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Australian Journal of Botany, 2007, 55, 336–344
Seed dormancy and germination stimulation syndromes for Australian temperate species D. J. MerrittA,B,C , S. R. TurnerA,B , S. ClarkeA,B and K. W. DixonA,B A
Kings Park and Botanic Garden, West Perth, WA 6005, Australia. School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Crawley, WA 6009, Australia. C Corresponding author. Email:
[email protected] B
Abstract. Understanding seed germination biology and the dynamics of seed dormancy is essential to developing reliable germination techniques. This paper presents some new data and reviews recent findings from germination studies on Australian species, with respect to the role of temperature and moisture in the control of dormancy and germination. A seed-burial experiment was conducted over a 1-year period (January–December) with seeds of Acanthocarpus preissii Lehm., Astroloma xerophyllum (DC.) Sond., Gahnia grandis (Labill.) S.T.Blake, Goodenia scaevolina F.Muell. and Tersonia cyathiflora (Fenzl) J.W.Green to determine the fluctuations in temperature and moisture seeds naturally experience in the buried environment. All seeds became hydrated during autumn (March) while soil temperatures were >15◦ C, suggestive of a period of warm stratification before the onset of cooler winter temperatures appropriate for germination. Evidence of rapid wetting and drying of seeds in the soil environment was also present. Laboratory experiments testing stratification as a means for dormancy loss showed that several weeks of warm stratification at 26/13◦ C or 33/18◦ C promoted germination of Lomandra preissii (Endl.) Ewart, Marianthus bicolor (Putt.) F.Muell. and Xyris lanata R.Br. seeds. X. lanata seeds also responded to several weeks of cold stratification at 5◦ C. By integrating this new data with other published data on germination of Australian species, diagrammatic models of germination timing, dormancy syndromes and propagation strategies for temperate Australian species are presented as working hypotheses to help direct future research.
Introduction Effective methods for seed germination are vital if seeds are to be used in conservation and restoration programs. To this end, considerable research effort has been directed towards understanding seed germination and dormancy traits in the Australian flora (Mott 1974; Dixon et al. 1995; Plummer et al. 1995; Roche et al. 1997; Bell 1999; Morris 2000; Tieu et al. 2001a, 2001b; Flematti et al. 2004; Baker et al. 2005a, 2005b; Turner et al. 2005, 2006a, 2006b; Merritt et al. 2006; Ooi et al. 2006). Despite considerable progress, there remains many prominent plant families in Australia, comprising species with largely unresolved seed-germination requirements (Table 1). For seed-based propagation of plants for restoration programs, dormancy-breaking treatments must be efficient, repeatable and readily applicable to large numbers of seeds. Such requirements present many challenges and seed dormancy is arguably the single greatest impediment to the effective use of native seeds in restoration. To solve these germination problems it is necessary to appreciate what has already been learned, identify knowledge gaps and evaluate research approaches that are either new, or yet to be applied to the Australian flora. In this paper, we consider some key areas of Australian seed research and some promising findings from recent germination studies, particularly with respect to the role of temperature and moisture in control of dormancy loss and germination. Important and necessary areas of research required to improve our ability to germinate seeds © CSIRO 2007
of Australian species are highlighted, and diagrammatic models of germination timing, dormancy syndromes and propagation strategies for temperate Australian species are presented as working hypotheses to help direct future research effort. Some key findings from germination studies on Australian species Across the last 30 years, studies on Australian seeds provide a wealth of information regarding germination. In particular, studies on the effects of incubation temperature and light conditions on germination, the role of temperature in overcoming physical dormancy and dry after-ripening requirements, and the use of germination-promoting agents such as smoke products, nitrates and gibberellins are well represented (reviewed in Bell 1999; Merritt and Dixon 2003). Dry after-ripening as a means of dormancy loss is particularly well documented for several Australian families, including Poaceae (Mott 1974; Lodge and Whalley 1981), Haemodoraceae, Stylidiaceae (Tieu et al. 2001a), Asteraceae (Sch¨utz et al. 2002), Apiaceae (Baker et al. 2005b) and Sapindaceae (Turner et al. 2006a). In most of these studies, after-ripening experiments have been conducted at ambient room temperatures (∼20–25◦ C) and the period of time for dormancy loss can range between 6 weeks (Sapindaceae) and >2 years (Haemodoraceae). In some species it is known that this dry after-ripening period can be circumvented by 10.1071/BT06106
0067-1924/07/030336
Seed dormancy of Australian temperate species
Australian Journal of Botany
Table 1.
