Plant and Soil 129, 277-281, 1990. t~) 1990 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 8482 Allelopathic potential of glucosinolates (mustard oil glycosides) and their degradation products against wheat Z. BIALY 1, W. OLESZEK 1, J. LEWIS 2 and G.R. FENWICK 2 1Institute of Soil Science and Plant Cultivation, PL-24-100 Pulawy, Poland and 2AFRC Institute of Food Research, Norwich Laboratory, Colney Lane, Norwich, NR4 7UA, UK Received 1 November1989. Revised June 1990 Key words: allelopathy, glucosinolates, isothiocyanates, mustards, weed Abstract A number of glucosinolates and isothiocyanates were tested for their allelopathic potential against wheat. Most of the glucosinolates showed no activity against wheat, with the exception of glucobrassicin which was moderately active, as was sinapine thiocyanate. Isothiocyanates showed high activity against wheat germination and seedling growth. The most active compound, 2-phenethyl ITC completely inhibited wheat germination at 500 ppm. Allyl ITC showed high activity whereas other isothiocyanates tested were only moderately active. The data is discussed in relation to the possible use of some mustard species for effective weed control. Introduction It is known that some cruciferous species have a harmful effect on subsequent small-grained crops (Kasting et al., 1973; Mason-Sedun and Jessop, 1988; Mason-Sedun et al., 1986; Waddington, 1978; Waddington and Bowren, 1978). It is therefore possible that, in line with current con- cerns to reduce or replace pesticide application by use of natural, phytotoxic plant residues (Put- nam and DeFrank, 1983), Brassica- or Sinapis species might be used as intercrops or forecrops for weed control. It would first, however, be necessary to identify both the active allelochemi- cals present in such species and those cultivars possessing highest levels of these compounds. The nature of the most active allelochemicals in the Cruciferae is not entirely known. How- ever, previously conducted tests (Mason-Sedun et al., 1986; Oleszek, 1987) on isolates derived from Brassica- and Sinapis species demonstrated large differences in allelopathic effect and indi- cated that the mustards, Brassica nigra and Bras- sica juncea were the most active. The suggestion that volatile degradation products of glucosino- lates, occurring in these crucifers, was respon- sible for the allelopathic activity received support from the findings of Tollsen and Bergstrom (1988) who examined the headspace volatiles collected from Brassica and Sinapis genera. Mac- erated leaves of B. juncea, B. nigra and B. carinata yielded large amounts of volatiles, in particular allyl isothiocyanate, a compound re- ported to be an allelopathic agent by Evenari (1949), Friedman and Waller (1983) and Putnam (1983). No extensive research was, however, carried out and neither the activity of this chemi- cal in relation to other allelopathic agents, nor the possibility of synergistic reactions between the components of the volatile fraction was ex- amined. Sinigrin (Ia, Fig. 1), the involatile precursor of allyl isothiocyanate, and its products have been suggested to be allelopathic (Evenari, 1949) and Kutacek (1964) claimed similar activity for the indole glucosinolate, glucobrassicin (Ig). Benzyl