Ecology Laboratory, BIO 320L Morehouse College
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Laboratory 6
Induction of Secondary Chemical Defenses
Objectives
1. Evaluate the presence of toxins in the leaves of tobacco plants (Nicotiana).
2. Perform an experiment to address the question: Does leaf damage, such as that caused by
herbivores, induce an increase in secondary compound toxin concentration in tobacco?
3. Address the question: Is there a difference in the response of tobacco plants to physical
damage alone and actual herbivore damage?
Introduction
At first glance, plant-herbivore interactions seem to be a highly unequal interaction
between a mobile and responsive predator attacking an immobile and helpless prey plant. Yet,
first impressions can be deceiving. Plants, in fact, are not helpless prey. Although they are
sessil, most plant produce two types of defenses, physical and chemical. The timing of life
history events, such as the production of flowers and fruits, can also be considered a form of
defenses (for example, seed masting). Physical defenses include increased tissue toughness by
means of cellulose and the production of defensive structures such as hairs, spines and thorns.
Chemical defenses are part of an extremely diverse collection of compounds that are not part of
the metabolic processes that plant require for their growth and maintenance in the absence of
herbivores. Given the accessory nature of these chemicals, which include non-photosynthetic
pigments and defensive chemicals, these compounds are termed secondary chemicals and
secondary chemical defenses.
Thousands of secondary chemicals have been identified in plants and many have clearly
demonstrated defensive functions (anti-herbivore, anti-microbial or anti-fungal activity) (Feeney
1992, Harborne 1993, Whittaker and Feeney 1971). These chemicals include nitrogen
compounds, terpenoids, and phenolics, and include chemicals that are important in human
affairs. Compounds traditionally used as spices are often anti-microbials (Billings and Sherman
1998). The pleasure we get from consuming the plant products coffee, tea and chocolate is
provided by a nitrogen compound, caffeine (an alkaloid), which is produced by plants to poison
their herbivores. Cocaine, morphine and nicotine are in this same class of secondary chemicals.
The complexity of many secondary chemical defense compounds and the use of limiting
