Chemical information transfer between plants:

Abstract

Chemical information conveyance between organisms has been well established for a wide range of organisms including protozoa, invertebrates, vertebrates and plant-parasitic plants. During the past 20 years, various studies have addressed whether chemical information conveyance also occurs between damaged and undamaged plants and many interesting pieces of evidence have been presented. To date, this research field has been restricted to the question whether and how plants (in general) are involved in plant-to-plant communication. However, apart from mechanistic questions, evolutionary questions should be addressed asking why plants do (or do not) exploit their neighbour's information and whether their strategy is affected by e.g. environmental conditions or previous experience. Recent progress in the field of chemical information conveyance between damaged and undamaged plants warrants an intensified study of this exciting topic in chemical ecology.

Introduction

All organisms are under selective pressure to maximize reproductive success. To exploit the prevailing environmental conditions to their full extent, organisms can take advantage of information. An important form of information consists of chemical cues. It has been well established that chemical information plays an essential role in the ecology of such diverse organisms as protozoa (Kuhlmann et al., 1999), crustaceans (Tollrian and Dodson, 1999), insects (Cardé and Bell, 1995; Roitberg and Isman, 1992), and vertebrates (Kats and Dill, 1998; Tollrian and Harvell, 1999). Likewise, there is a rich literature on the emission of chemical information by plants and its use by arthropods (Schoonhoven et al., 1998; Chadwick and Goode, 1999), by fungi (Nagashi and Douds, 1999) and by plant-parasitic plants (Estabrook and Yoder, 1998). In addition, allelopathic effects of plant compounds on neighbouring plants have been amply documented (Pellisier and Souto, 1999; Mallik and Romeo, 2000). The role of chemical information in interactions between damaged and undamaged plants, however, remained controversial ever since the first scientific publications appeared in the early 1980s (Baldwin and Schultz, 1983; Rhoades, 1983; Fowler and Lawton, 1985). Some studies found no evidence for transfer of information between damaged and undamaged plants (Myers and Willams, 1984; Williams and Myers, 1984; Fowler and Lawton, 1985; Lin et al., 1990; Preston et al., 1999). Many others presented evidence supporting the hypothesis of information conveyance between damaged and undamaged plants (Rhoades, 1983; Haukioja et al., 1985; Rhoades, 1985; Zeringue, 1987; Dicke et al., 1990; Farmer and Ryan, 1990; Bruin et al., 1992; Shulaev et al., 1997; Arimura et al., 2000; Dolch and Tscharntke, 2000; Karban et al., 2000) and several stimulating reviews have been published in the past five years (Bruin et al., 1995; Shonle and Bergelson, 1995; Karban and Baldwin, 1997).

Yet, studies on plant-to-plant communication are often received with scepticism. The major issues raised by critics are, in random order: (1) data suffer from statistical flaws such as pseudoreplication, (2) the dose of the chemical cues applied in experiments was unrealistically high, (3) the mechanism is unknown or alternative mechanisms may explain the data, (4) ubiquitous cues cannot be meaningful information in interactions between damaged and undamaged plants, and (5) experiments under realistic field conditions are lacking (Fowler and Lawton, 1985; Firn and Jones, 1995; Karban and Baldwin, 1997). Although related to very different aspects of experimental studies, each of these issues is important and should be considered in studies on information conveyance between damaged and undamaged plants. They should stimulate scientists to improve their experimental protocols, to investigate alternative mechanisms, to determine costs and benefits, and to assess the impact on population dynamics—in short they should stimulate continual investigation of a phenomenon with good potential. After all, there is abundant evidence that chemical information from damaged plants is available to undamaged plants.