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Tobacco Hornworms


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The Effect of Elevated CO2 on Tobacco Hornworm Herbivory

--Introduction--

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 major players.

Element
Proportion of Leaf Dry Weight
AtmosphericSource
Proportion of Atmosphere

carbon

45 %
CO2
0.03 %

oxygen

45 %
O2
21 %

nitrogen

5 %
N2
78 %

hydrogen

1-3 %
H2
trace

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 entirely carbon-based.

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 cellulose.

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.

 

Background Paper:

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. 37:552-555.

 

Investigating the effect of elevated CO2 on herbivory:

Overview.


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July 1999