Karrikinolide residues in grassland soils following fire: Implications on germination activity

Smoke plays a positive role in promoting seed germination and enhancing post-germination processes. The compound in smoke is 3-methyl-2H-furo[2,3-c]pyran-2-one (KAR1). Recently a structurally related butenolide [3,4,5-trimethylfuran-2(5H)-one, (trimethylbutenolide, TMB)], which inhibits germination and reduces the effect of KAR1, was isolated. The mechanisms of action and interaction of these karrikins are unknown. In addition, the ecological significance of fires in altering soil-smoke-chemistry and the spatial dimensions of the influence on burnt sites and neighbouring areas are undetermined. This study quantified KAR1 and TMB residues in soils following fire and assessed the germination activity of burnt soil extracts. Soil samples from 0 to 2, 2 to 4, 4 to 6 and 6 to 8 cm depths were extracted using dichloromethane and bioassayed using Lactuca sativa L. achenes (seeds). At all soil depths, L. sativa seeds exhibited significantly greater percentage germination when treated with burnt soil extracts compared to the no-burn soil (control). The L. sativa seeds also showed significantly greater percentage germination when treated with soil extracts from the adjacent plots. Compared to the no-burn soil, higher concentrations of KAR1 and TMB were detected in the surface layers of the burnt soils. Considerable concentrations of KAR1 and TMB were also detected in no-burn soil indicating that sources other than fire may also generate karrikins. Findings of this study imply that post-fire increases in KAR1 residues in the soil may influence soil seed bank stimulation of certain smoke-responsive plant communities in both burnt and adjacent non-burnt areas.