plant. Once known as secondary metabolites, they are
now usually termed defense compounds and collectively
called allelochemicals. Daniel Janzen (1975) put it well:
“The world is not coloured green to the herbivore’s
eyes, but rather is painted morphine, L- DOPA, calcium
oxalate, cannabinol, caffeine, mustard oil, strychnine,
rotenone, etc.” Many familiar drugs ranging from
caffeine to curare to cocaine (chapter 17) are derived
from the allelochemicals of plants. There really is a kind
of “Neotropical pharmacy” in the rain forest.
Defense compounds likely originated as genetically
based accidental metabolic by- products or chemical
wastes that, by chance, conveyed some measure of
protection from attacks by microbes or herbivores.
Such mutations would thus confer fitness, and natural
selection would favor their rapid accumulation. Most
plant species contain a pharmacopoeia of secondary
metabolites that could function as potential defense
compounds. Some defense compounds function
principally to protect against herbivores, some to
protect against bacteria and fungi. What follows is
a brief introduction to some of the more prevalent
groups of defense compounds. It is not meant to be
comprehensive, but it should teach you enough that
you will not be very tempted to make a salad composed
of leaves taken randomly from tropical trees and
shrubs. On the other hand, be aware that the science
of ethnobotany is derived from the knowledge that
humans began to acquire long ago when they realized
the potency contained within the leaves, stems, and
roots of numerous species of tropical plants and began
to put plants to use. Ethnobotany is discussed more in
chapter 17.
Alkaloids
Alkaloids are a group of organic bases, all containing
nitrogen. The alkaloid group includes some familiar
and often addictive drugs. Some, such as the frog
toxins described above, are highly toxic. Others are less
so. Cocaine (from coca), morphine (from the opium
poppy), cannabidiol (from hemp), caffeine (from
teas and coffee), and nicotine (from tobacco) are each
alkaloids. Taken together, there are more than 4,000
known alkaloids globally distributed among 300 plant
families and over 7,500 species. A single plant species
may contain nearly 50 different alkaloids. Alkaloids are
found not only in leaves but almost anywhere in the
plant, including seeds, roots, shoots, flowers, and fruits.
Most alkaloids taste bitter. In mammals, depending
on dosage level, they may act as a stimulant (think
about caffeine, for example) but they also may
interfere with liver and cell- membrane function, as
well as numerous other metabolic functions. The bitter
taste combined with the potential negative effects on
digestion and liver function may discourage animals
from consuming alkaloid- rich vegetation, though the
evidence for this is not widespread. Perhaps more
significantly, alkaloids act to toughen leaves, and that
alone acts to discourage herbivory, since it imposes an
added cost to chewing and digestion.
In general, alkaloid- containing plant species are
more abundantly and disproportionally represented in
tropical latitudes than in higher latitudes. This pattern
suggests that there has been stronger selection pressure
in the tropics to evolve alkaloids as plant protective
compounds.Phenolic Compounds, Including Tannins
Phenolic compounds represent a diverse group of
organic chemicals common in plants of all kinds.
Some add pungency to spices. Some appear to function
as defense compounds. Phenolics are compounds
containing a hydroxyl group (−OH) directly bonded
to an aromatic hydrocarbon group. The compound
phenol (C6H5OH) is an example. Phenolics are stored
in cell vacuoles, which break when an insect or other
herbivore bites the leaf. Upon release, the phenolics
combine with various proteins, including those
enzymes necessary for splitting polypeptides (parts of
proteins) in digestion. This makes it more difficult for
the herbivore to digest protein. Leaf damage by insects
or pathogens may stimulate production of phenolics.
Tannins represent an important group of phenolic
compounds that are structurally more complex. They
are complex polyphenols with abundant hydroxyl
and carboxyl groups that bond with proteins. Tannins
are abundant in temperate and tropical oak leaves as
well as in many other tropical plant species, including
various mangroves. They provide the basic compounds
used in tanning leather. Most people who enjoy wine
know that tannic compounds are responsible for the
astringency of certain red wines.
Research by Phyllis Coley on Barro Colorado
Island, Panama, that focused on the early successional
tree Cecropia peltata showed that tannins are heavily
concentrated in young trees of this species but decline in188 chapter 11 evolutionary arms races: more coevolution, more complexity