The generic delimitation of Magnoliaceae has long been debated. Figlar and Nooteboom (2004) classified only two genera in the family, Magnolia and Liriodendron. However, Xia (2007) has comprehensively revised the family (particularly for the Chinese taxa), and 17 genera have been recognized. Manglietiastrum sinicum was proposed as a species of a monotypic genus in the Magnoliaceae family (Law, 1979). It also was treated taxonomically as Manglietia sinica (Chen and Nooteboom, 1993), Magnolia sinica (Cicuzza et al., 2007; Figlar and Nooteboom, 2004), and Pachylarnax sinica (Xia, 2007). The genus Manglietiastrum has been widely accepted in China (Kunming Institute of Botany, 2006), and morphologically it is highly distinguishable; therefore, M. sinicum is used in this article. In the past two decades, field surveys have been continuously conducted and only approximately 10 large mature trees have been found in the broadleaved evergreen montane forests in southeastern Yunnan Province of China (Cicuzza et al., 2007). Huagaimu has attractive fragrant flowers, a beautiful crown, and shiny leaves, and as such, it is an ideal landscaping tree. Also, the species has a straight trunk and silky textured wood, and in the past has been used as a timber tree. Because of its rarity, habitat destruction, and botanical importance, huagaimu has been proposed as the first-ranked priority for China's National Protection (National Forestry Bureau and Agriculture Ministry of China, 1999), and it is also currently being evaluated as a critically endangered species globally (Cicuzza et al., 2007).
Propagation from seeds is important for huagaimu. During the investigations for carrying out the Fauna and Flora International (FFI)-China Magnolia Program, we found about 5000 saplings of huagaimu in several local nurseries. The saplings were from seeds commonly sown in native loess, and the seeds normally took almost half a year to germinate. However, the relevant information of seed biology and germination physiology have not yet been recorded.
Seed germination is an intricate biochemical process involving a complex of morphological, physiological, and biochemical changes in the embryo. Failure to germinate when environmental conditions are adequate is called dormancy (Bewley, 1997). Seed dormancy is an adaptive trait common to many plant species. The extent and persistence of dormancy is genetically controlled and highly dependent on environmental conditions before and after seed maturation (Bethke et al., 2004). Various dormancy breaking and germination stimulating treatments have been tried with seeds of a wide range of species. In this respect, plant growth regulators (PGRs) and low temperatures have been studied intensively. PGRs are essential in all physiological and developmental processes occurring during plant growth. Levels of endogenous PGRs such as gibberellic acids (GAs), cytokinins (CTKs), and ethylene are believed to play a major role in breaking seed dormancy. Numerous researchers have found that exogenously applied GAs can overcome dormancy in many plant species (Arnold et al., 1996; Delanoy et al., 2006; Gupta, 2003; Nadjafi et al., 2006; van Staden, 1973). In addition, GAs are thought to stimulate germination by promoting the mobilization of stored food reserves (Adkins et al., 2002). Although more than 100 members of GAs are now known, GA3, GA4, and GA7 are most frequently used exogenously to break seed dormancy. CTKs usually display low activity in dormancy and germination control compared with GAs; however, they are more effective than GAs in counteracting inhibitors of various GA-sensitive processes (Leadem, 1987). Webb and Wareing (1972) also found that exogenous application of kinetin to dormant sycamore seeds increased germination whereas GA3 had no effect. On the other hand, in light-requiring seeds, GAs often induce high germination in darkness, whereas CKs generally require some irradiation or the presence of GAs (Thomas, 1992). Therefore, different mechanisms of breaking seed dormancy might exist between GAs and CTKs. Khan (1975) reported that GAs and CTKs take on primary and permissive roles, respectively, in regulating seed germination. Auxins play a major role in a variety of growth and developmental processes in plants by regulating cell division, elongation, and differentiation (Becker and Hedrich, 2002). Nevertheless, there are very few reports on the role of synthetic auxins in controlling seed dormancy. It has been pointed out that auxins such as 2,4-dichlorophenoxyacetic acid (2,4-D) and α-naphthaleneacetic acid (NAA) can promote biosynthesis of ethylene (Arteca, 1982; Balagué and Pech, 1985), and ethylene is involved in the promotion of seed germination (Gniazdowska et al., 2007; Kępczyński et al., 2003, 2006a, 2006b). Therefore, in the present study, NAA and 2,4-D were employed to test their effects on release of seed dormancy of huagaimu. Moist chilling, as an external stimulus, is often effective on breaking seed dormancy and results in enhanced seed germination, seedling emergence, or both. Bradbeer (1968) reported that the essential effect of chilling on intact hazel seeds may be to activate the mechanism for gibberellin synthesis, and Villiers and Wareing (1960) observed that the embryo of ash (Fraxinus excelsior) produced stimulating substances counteracting the inhibitors present in the embryo and endosperm during the chilling treatment.
Seed dormancy is a common phenomenon in the Magnoliaceae family, and our previous observations indicated that seeds of huagaimu also exhibit dormancy (Zheng et al., 2008). The reports indicated that dormancy of some species from Magnoliaceae can be effectively broken by GAs and low-temperature treatments (Guo et al., 2006; Han, 2008; Zhou, 1991). The objective of this study was to test whether exogenous PGRs and moist chilling could break the seed dormancy of huagaimu and to find a practical protocol of speeding the seed germination of the species.
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