Do heat and smoke affect the permeability of the Grevillea seed
coat to large molecular weight compounds?
E. Charles Morris, and Candida Briggs
School of Science, Food and Horticulture, University of Western Sydney Hawkesbury
Grant details Final report
Seeds of east Australian Grevillea species germinate in response to fire-related
cues such as heat and smoke. The seed coat is responsible for
dormancy in G. linearifolia: a possible dormancy mechanism
is the existence of barriers to diffusion of large molecular weight
compounds in the seed coat. Such internal barriers are known to be involved in the dormancy of fire-responsive seeds overseas eg Emmenanthe
pendulioflora, and the permeability of these barriers is altered
by smoke. This model was tested for G. linearifolia by
investigating the permeability of the seed coat to diffusion of
large molecular weight compounds, and whether this changed after
exposure to fire cues. The germination characteristics of seeds to heat,
and combined exposure was tested. The penetration of the dye Lucifer
Yellow into intact seeds was examined after 24 and 48 hours of
exposure, and the penetration of the dye from the inside of the seed coat outwards was examined after 24 hours.
Histochemical staining with Nile Red and Acridine Orange was used to
locate cuticles, suberin and lignin.
About one-fifth of untreated seeds germinated; both heat by itself, and smoke by itself, increased germination; greatest germination (up to 80%) was observed after treatment with both fire cues. The seed coat of untreated seeds was impermeable to Lucifer Yellow, irregardless of whether the dye was diffusing inwards or outwards, and 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.
The Emmananthe model was not supported: fire cues did not significantly decrease barriers to diffusion of large molecular weight compounds in the seed coat of Grevillea, and must be breaking dormancy by another mechanism.
Publication from Grant: Briggs, Candida L., Morris, E. Charles. (2008) Seed-coat dormancy in Grevillea linearifolia: little change in permeability to an apoplastic tracer after treatment with smoke and heat. Annals of Botany 101: 623 – 632.