Does fire drive fatty acid composition in seed coats of physically dormant species?

Seed dormancy is the key driver regulating seed germination, hence is fundamental to the seedling recruitment life-history stage and population persistence. However, despite the importance of physical dormancy (PY) in timing post-fire germination, the mechanism driving dormancy-break within seed coats remains surprisingly unclear. We suggest that seed coat chemistry may play an important role in controlling dormancy in species with PY. In particular, seed coat fatty acids (FAs) are hydrophobic, and have melting points within the range of seed dormancy-breaking temperatures. Furthermore, melting points of saturated FAs increase with increasing carbon chain length. We investigated whether fire could influence seed coat FA profiles and discuss their potential influence on dormancy mechanisms. Seed coat FAs of 25 species within the Faboideae, from fire-prone and fire-free ecosystems, were identified and quantified through GC-MS. Fatty acid profiles were interpreted in the context of species habitat and interspecific variation. Fatty acid compositions were distinct between species from fire-prone and fire-free habitats. Fire-prone species tended to have longer saturated FA chains, a lower ratio of saturated to unsaturated FA, and a slightly higher relative amount of FAs compared to fire-free species. The specific FA composition of seed coats of fire-prone species indicated a potential role of FAs in dormancy mechanisms. Overall, the distinct FA composition between fire-prone and fire-free species suggests that chemistry of the seed coat may be under selection pressure in fire-prone ecosystems.

Evidence for physiological seed dormancy cycling in the woody shrub Asterolasia buxifolia and its ecological significance in fire-prone systems.

Dormancy cycling is a key mechanism that contributes to the maintenance of long-term persistent soil seed banks, but has not been recorded in long-lived woody shrub species from fire-prone environments. Such species rely on seed banks and dormancy break as important processes for post-fire recruitment and recovery. We used germination experiments with smoke treatments on fresh seeds and those buried for 1 year (retrieved in spring) and 1.5 years (retrieved the following late autumn) to investigate whether Asterolasia buxifolia, a shrub from fire-prone south-eastern Australia with physiologically dormant seeds, exhibited dormancy cycling. All seeds had an obligation for winter seasonal temperatures and smoke to promote germination, even after ageing in the soil. A high proportion of germination was recorded from fresh seeds. but germination after the first retrieval was significantly lower, despite high seed viability. After the second retrieval, germination returned to the initial level. This indicates a pattern of annual dormancy cycling; one of the few observations, to our knowledge, for a perennial species. Additionally, A. buxifolia's winter temperature and smoke requirements did not change over time, highlighting the potential for seeds to remain conditionally dormant (i.e. restricted to a narrow range of germination conditions) for long periods. For physiologically dormant species, such as A. buxifolia, we conclude that dormancy cycling is an important driver of successful regeneration, allowing seed bank persistence, sometimes for decades, during fire-free periods unsuitable for successful recruitment, while ensuring that a large proportion of seeds are available for recruitment when a fire occurs.