Impacts of fire cues on the germination of Brassica napus L. seeds with high and low levels of secondary dormancy

Agricultural burning is used in farm management operations however, information about the impacts of fire cues on the release and /or induction of secondary dormancy in crop seeds is scarce. Seeds from two oilseed rape cultivars were induced for high (HD) and low (LD) secondary dormancy by polyethyleneglycol (PEG) pre-treatment and their germinations after exposure to various fire cues were compared to control PEG pre-treated and also non-dormant seeds. Non-dormant seed germination remained unaffected by various fire cues. Low doses of aerosol smoke released secondary dormancy in HD seeds while higher doses increased dormancy of LD seeds. Diluted smoke water also released HD seed secondary dormancy but the concentrated one enhanced dormancy in both LD and HD seeds. The concentrated aqueous extracts from charred oilseed rape straw promoted only the germination of HD seeds while after dilution inhibited LD seed germination. Heat shock (80 degrees C, 5 min) released secondary dormancy in HD seeds however, higher temperatures and /or increased exposure time was associated with seed death. GC-MS analyses of smoke water revealed two butenolides and an array of monoaromatic hydroxybenzene compounds with potential germination inhibitor or promoter activities. The extent of secondary dormancy induction in seeds affects their subsequent responses to fire cues. Both aerosol smoke and smoke water contain germination promoter and inhibitor activities. Free from any butenolides, aqueous extracts from charred straw contained a potential germination stimulating steroid i.e. ergosterol. The significance of fire-derived cues on behavior of oilseed rape seeds in the soil seed bank has been discussed.