Crop allelopathy types: Current research status and prospects in China

In this review, we present details of recent advances in research on different types and action modes of crop allelopathy in China. In particular, we focus not only on direct and indirect allelopathy but also on intra- and interspecific interactions, with the aim of informing international peers of the ongoing developments in this field. The term crop allelopathy was first defined in 1984, and since that time, substantial progress has been made in this area, during which the concept of crop allelopathy has been broadened to encompass plant-soil-microbial interactions, including amensalism, autotoxicity, and facilitation in cropping systems. Recent studies have revealed that donor plants are able to trigger the expression of defense-related genes, resulting in the release of specific metabolites (allelochemicals) into the environment. In particular, allelopathic crops have been found to secrete these chemicals into the soil environment via root exudation in response to stresses induced by target plants (such as weeds), which in turn results in allelopathic amensalism and allelopathic commensalisms in cropping systems. The amensalistic and commensalistic components of crop allelopathy can be further divided into intra- and interspecific interactions based on mode of action. Interspecific interactions involve the inhibitory or facilitative effects of donors on recipient plants, depending on the types, concentrations, and bio-activity of allelochemicals; whereas intraspecific interactions include auto-promotive and auto-toxic effects, which can be either positive or negative. The current consensus indicates that both positive and negative allelopathic interactions are mediated via changes in rhizosphere soil microbial composition and structure in response to root allelopathic secretions. In this regard, however, there is often an imbalance in the composition of soil microbial communities, which is largely attributable to an increase in pathogen populations and reduction in those of beneficial bacteria as a consequence of consecutive monoculture cropping. Such imbalances inevitably lead to three undesirable outcomes in continuous cropping systems, namely, soil nutrient sequestration, soil acidification, and the outbreak of soil-borne diseases, thereby resulting in reduced crop yields and quality. Conversely, in the case of positive allelopathic interactions, continuous cropping can contribute to promoting increases in microbial diversity mainly as a consequence of increments in the populations of beneficial bacteria and corresponding reductions in pathogenic microorganisms, thereby enhancing soil micro-habitats, and thus increasing crop yield and quality. Given these responses, a key priority for future research is more in-depth studies of the structure and function of rhizosphere microbial communities, and appropriate modification of rhizosphere habitats, with the aim of producing high-yielding good quality crops for sustainable agricultural development.