Autotoxic Compounds from Rhizosphere Soil of Humulus lupulus L. Extracts: Identification and Biological Activity

Hop (Humulus lupulus L.) is a perennial herb that has been widely used in brewing around the world. Autotoxicity has been reported as one of the major problems hindering continuous hop cultivation. In this study, the autotoxic compounds from rhizosphere soil of hop fields at different time periods of hop cultivation were separated and identified by gas chromatography–mass spectrometry (GC–MS). From those compounds, one of the suspected bioactive compounds, 2,4-bis(1,1-dimethylethyl)-phenol was selected, and its biological activity on hop seedling growth and effect on soil microbial populations were determined. The results of GC–MS analysis showed that the types of compounds found and their amount increased with the number of years hops had been cultivated. The results of a pot experiment showed that 2,4-bis(1,1-dimethylethyl)-phenol significantly affected photosynthesis and hop seedling growth in a concentration-dependent manner; generally, low concentrations enhanced photosynthesis and growth while high concentrations limited plant growth. The results of the pot experiment also indicated that 2,4-bis(1,1-dimethylethyl)-phenol significantly decreased the numbers of culturable bacteria, fungi, actinomycetes, and the soil microbial community diversity index in the rhizosphere soil by the 7th d; however, the numbers of culturable bacteria, fungi, and actinomycetes of the rhizosphere soil significantly increased by 35 d after treatment. Furthermore, we verified the concentration of 2,4-bis(1,1-dimethylethyl)-phenol by high performance liquid chromatography, which was in line with the concentration determined to be bioactive. These results suggest that 2,4-bis(1,1-dimethylethyl)-phenol is involved in hops autotoxicity and that autotoxicity is the main deterrent to the success of hops replanting.