Seed fatty acid composition and physical dormancy in fire-prone ecosystems.

Background and aims: The maintenance of seed banks and timing of germination are fundamental to ensuring population persistence. Physical dormancy (PY) in disturbance-prone environments contributes to these processes via an impermeable seed coat. Dormancy is broken often by heating, which in fire-prone regions is determined by species-specific threshold temperatures. However, the mechanisms by which seeds persist or control dormancy-breaking thresholds in such environments is unclear. We determined whether unsaturated and saturated fatty acids (FAs; within triacylglycerols), a common lipid linked to heat-stress resilience, might contribute to seed coat dormancy and overall seed persistence, and whether fire selects for different FA compositions and drives PY function in fire-prone regions.

Methods: We characterised seed FA compositions of 26 Fabaceae species from fire-prone and fire-free ecosystems through gas chromatography-mass spectrometry. We compared FA saturation, total relative FA content and the highest melting point FA of each species across seed tissues (seed coat vs internal tissues) and habitat type (fire-prone vs fire-free) and, for fire-prone species, tested for a relationship with species-specific dormancy-breaking thresholds.

Key results: No relationship between FA composition and species-specific dormancy-breaking thresholds was found. Seeds of fire-free species had more saturated FAs than fire-prone species, particularly for internal tissues. FA saturation was higher in seed coats than internal tissues across both habitat types. Relative FA content was similar in internal tissues across habitat type but differed for seed coats, with fire-prone species having marginally more FAs.

Conclusions: While no correlation existed between FA composition and dormancy-breaking thresholds in fire-prone species, the consistent differences between seed tissue types we found highlights a similar role for FAs in seed coats across habitats, likely linked to maintaining impermeability. Some evidence supports fire selecting for greater total FA content in seed coats, however further work is needed to test its relationship with temperature thresholds.