Do heat and smoke affect the permeability of the Grevillea seed coat to large molecular weight compounds? Flowers and ripening fruits of Grevillea linearifolia

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, smoke, 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.