Variation in allelopathic activity over 100 years of barley selection and breeding

Barley (Hordeum vulgare L.) landraces and cultivars from Finland, Sweden, Denmark and the Baltic states, in total 127, were analysed for allelopathic activity against ryegrass (Lolium perenne L.). The germplasm covered most of the gene pool used during 100 years of barley breeding. An agar-based bioassay developed for screening allelopathic activity was used. In the germplasm from all countries a decreasing trend in allelopathic activity was observed with the introduction of new cultivars from 14% for the Swedish to 31% for the Finnish cultivars. In the Finnish, Swedish and Baltic cultivars an increase in root growth was noticed, and changes in root biomass therefore cannot explain the decrease in allelopathy. In fact, the allelopathic activity per milligram root decreased with 32-85%. In contrast, root growth decreased over time in the Danish collection. In the Finnish cultivars both two- and six-row barley cultivars were tested with similar results. Allelopathic activity of barley probably originated from different landraces, and in most cases from a specific landrace from the Swedish island of Gotland. We suspect that more than 100 years of selection and breeding have resulted in a dilution of the genes from landraces and consequently a declining allelopathic activity. In the Swedish collection, two cultivars did not follow the general trend and in both cultivars, several landraces had been combined.