Examples of Australian plant families containing species with difficult-to-germinate seeds Families are illustrative of those comprising many difficult-to-germinate species and/or those with species commonly required for restoration programs Anthericaceae Asteraceae Campanulaceae Cyperaceae Dasypogonaceae Dilleniaceae Ericaceae Euphorbiaceae
Goodeniaceae Gyrostemonaceae Haloragaceae Iridaceae Lamiaceae Myoporaceae Onagraceae Phormiaceae
Pittosporaceae Poaceae Portulacaceae Proteaceae Ranunculaceae Restionaceae Rutaceae Violaceae
exposing seeds to high temperatures (80–120◦ C) for short periods of time (10–180 min) (Tieu et al. 2001b; Morris 2000) and such ‘heat-shock’ techniques can be useful propagation tools, particularly when used in combination with gibberellic acid or smoke products. When considering the dormancy types and classification schemes proposed by Baskin and Baskin (2004a), it can be concluded that these seeds have a physiological dormancy component. Physiological dormancy is the most common and phylogenetically widespread form of dormancy in the angiosperm flora (Baskin and Baskin 2003) and is, therefore, likely to also be a predominant dormancy class in the Australian flora. Physiological dormancy is known to be alleviated not only by after-ripening in dry storage, but also by stratification of imbibed seeds at warm (≥15◦ C) or cold (0–10◦ C) temperatures (Baskin and Baskin 2003, 2004a). While the effects of temperature on dormancy loss of dry seeds has been, and continues to be, a focus of research in the Australian flora, the effects of temperature on dormancy loss of moist seeds (i.e. stratification) are sparsely documented. These stratification studies are limited to a few, reporting increased germination following cold stratification of e.g. Eucalyptus pauciflora Sieber ex Spreng. (Beardsell and Mullett 1984), Allocasuarina verticillata (Lam.) L.A.S.Johnson (Moncur et al. 1997) and Xanthorrhoea australis R.Br. (Curtis 1996), and the role of warm stratification appears little explored. Yet there is emerging evidence suggesting that stratification may play an important role in dormancy loss of Australian species with geosporous seeds. It is well known that improved seed germination can be achieved for several difficult-to-germinate Australian geosporous species following a period of soil burial, particularly when smoke is applied following burial (Roche et al. 1997; Tieu et al. 2001a; Baker et al. 2005a), and recently the cyclical nature of seed dormancy in the soil seedbank has been demonstrated for some Australian species (Tieu et al. 2001a; Baker et al. 2005b). This indicates that seeds are responding to cues in the soil seedbank to control the timing of dormancy loss/induction and germination—cues that we are yet to replicate to achieve reliable germination under laboratory conditions for many species. These cues would naturally include fluctuations in temperature and moisture analogous to stratification and afterripening. Certainly, studies on species from temperate Northern Hemisphere climates modelling dormancy loss under field and laboratory conditions reveal that both temperature and seed moisture play key roles in regulating seasonal changes in seed-
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dormancy status, with temperature perhaps being the more critical factor (Bouwmeester and Karssen 1992, 1993; Batlla and Benech-Arnold 2004, 2006). Thus, experiments that investigate (a) the role of fluctuations in temperature and moisture under natural (field) conditions; (b) temperature and moisture cycles that mimic these natural processes in the laboratory, and (c) the timing of application of germination stimulants such as smoke are likely to help resolve germination difficulties in the Australian flora. Seed burial as a means for understanding the natural cycles of temperature and moisture that affect dormancy release To gather information on the variations in soil temperature, soil moisture content and seed moisture content seeds are exposed to in the soil seedbank, a seed-burial experiment was conducted in typical bushland soil over a 1-year period (January– December) in 2005 (Fig. 1). Five species, Acanthocarpus preissii (Laxmanniaceae), Astroloma xerophyllum (Ericaceae), Gahnia grandis (Cyperaceae), Goodenia scaevolina (Goodeniaceae) and Tersonia cyathiflora (Gyrostemonaceae), were selected as representatives of genera producing water-permeable seeds, but with poorly known germination requirements. During this 1-year period, soil moisture was
