Allelochemicals in wheat and barley: role in plant—insect interactions

Abstract

Allelochemicals in plants may be important in allelopathy, plant—insect interactions, plant—herbivore interactions, etc. (Waller, 1987). Wheat is known to accumulate several hydroxamic acid glycosides during its early stages of development (Fig. 9.1) (Argandoña et al., 1981; Wahlroos and Virtanen, 1959; Willard and Penner, 1976). Barley seedlings accumulate gramine, an indole base (Hanson et al., 1983; Smith, 1977). Although the concentrations of hydroxamic acids in wheat and of gramine in barley reach up to about 5 mmol kg^-1 of fresh mass, little is known about the importance of these compounds for the plant. Hydroxamic acids appear to determine the resistance of maize to the European corn borer Ostrinia nubilalis (Klun et al., 1967) and of several members of Gramineae to aphids (Argadoña et al., 1980; Corcuera et al., 1982; Long et al., 1977). Gramine may be responsible for possible allelopathic effects attributed to barley (Overland, 1966) and may cause toxicity in ruminants feeding on other members of Gramineae and Leguminoseae (Gallagher et al., 1964; Marten et al., 1981). Gramine also increases the resistance of barley to aphids (Zúñiga and Corcuera, 1986; Zúñiga et al., 1985). The existing evidence for the participation of hydroxamic acids and gramine in the resistance of wheat and barley to aphids will be discussed in this chapter.