While the process of photosynthesis is likely familiar to everyone, there are some details and
technical terms that are often misunderstood. Here are three of them.
assimilation The uptake of CO2 by a plant in order to perform photosynthesis according to (1).
Carbon dioxide enters into a leaf via small holes called stomata. We will use the variable A
CO2
to quantify assimilation with the units of mol
.
m2 s
photosynthetically–active radiation (PAR) Not all light from the Sun is used in photosynthesis. In (1), “light” is actually just the photosynthetically–active radiation, or PAR for
short. Note that green light is not photosynthetically–active, which is why plants that are
conducting photosynthesis scatter this wavelength of light and look “green” to us.
transpiration The loss of H2 O from a plant. Water taken by the plant from the soil via the roots
makes its way to the leaves via the plant’s vascular system. Some of this water is used in
photosynthesis according to (1). However, when the stomata open to take in carbon dioxide,
some of the left–over water evaporates and leaks out. Notice that this evaporated water does
not appear in (1)! However, it is always happening when photosynthesis occurs, because if
the stomata are open to take in CO2 , H2 O will always be lost! We will use the variable T to
H2 O
quantify transpiration with the units mol
.
m2 s
In this data collection activity we will make measurements relevant to photosynthesis, both at
the scale of an individual leaf, and for an entire plant canopy (collection of many plants).
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Figure 1: The LI-6400XT is able to measure transpiration, T , and assimilation, A, by keeping track
of what goes into the control volume, and what goes out of the control volume.
Objective
There are two objectives for this laboratory.
1. Measure A and T while varying the following:
(a) ambient carbon dioxide (CO2 ) concentration;
(b) ambient water vapor (H2 O) concentration (humidity);
(c) incident photosynthetically–active radiation (PAR); and
(d) temperature.
2. Measure the area of leaves of various sizes and shapes.
We will use a special instrument called the LI-6400XT Portable Photosynthesis System to
measure A and T , and use three different methods to measure leaf area. This equipment belongs
to the crop production and plant physiology group in our department, and is used by many to
collect data for research projects. We will be joined today by to members of that group, Dr. Emily
Heaton, a faculty member in our department, and Dr. Nic Boersma, a research scientist working
with Dr. Heaton.
The data we collect will enable you to answer questions like the following.
How does carbon assimilation change as the amount of incident light is increased?
How does transpiration change as the amount of incident light is increased?
What is the optimum level of ambient CO2 concentration?
Do two methods of leaf area measurement give the same answer?
Can measurements of leaf length and width be used to estimate leaf area?
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Some Background
How can an instrument like the LI-6400XT measure A and T ? Consider Figure 1. Think of the
box labeled “control volume” as an imaginary boundary surrounding a leaf. It is called a control
volume because we will keep track of what goes into the box, what goes out of the box, and use
the principle of the conservation of mass (mass is neither created nor destroyed) to determine A,
T , and also O, the release of O2 created through photosynthesis from the stomata as shown in (1).
O2
The units of O are mol
.
m2 s
First, think about all of the air going in and out of the control volume. The air contains many
types of gases. We are primarily concerned with three of them: H2 O, CO2 , and O2 . If fi is the
flow rate of air into the control volume with units mols air , and fo is the flow rate of air out of the
control volume also with units mols air , then:
fo = fi + s T
(2)
where s is the area of the leaf that is transpiring. Assuming the units of s are m2 , then the quantity
s T has units of mol sH2 O . What (2) says is that the rate at which moles of air (particularly H2 O,
CO2 , and O2 ) are coming into the control volume must equal the rate at which moles of air are
going out. Note that H2 O comes into the control volume in two ways. The first is with the flow of
air associated with fi (assuming the air is not perfectly dry). The second is through the stem of
the leaf. This water entering through the stem is released into the control volume via T and adds
to the moles of gas inside the control volume.
Why don’t we consider A and O from Figure 1 in (2)? It’s because the same amount of gas
taken up by the leaf (moles of CO2 ) is released by the leaf (moles of O2 ): for every molecule of CO2
assimilated, a molecule of O2 is produced according to (1). So the net effect of photosynthesis, in
terms of gas molecules, is only to add water vapor to the control volume.
Next, think about the moles of H2 O going into and out of the control volume in Figure 1. If
H2 O
wi is the mole fraction of water vapor coming in with units mol
mol air , and wo is the mole fraction of
H2 O
water vapor going out also with units mol
mol air , then we can write the following.
fo wo = fi wi + s T
(3)
H2 O
mol H2 O
Each term in (3) has the units mols air × mol
.
mol air =
s
Finally, consider the moles of CO2 coming in and out of the control volume in Figure 1. If ci
CO2
is the mole fraction of carbon dioxide coming in with units mol
mol air , and co is the mole fraction of
mol CO2
carbon dioxide going out also with units mol air , then we can write the following.
fo co = fi ci − s A
(4)
CO2
mol CO2
Each term in (4) has the units mols air × mol
. Also notice the minus sign in (4): in
mol air =
s
this case the leaf is removing CO2 from the air inside the control volume.
