The Biotic Environment

since the Cretaceous period (Chapter 2, Section 4.2). All parts of a plant may be exploited 695

as a result of the activities of grazing, sucking, and boring insects. As might be anticipated in view of the length of time over which this coevolution has occurred, some of the relationships between herbivorous insects and angiosperms are extremely intimate and refined, though essentially the relationships have a common theme. Insects gain energy (food) at the expense of plants, whereas plants attempt to defend themselves (conserve their energy) or at least to obtain something in return for the energy that insects take from them. Though the theme remains constant through time, the relationships themselves are always changing as a result of natural selection. Insects strive to improve their energy-gathering efficiency (most often by concentrating on energy in a particular form and from a restricted source and by specialization of the method used to collect the energy) while plants concurrently improve their defenses. Most authors, for example, Price (1997) view this relationship as "constant warfare" between insects and plants, which forms the basis of their coevolution. Other authors such as Owen and Wiegert (1987) believe that herbivory is a form of mutualism. They point out that, in analogy with pruning, mowing, and similar activities carried out by humans, a frequent effect of insects grazing on newly formed plant tissues is to stimulate the plant to produce more branches and, eventually, more reproductive structures and seed; in other words, the plant is making an adaptive, mutualistic response to the herbivore.

The most common method used by plants as defense against insects (and other herbivorous animals) is production of toxic metabolites. Plants produce a wide array of such chemicals in secondary metabolic pathways (i.e., those not used for generation of major components such as proteins, nucleic acids, and carbohydrates). Particular types of secondary plant compounds are commonly restricted to specific plant families, for example, glucosinolates in Brassicaceae (crucifers), cardenolides (mainly cardiac glycosides) in Asclepiadaceae (milkweeds), and cucurbitacins in Cucurbitaceae (Panda and Khush, 1995; Schoonhoven etal., 1998). Moreover, the compounds often accumulate within specific tissues or areas of the plant, for example, trichomes (terpenes), the wax layer (phenolics), vacuoles (alkaloids), and seeds (non-protein amino acids) (Bernays and Chapman, 1994). The reproductive parts of plants, which represent concentrated stores of energy, are especially attractive to herbivores and often serve as a sink for secondary metabolites. For example, Hypericum perforatum (Klamath weed) produces the toxic quinone hypericin. The concentration of hypericin is 30 |ag/g wet weight in the lower stem, 70 |ag/g in the upper stem, and 500 |ag/g in the flower (Price, 1997). Typically, the toxins are chemically combined with sugars, salts, or proteins to render them inactive while in storage. When the plant tissue is damaged, enzymes release the toxin from its conjugate, allowing a localized effect at the site of the wound (Bernays and Chapman, 1994).

The evolutionary origin of these secondary metabolites remains a matter of speculation. An early view was that the chemicals arose as waste products of a plant's primary metabolism, and the plant, being unable to excrete the molecules, simply retained them within its tissues. This idea is now considered unlikely given the highly complex nature of some of these compounds and, therefore, the amount of energy required for their synthesis. A more likely possibility is that originally the metabolites were simply short-lived intermediates in normal biochemical pathways within plants and/or provided a means of storing chemical energy for later use by the plant. In other words, the original function(s) of these compounds may have been unrelated to the occurrence ofherbivores. An example of such a compound might be nicotine produced by the tobacco plant (Nicotiana spp.). Radioisotope studies have shown that, although about 12% of the energy trapped in photosynthesis is used

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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