Induction

Furanocoumarin synthesis is inducible by mechanical damage, herbivore
feeding, and pathogen attack.

	An increase in furanocoumarin concentrations, comparable to the increase elicited by caterpillar feeding, follows mechanical damage, such as that shown at right. This damage was inflicted by pressing a regular arrangement of straight pins through the leaflet lamina. Damage to just one the several leaflets of a compound leaf in wild parsnip causes a localized induction of furanocoumarin production..


		Localized induction. Once damaged, the concentration of furanocoumarins in the damaged leaflet increases rapidly, reaching maximum values within 3 hours. Other leaflets on the same leaf are largely unaffected (see cartoon at left), although occasional increases of a lesser magnitude are sometimes manifested in adjacent leaflets on the same side of the midvein, including one half of the terminal leaflet.

	Anatomical Localization In foliage, the furanocoumarins are localized within oil tubes, which are closely associated with xylem and phloem vessels. Finely feeding first instar cabbage loopers, Trichoplusia ni, can remove virtually all of the interveinal tissue and yet furanocoumarin levels will increase up to ten-fold.

Only a handful of chemicals are significantly altered by damage (blue bars) --furanocoumarins are consistently increased by damage.

	Effect of induction on cabbage loopers. Caterpillars that were fed intact parsnip foliage, consumed more of the foliage and converted more of it to body mass than caterpillars fed lightly damaged tissue.
	This effect can be replicated by incorporating the furanocoumarin xanthotoxin into an artificial diet at levels comparable to intact and damaged foliage

Genetic variation in inducibility. Like furano- coumarins in intact tissue, inducibility of certain furanocoumarins, xanthotoxin and sphondin, is genetically variable. As such, inducibility is a trait that is subject to change due to natural selection. For more information on the genetics of furanocoumarin production see coevolution.

Inducibility and optimal defense Optimal defense theory suggests that natural selection will shape defenses in such a way that the benefit-cost ratio of defense allocation is optimal. It has been hypothesized that inducible defenses defer the cost of defenses until they are needed.

If the likelihood of an attack is remote, a plant is expected to rely almost exclusively on inducible defense, while a plant that is certain to be attacked is expected to rely on a high level of round-the-clock (constitutive) defenses. The same theory can be applied to parts of a plant. In wild parsnips, constitutive levels of furanocoumarins are high in the fruits, intermediate in the leaves and low in the roots. In contrast, the degree of furanocoumarin inducibility is highest in the roots, intermediate in the leaves, and non-existent in the fruits.


	If optimal defense theory is valid, the probability of attack of these plant parts should vary. Specifically, roots would be attacked infrequently while reproductive parts would be attacked with high frequency. Observations of attack rates (evidence of feeding damage) were observed for all three plants parts, in three populations, and over two consecutive years. These data confirmed that roots are attacked much less frequently that reproductive tissues.

However, leaves experience a likelihood of attack similar to that of reproductive parts. Why is the defense system of leaves intermediate between roots and fruits? In addition to scoring presence or absence of damage in 1994, we also scored severity of attack (proportions of the plant part category exhibiting damage) and found that leaves were subject to less intense attacks.

last updated 1/11/2000