Combining (2) with (3) yields the following expression for transpiration from the leaf inside the
control volume.
fi (wo − wi )
T =
(5)
s (1 − wo )
And combining (2) with (4) yields the following expression for assimilation inside the control volume.
A=
fi
(ci − co ) − T co
s
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(6)
Figure 2: To get transpiration or assimilation for an entire plant canopy (collection of plants), you
need to know how many leaves are conducting photosynthesis, and their area.
So what the LI-6400XT does is form a control volume around a leaf, which defines s, and then
measures fi , wo , wi , ci , and co . Then (5) and (6) can be used to find T and A, respectively.
The LI-6400XT can measure A and T for individual leaves. But how much CO2 is being taken
up and how much H2 O is being released by a canopy of many plants? Look at Figure 2. To scale
up A and T from a single leaf to a canopy, you must determine the total area of leaves conducting
photosynthesis. Therefore it is important to measure leaf area. We will be using three methods to
determine leaf area.
• A laboratory method for which leaves must be detached from a plant (destructive leaf area).
• A field method that can be done while a leaf is still attached to a plant (nondestructive leaf
area).
• A manual measurement in which grid paper will be used to find the area of individual leaves.
General Instructions
Each team will rotate through four data collection centers. See Figure 3. One team will move from
each center to the next center every 10 minutes.
There are four individual stations in the “assimilation and transpiration” center, so during each
10–minute period there will be four teams, one at each station, at the assimilation and transpiration
center. It is at this station teams will be using LI-6400XT instruments to measure A and T .
Instructions and data sheets for each station can be found on Blackboard.
The team that leaves the fourth station at the assimilation and transpiration center will next
go to the “destructive leaf area” center. Only one team at a time works in this center, and only
10 minutes is needed to complete the activity. The instrument at this station is the LI-3100C Area
Meter. The manual can be found on Blackboard.
From the destructive leaf area center teams will go to the “manual leaf area” center and remain
there for approximately 80 minutes. At this center teams will do the following.
1. Cut out your paper leaves.
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Figure 3: Rotation among data collection centers.
2. Label each leaf using the naming convention on Blackboard.
3. Use the graph paper to find the area of at least three leaves. Pick at least one small leaf, at
least one medium–sized leaf, and at least one large leaf. Note that each grid square is 1 mm2 .
4. Measure the length and width of each leaf.
It may take your team longer than others to complete the tasks at this center. In any case, one
team must leave this center every 10 minutes. If you do not finish your tasks, then complete them
when your team rotates back through.
From the manual leaf area center, teams will go to the “nondestructive leaf area” center. Here
you will use one of two LI-3000C instruments. See the manual on Blackboard. Measure as many
leaves as you can: some will be too small. Again, one team must leave this center every 10 minutes.
If you do not finish your measurements with the LI-3000C, you may have an opportunity to do so
if you rotate back through.
Equipment
Your team will check out the following equipment.
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1. A set of artificial leaves for each team member.
2. Three sheets of graph paper for each team member.
3. A tablet computer to record data, if so desired.
At each center there will be one or more instruments that you will use. For carbon assimilation
and transpiration, your team will use a LI-6400XT Portable Photosynthesis System. For destructive leaf area measurement, your team will use a LI-3100C Area Meter. For nondestructive leaf
area measurement, your team will use a LI-3000C Portable Area Meter. For manual leaf area
measurement, your team will use our 1.5-m measuring poles and graph paper.
Minimum Set of Measurements
Measure all of each team member’s leaves with the LI-3100C at the destructive leaf area center.
Measure as many of each team members leaves with the LI-3000C at the nondestructive leaf area
center. Follow the instructions and record the data as directed at the assimilation and transpiration
center. Measure the length and width of all of each team member’s leaves and have each team
member find the area at least three leaves using the graph paper at the manual leaf area center.
Location
We will meet in our normal classroom at the Hansen Agriculture Student Learning Center and
then conduct some of our work in the atrium of Hansen.
Tentative Timeline
9-9:15 Determine group roles. Managers check out tablet computer from Kelsie, if you would like,
and your group’s leaves. Brief overview of data collection activities. Review instructions and
ask questions through your Communicator.
9:15-11:45 Measurement period. Four teams (Roush, Castillo, Carmenate, Knapp) will begin at
the carbon assimilation and transpiration center (one at each station), one team (Hugen) at
destructive leaf area, eight teams (Schnicker, Magallanes, Reicks, Schumer, Weathers, Bloome,
Korhonen, Geissinger) at manual leaf area, and two teams (Conner, Fevold) at nondestructive
leaf area. See Figure 3 for the rotation among centers.
11:45-11:50 Recorders upload data to CyBox. Managers return tablet computers to Kelsie.
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