Tolerance of thermal shock in seeds

Tolerance to high temperatures is an important attribute of seeds of species that inhabit regions that periodically burn. In some species, seeds not only survive fire, but require a heat shock in order to germinate. Twenty-one taxa, representing a range of ecological attributes, were tested for germination following scarification and boiling-water treatments of up to 90 min duration. The black-coloured seeds from the serotinous fruits of Dryandra sessilis (Knight) Domin., Hakea lissocarpha R.Br. and four species of Eucalyptus were vulnerable to temperatures of 100 degrees C with increasing duration of exposure associated with reduced germination percentage. Seeds of Xanthorrhoea preissii Endl., a species that flowers in response to fire and disperses seed into a post-fire environment unlikely to experience a second fire, also had few seeds that could tolerate even 30 s of boiling-water treatment. The reddish coloured, soil-borne, leguminous species seeds showed enhanced germination after short durations of 100 degrees C, but gradually decreasing proportions of seeds germinated as heat-shock durations were lengthened to 90 min. Within the genus Eucalyptus, seeds of relatively large mass and thick seed coats survived longer than seeds of small mass and thin seed coats. In contrast, within the legumes, seed mass, wall thickness and cellular structure were not strong indicators of thermal-shock tolerance. Differences in germination percentage between scarified and short-term heat-shock treatments indicated that the heat required to break dormancy in Australian legumes may not always be related to breaking an impervious seed coat. Australian native legumes appear more tolerant to thermal-shock than species reported from other regions of the world.