Seed-coat Dormancy in Grevillea linearifolia: Little Change in Permeability
to an Apoplastic Tracer after Treatment with Smoke and Heat
Candida L. Briggs1 and E. Charles Morris2,
1School of Natural Sciences, University of Western Sydney, Locked Bag 1797, South Penrith DC 1797, Australia and
2Ecology and Environment Research Group, Hawkesbury Campus (K12), University of Western Sydney, Australia
Annals of Botany 101: 623–632, 2008 Grant details
Background and Aims Seeds of Grevillea
linearifolia germinate following
fire, and have seed-coat dormancy
broken by smoke and heat shock. Smoke breaks seed coat dormancy in Emmenanthe penduliflora by altering the
permeability of the seed coat to an internal germination inhibitor,
which subsequently escapes. This model was
tested for in G. linearifolia by investigating the permeability
of the seed coat to diffusion of high-molecular weight
compounds, and whether this changed after exposure to fire cues.
Methods Germination response of the seeds to heat shock, smoke or heat & smoke was tested. Penetration of Lucifer Yellow dye into intact seeds was examined after 24 and 48 h of exposure, and penetration of the dye from the inside of the seed coat outwards was examined after 24 h. Histochemical staining with Nile Red and Acridine Orange was used to locate cuticles, suberin and lignin.
Key Results Twenty-three per cent of untreated seeds germinated; heat shock and smoke increased germination additively up to approx. 80% for both cues combined. Lucifer Yellow did not penetrate fully through the seed coat of untreated seeds, whether diffusing inwards or outwards. Three barriers to diffusion were identified.
Treatment with heat or smoke slightly increased penetration of the dye, but did not completely remove the barriers.
Suberin was identified in secondary walls of exotestal and mesotestal cells, and was absent from primary cell walls.
Movement of Lucifer Yellow occurred through the middle lamella and primary cell wall of suberized cells; movement of the dye was impeded where suberin was absent.
Conclusions Fire cues did not significantly decrease barriers to diffusion of high-molecular-weight compounds in the seed coat of Grevillea, and must be breaking dormancy by another mechanism.