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The Effect of Elevated CO2 on Tobacco
The purpose of this lab is to investigate the ways that
organisms might respond to the increasing concentration of
carbon dioxide in the Earth's atmosphere. Specifically,
you'll be testing the prediction that the rate of insect
herbivory will be changed by testing how Manduca sexta
larvae respond to feeding on tobacco plants grown in an
enriched carbon-dioxide atmosphere.
In this lab we will be concerned with both development
and physiological responses of organisms to human-caused (or
anthropogenic) changes in the atmosphere. Such changes are
currently of intense interest, as demonstrated by frequent
newspaper headlines about ozone depletion and global
warming. One change in the atmosphere that is
well-documented is the increase in carbon dioxide
concentration. Carbon dioxide now makes up only about 0.03%
of the atmosphere, but it has been increasing steadily since
the middle of the 19th century. There is controversy about
the exact cause of the increase, though there is general
agreement that it is caused by human activities. The two
main hypotheses for the increase are 1) burning of fossil
fuels and 2) deforestation, particularly of tropical
rainforests. Carbon dioxide is one of the "greenhouse"
gasses, and many scientists predict that more carbon dioxide
in the atmosphere will lead to a warming of the global
climate. Because of the extreme complexity of the global
biosphere/atmosphere system, it is difficult to make exact
predictions of how the climate will change in response to
carbon dioxide increase.
This lab will concentrate on the direct (non-climatic)
effects of an increase in atmospheric carbon dioxide on
insect herbivory on plants. At an ecosystem level, changes
in patterns of herbivory on plants could have widespread
impacts on both natural and agricultural ecosystems in all
parts of our planet.
Although CO2 is only 0.03% of the atmosphere,
as mentioned above, carbon makes up 45% of the dry weight of
a leaf. This should make it clear that plants are very
successful at getting carbon out of the air, and "fixing"
the carbon from carbon dioxide into forms it can use (like
sugars). The table below gives you an idea of the other
Proportion of Leaf Dry Weight
Proportion of Atmosphere
In order to understand the effects of increased
CO2 on plants, and the subsequent effect on
herbivores, you need to understand how plants use
CO2. Use your text and the descriptions below to
start thinking about how increased CO2 might work
to change the physical characteristics you'll be looking at
this week in lab.
What do plants do with CO2?
- CO2 serves as a source of energy for
respiration; plants use sunlight to "fix" carbon into
sugars, which serve as long term energy currency.
- CO2 also provides carbon for structural
elements in plants; cell walls, for example, are almost
What do plants do with O2?
- O2 is necessary for cellular respiration
to occur; it serves as the final electron acceptor in the
electron transport chain, which provides the cell with
most of its ATPs.
- O2 provides oxygen molecules which are
incorporated into structural molecules like
What do plants do with N2?
- Plants can NOT use N2 in its gaseous form.
They rely on microorganisms to convert N2 into
a form they can use.
- Nitrogen is vital for plants to make all the proteins
they need. The only way (most) plants get amino acids is
by making them "from scratch," which requires nitrogen.
Nitrogen is a common ingredient in fertilizer, and is
often a limiting factor in plant growth.
You will use tobacco plants (Nicotiana tabacum) grown in
growth chambers of two types. One chamber type had ambient
levels of carbon dioxide while the other type had extra
carbon dioxide pumped directly into it; the two types of
chambers were otherwise matched for all environmental
variables (temperature, light, etc.). We have been
monitoring carbon dioxide levels in the chambers daily, and
will provide you with data on the exact levels.
Rufty, T. W. Jr., D. M. Jackson, R. F. Severson, J. J.
Lam, Jr., M. E. Snook. 1989. Alterations in growth and
chemical constituents of tobacco in response to
CO2 enrichment. J. Agric. Food Chem.
Investigating the effect of elevated CO2 on