| Most theories of plant chemical defenses have defined "defense" in the context of herbivory. Increased light intensity, particularly in the ultraviolet region, may cause oxidative damage to leaves. In this study, I examined changes in plant primary and secondary chemical production resulting from exposure to increased light stress, and how these changes affect insect herbivores.; Total antioxidant content was measured in a community-level screening of tropical rainforest plants under high and low light conditions. A subset of the species was analyzed for water- and fat-soluble antioxidant activity. Antioxidant content of tropical plant foliage was higher in plants from high light conditions, with the elevated antioxidant levels due primarily to increased water-soluble antioxidants.; In a second study, antioxidants, UV-absorbing compounds, and antiherbivore defenses were analyzed in wild parsnip, Pastinaca sativa L., grown under two light conditions simulating the UV-B exposure of full sun and shade. In wild parsnip, fat- (61.5% increase) and water-soluble (148% increase) antioxidant levels were higher in the high light samples, with ascorbic acid accounting for 34-44% of the increase. UV-absorbing compound concentrations in the high light treatment were twice that of the low light treatment. Of the seven furanocoumarins and all other compounds assayed as antiherbivore defenses, only two furanocoumarins, bergapten and psoralen, differed between treatments, with lower concentrations in the high light treatment.; In a third experiment, growth and survival were assayed for a specialist herbivore, the parsnip webworm (Depressaria pastinacella) and a generalist herbivore, the cabbage looper (Trichoplusia ni) raised on artificial diets simulating levels of antiradiant and a furanocoumarin measured from the wild parsnip in the second study. Virtually no effects were seen on growth or on survival for either species as a result of diet treatments, although high light levels reduced survival and pupal mass and accelerated growth irrespective of diet.; Plants growing in high light conditions respond with allocation of resources to antioxidant and anti-UV compounds. This shift of resources, while drawing from allocation to other purposes, does not necessarily alter the plant's resistance to herbivores. |
