2-methylisoborneol and geosmin,the main sources of soil odor,inhibit the germination of brassicaceae seeds

The effects of 2-methylisoborneol (MIB) and geosmin, the main sources of soil odor produced by microorganisms, on plant seed germination were examined for 15 kinds of Brassicaceae seeds including radish (Raphanus sativus var. radicula). Authentic MIB or geosmin was used at the concentration of 100 mg L^-1 in the assay, the effects were quantitatively indexed based on four parameters of cumulative seed germination: the final germination percentage, germination onset, weighted mean rate, germination index. MIB and geosmin inhibited germination of all kinds of seeds tested for these parameters. Radish variety “Comet” was the most affected of all of the tested plant seeds. The estimated 50% inhibitory concentration values on germination of “Comet” were 70.5 and 7.5 mg L^-1 for MIB and geosmin, respectively. We also conducted the primary characterization of the inhibition by MIB and geosmin on “Comet” seeds. Our findings were as follows: (1) onset of seed imbibition was not affected; (2) germination of seeds pre-imbibed in the control solution until approx. 2 h before germination onset was inhibited by subsequent MIB or geosmin treatment; (3) growth of seedlings was not significantly affected; (4) the germination was restored by stratification, gibberellin A₃ treatment, or seed coat removal; and (5) light irradiation affected the degree of inhibition. These results suggest that inhibition by MIB and geosmin is restricted to the germination process, at least in the early stage of growth, that they act as growth -regulating substances.