Evidence for allelopathic activity in barley and the major temperate cereal crops is reviewed. We believe that the secondary metabolites, gramine and hordenine, produced by barley (Hordeum spp.) play a role in defending the producing plant against interference from other organisms.Our recent work has shown inhibitory effects on a fungus (Drechslera teres) and on armyworm (Mythimna convecta) larvae as well as on a number of plant species. This work and that of others showing activity against bacteria, aphids and mammals suggests a possible physiological resistance to these organisms which may be exploitable through plant breeding. A survey of 43 lines of barley including ancestral and modern types indicates that hordenine production by modern cultivars may already have been inadvertently favored by selection for agronomic traits while the ability to produce gramine may have been reduced or lost during this process.Increasingly, the recognition of the defensive-come-communicational role of allelochemicals has led to investigation about their potential in natural resistance to pests and, hence, in crop protection, with special reference to biological control (see, for example, 7-5). Activity of allelochemicals against insects and other organisms has long been identified with such widely distributed families of secondary metabolites as the alkaloids, terpenes, phenolics and cyanogenic glycosides (6). Notwithstanding the plethora of naturally-occurring compounds which is available, relatively few have been deployed in the cause of crop protection. Thus, in respect of activity against insects, Jacobson (7) noted that, from early Roman times to the middle of the present century only pyrethrum, rotenone, nicotine, sabadilla and quassin were widely used as insect repellents and toxicants, at least, in the Western Hemisphere. He concluded 0097-6156/95/0582-0170$08.00/0
A method was devised for the extraction and quantification of hordenine and gramine from barley (Hordeum vulgare) tissue using HPLC techniques. Quantification was by peak area, the relationship between peak area and concentration of authentic standards being linear for both hordenine and gramine. Significant differences in the ability of three lines of barley to produce hordenine and gramine were detected using this method.
Forty-three lines of barley, including ancestral (wild), landraces, Middle Eastern lines, and modern cultivars, were grown under two different sets of environmental conditions. Hordenine production in barley roots was determined at the one-leaf stage by HPLC analysis and, in two lines only, over a period of 35 days. Forty-two of the 43 lines produced significant amounts of hordenine, although there was no variation among groups. Middle Eastern lines had the highest production with 327 µg/g on a dry weight basis. Production was, however, determined more by environmental conditions during growth than by genetic factors. Hordenine production was up to seven times higher in plants grown under lower light intensities.
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