Allelopathy is a naturally occurring biological process involving the release of chemicals from the plant roots that can affect both biological and agricultural systems. The Austrian plant physiologist Hans Molìsch first coined the term ‘allelopathy’ (Molísch, 1937), and defined it as ‘any direct or indirect harmful effect by one plant (including microorganisms) on another through the release of chemicals that escape into the environment’. It encompasses both direct (e.g. root exudates) and indirect (e.g. volatiles) chemical interactions between plants. Such chemical compounds are usually secondary metabolites or byproducts of the major metabolic pathways in plants.