Plant allelopathy types and their application in agriculture

Since allelopathy was defined by Rice in 1984, important research progress has been made on the plant allelopathy. Previous studies demonstrated that plant allelopathy has four types (or action modes), amensalism, autotoxicity, stimulation and facilitation. Plant allelopathy was widely understood as the harmful effects (amensalism) of one plant on another plant due to chemicals that were released into the environment, especially into the soil environment. Increasing studies showed that plant allelopathy was the result of the comprehensive effect of plant-microbe interactions mediated by root exudates. Amensalism has been applied in weeds control, and great achievements have been made in the practice. Allelopathic autotoxicity, also known as consecutive monoculture problems, was shown as serious soil disease in plants, consequently resulting in decreased biomass and quality of plant under consecutive monoculture. The allelopathic autotoxicity of medical herbs, especially those with tuberous roots for medicinal use, was just the case with much more sensitive reaction. Recent researches has displayed that the ecological effects of soil microbial functional disorder mediated by root exudates were the main reasons for the consecutive monoculture problems. The findings suggested that the improvement of disease-conducive soil environment was crucial to the remediation of unbalanced microbial community consturcture, and increase of ecosystem functions to overcome the problems. Positive allelopathic stimulation referred to that the monocultured plants graw well, showing higher biomass and better quality with the increasing years of consecutive cropping. Although the mechanism for positive allelopathic stimulation was not deeply understood, it suggested that the ecological phenomena mainly resulted from the improvement of soil microbial community constructure and their interaction mediated by root exudates, including positive and negative facilitation among microorganisms in rhizosphere soil, and hence increased soil fertility and nutrient supply ability, thereby, enforced root resistance to disease, consequently resulted in enhanced plant yield and quality. Achyranthes bidentata Blume was found to be a typical medicinal plant with positive allelopatic effects after continuously monocultured, and had been used as a rotation/intercropping crop with other medicinal plants sensitive to consecutive cropping to keep sustainable production of medicinal herbs. Positive interspecific interactions (facilitation) including commensalism, mutualism and protocooperation, have become a hotspot at home and abroad. Many scholars found that the mutually beneficial relationships in rhizosphere between plant species, especially the plant-microbe interactions mediated by root exudates, played important roles in the yield increase of intercropping system and efficient utilization of soil nutrients. Finally, the authors emphasized that the compositon of metagenome of the complex plant-associated microbial community existing in rhizosphere was the key to the crop plants whether they can healthily grow in the monoculture system. More in-depth studies in the second genome and oriented control of rhizospheric biological processes based on the principle and technology of modern synthetic biology would become the priority research areas to promote the sustainable development of agricultural production.