Box 11.1. Induced defenses
Plants contain various chemicals that may deter, or at least reduce their suitability to, some herbivores. These are the secondary plant compounds (noxious phytochemicals, or allelochemicals). Depending on plant species, such chemicals may be present in the foliage at all times, only in some plant parts, or only in some parts during particular stages of ontogeny, such as during the growth period of new leaves. Such constitutive defenses provide the plant with continuous protection, at least against non-adapted phytophagous insects. If defense is costly (in energetic terms) and if insect damage is intermittent, plants would benefit from being able to turn on their defenses only when insect feeding occurs. There is good experimental evidence that, in some plants, damage to the foliage induces chemical changes in the existing or future leaves, which adversely affect insects. This phenomenon is called induced defense if the induced chemical response benefits the plant. However, sometimes the induced chemical changes may lead to greater foliage consumption by lowering food quality for herbivores, which thus eat more to obtain the necessary nutrients.
Both short-term (or rapidly induced) and long-term (or delayed) chemical changes have been observed in plants as responses to herbivory. For example, proteinase-inhibitor proteins are produced rapidly by some plants in response to wounds caused by chewing insects. These proteins can significantly reduce the palatability of the plant to some insects. In other plants, the production of phenolic compounds may be increased, either for short or prolonged periods, within the wounded plant part or sometimes the whole plant.
Alternatively, the longer-term carbon—nutrient balance may be altered to the detriment of herbivores.
Such induced chemical changes have been demonstrated for some but not all studied plants. Even when they occur, their function(s) may not be easy to demonstrate, especially as herbivore feeding is not always deterred. Sometimes induced chemicals may benefit the plant indirectly, not by reducing herbivory but by attracting natural enemies of the insect herbivores (section 4.3.3). Moreover, the results of studies on induced responses may be difficult to interpret because of large variation in foliage quality between and within individual plants, as well as the complication that minor variations in the nature of the damage can lead to different outcomes. In addition, insect herbivore populations in the field are regulated by an array of factors and the effects of plant chemistry may be ameliorated or exacerbated depending on other conditions.
An even more difficult area of study involves what the popular literature refers to as “talking trees”, to describe the controversial phenomenon of damaged plants releasing signals (volatile chemicals) that elicit increased resistance to herbivory in undamaged neighbors. Whether such interplant communication is important in nature is unclear but within-plant responses to her- bivory certainly can occur at some distance from the site of insect damage, as a result of intraplant chemical signals. The nature and control of these systemic signals have been little studied in relation to herbivory and yet manipulation of such chemicals may provide new opportunities for increasing plant resistance to herbivorous insect pests.