Annals of Botany

Dyeing to escape

As I glance out of my window at the rain being driven by 100-kph winds it is difficult to imagine ecosystems where fire is an essential factor. In such ecosystems, one of the effects of fire is the breakage of seed dormancy of many fire-dependent species. This phenomenon is discussed by Briggs and Morris, University of Western Sydney, Australia (pp. 623–632). Our knowledge of the mechanisms by which fire breaks seed dormancy is very ‘patchy’. For some species, for example, it is clear that smoke alters a seed-coat barrier so that a germination inhibitor may escape, but for many other species no obvious mechanism has been described. One such is Grevillea linearifolia, a native of eastern Australia. Dormancy is imposed by the seed coat; high germination rates are achieved in the absence of fire if the seed coat is removed. In the authors’ experiments, 23 % of whole, untreated seeds germinated. This increased to 50 % in seeds exposed to heat, and to 67 % in seeds exposed to smoke. Seeds treated with both heat and smoke exhibited 80 % germination. The approach to detection of cell wall permeability changes was simple and effective, making use of the dye Lucifer Yellow (LY). The dye was applied either to the outside of the seed coat of whole seeds or to the inside of seed coats of partly dissected seeds. Permeation of the dye was observed by light microscopy in seed coats from control seeds and from seeds exposed to heat and/or smoke. The results were clear: smoke and/or heat treatment did not make the complex seed coat of G. linearifolia permeable to LY. The authors have therefore turned their attention to the possibility that heat and/or smoke alter the physical properties of the seed coat cells, as suggested by the ‘mechanical constraint’ model of seed coat dormancy.