Identification of chemicals in potato root exudates under different years of continuous cropping and their biologic effects

In order to explore possible obstruction mechanisms in continuous potato cropping systems, potato root exudates were collected in 1-5 years (CP₁-CP₅) continuous cropping systems of potato under field conditions. Chemical composition of root exudates was determined by the GC-MS method and biological effects of the exudates were detected by biological monitoring. The results showed that the main components of potato root exudates in CP₁-CP₅ treatments included organic acids, glucides, amines, alcohols, esters and pyrimidines, although the chemicals occurred in different quantities and contents. Palmitic acid occurred in all the CP₁-CP₅ systems, with relative contents of 0.55%, 0.87%, 1.24%, 1.05% and 0.95%, and concentrations of 7.12 mg·L^-1, 7.39 mg·L^-1, 9.46 mg·L^-1, 8.38 mg·L^-1 and 8.02 mg·L^-1, respectively. Biological analyses showed that potato root exudates significantly inhibited potato growth, with inhibition expression of CP₅ > CP₄ > CP₃ > CP₂ > CP₁. The inhibition of potato growth by palmitic acid had obvious concentration effects, with inhibition expression of 100 mg·L^-1 > 50 mg·L^-1 > 10 mg·L^-1 > 5 mg·L^-1. Potato root exudates obviously promoted the growth of Rhizoctonia solani. The colony diameter and mycelia fresh weight of R. solani were highest in CP₃ and lowest in CP₁. No significant differences were observed between CP₂, CP₄ and CP₅. Palmitic acid promoted the growth of R. solani. The colony diameter and mycelia fresh weight of R. solani increased firstly and then later decreased along with increasing concentration of palmitic acid. Colony diameter and mycelia fresh weight of R. solani were highest at 10 mg·L^-1. The study showed that potato root exudates significantly inhibited potato growth, and the inhibiting effect increased with increasing number of years of continuous cropping. The promotion of potato root exudates to R. solani growth exacerbated the obstacles of continuous cropping. Palmitic acid is the allelochemicals secreted by potato roots.