Promoting Germination in Ornamental Palm Seeds Through Dormancy

embryos that need more time to grow Promoting Germination in Ornamental Palm before germination can occur are Seeds through Dormancy Alleviation morphologically dormant. Finally, seeds may possess combinations of dormancy. For example, some seeds Hector E. Pe´rez1 possess morphological and physiological (morpho-physiological) or physical and physiological (combina- ADDITIONAL INDEX WORDS. Arecaceae, embryo, imbibition, morpho-physiological, operculum, stratification tional) dormancy. Physiological and morpho-physiological dormancy can SUMMARY. Delayed and inconsistent seed germination often hampers commercial be broken down further into different production of palms (Arecaceae). Such sporadic germination is commonly due to levels of dormancy, which are based seed dormancy. Mature, freshly shed seeds of palms typically display a combination on requirements for dormancy allevi- of underdeveloped embryos (morphological dormancy) and the inability of devel- ation (Baskin and Baskin, 2001, oping embryos to rupture covering structures (physiological dormancy). Fruit and seedcoats are capable of imbibing water. Therefore, dormancy due to water- 2004). impermeable fruit or seedcoats (physical dormancy) does not occur. Removal of Palm fruit and seed embryo covering structures, such as the pericarp and operculum, followed by incubation under moist, warm (25–35 °C) conditions promotes rapid and complete morphology germination. Complete burial in soil promotes germination of seeds in intact fruit Generally, palm fruit consist of of loulu palm (Pritchardia remota). three layers, collectively referred to as the pericarp (Uhl and Dransfield, ith over 2000 species, the germination when conditions are 1987). The exocarp, or outer cover- palm family (Arecaceae) is suitable for germination but not for ing, is usually smooth, although the Wdiverse (Uhl and Dransfield, seedling establishment (Fenner and exocarp of some species may possess 1987). This diversity is manifest in Thompson, 2005), for example, in a hairs, warts, or prickles. The meso- numerous growth forms, textures, sudden warm period during an other- carp is usually fleshy or fibrous and colors, and cultivation requirements. wise cold winter with frequent may be dry at fruit shedding. The Palms are used in various interior and freezes. Baskin and Baskin (2004) inner fruit layer, or endocarp, is very exterior landscape settings because of consider seeds to be dormant if thick and woody in most species. their assortment of features that add a freshly matured, viable seeds do not Fruit typically consist of one large touch of elegance and stimulate the germinate in about 4 weeks over a seed enclosed within the endocarp senses. There is a high demand for range of environmental conditions. (Uhl and Dransfield, 1987). Palm palms in the trade. For example, Moreover, five classes of dormancy seeds may possess a relatively thick yearly sales of ornamental palms in have been proposed. These include: endocarp and testa (Figs. 1 and 2). A Florida reached $220 million (U.S. physical, physiological, morphologi- germination lid, or operculum, may Department of Agriculture, 2007). cal, morpho-physiological, and com- be observed on the testa or endocarp When used, palms typically represent binational (Baskin and Baskin, 2001, of some species. The small, usually the largest dollar investment in plant 2004; Fenner and Thompson, 2005). cylindrical embryo is found under the material for landscaping projects According to Baskin and Baskin operculum (Fig. 3). For the most (H.E. Pe´rez, personal observation). (2001), seeds with physical dor- part, palm seeds are shed with con- Although a few palms are ame- mancy possess fruit coats (pericarps) siderable amounts of hard endosperm nable to propagation through divi- or seedcoats (testa) that are water (Uhl and Dransfield, 1987). sion of rhizomes, offshoots, or tissue impermeable. The nature of the culture, the vast majority of landscape impermeability may be due to multi- Seed dormancy in palms palms are propagated by seeds. How- ple layers of tightly spaced, thick- Although seeds of a few palm ever, commercial production of many walled cells in the pericarp and testa, species such as areca palm (Chrysalido- palm species is hampered by delayed the presence of waxes, lignins, and carpus lutescens), sabal palm (Sabal and inconsistent germination. Such pectins, or a combination of these palmetto), washington palm (Washing- sporadic germination is often due to factors. Seeds with physiological dor- tonia robusta), and date palm (Phoenix seed dormancy mechanisms. There- mancy possess embryos that cannot dactylifera) complete germination in fore, the intent of this article is to generate sufficient force to rupture 28 d or less (Broschat and Donselman, review dormancy mechanisms in embryo-covering structures, such 1986; Broschat and Meerow, 2000; palms and to communicate methods as endosperm, testa, or pericarps. Brown, 1976; Loomis, 1958; von to promote germination in this val- Mature seeds with underdeveloped Fintel et al., 2004; Wagner, 1982), uable group of plants. Classes of seed dormancy Units From a functional perspective, To convert U.S. to SI, To convert SI to U.S., seed dormancy is a trait that prevents multiply by U.S. unit SI unit multiply by University of Florida, Department of Environmental 2.54 inch(es) cm 0.3937 Horticulture, P.O. Box 110675, Gainesville, FL 25.4 inch(es) mm 0.0394 32611-0675 1 micron mM 1 1Corresponding author. E-mail: heperez@ufl.edu. (°F – 32) O 1.8 °F °C(1.8·C) + 32 682 • October–December 2009 19(4) the majority of species germinate slowly. For example, germination commenced about 5 weeks after sowing and continued sporadically over the next 17 weeks in fruit of loulu palm (Pritchardia remota)(Pe´rez et al., 2008). Koebernik (1971) lists 126 species that completed germination more than 100 d after sowing. Likewise, Tomlinson (1990) esti- mated that 25% or more of all palm species required more than 100 d to germinate. Finally, Orozco-Segovia et al. (2003) reported that many palms began germinating about 17 weeks after shedding, suggesting that a period of dormancy may be present. It is often assumed that the thick endocarp and testa of many palms are impermeable to water (Orozco- Segovia et al., 2003). For example, Moussa et al. (1998) suggested that the thick pericarps of doum palm Pritchardia remota Fig. 1. Anatomy of mature loulu palm ( ) endocarp. (A) Note the (Hyphaene thebaica) are water imper- multiple layers comprising the endocarp. The middle endocarp (me) is thick and composed of thick-walled, pitted sclerids with lignin deposits. (B) and (C) are meable. Yet, these authors provided details of the outer (oe) and inner (ie) endocarp, respectively. The oe and ie are no evidence that imbibition is blo- composed of tangentially stretched cells with some lignin deposits. Tissues were cked due to the pericarp. On the stained with phloroglucinol. Slides viewed and images captured at ·6.3 in (A) and contrary, imbibition may occur in ·16 in (B) and (C). All sections are 30 mM (microns); 1 mm = 0.0394 inch. palm seeds, albeit relatively slowly (Ehara et al., 2001; Robertson and Small, 1977; Wood and Pritchard, 2003) or rapidly (Brown, 1976), suggesting that the pericarp or testa are permeable. Pe´rez et al. (2008) demonstrated that endocarps, seeds, and embryos of loulu palm (P. remota) become hydrated to similar levels whether endocarp or testa are mechanically abraded or not. Imbibi- tion occurred, but at a slow rate in all treatments. Moreover, endocarp and testa anatomy in various palms species did not resemble the anatomy typically found in seeds of nonpalm species with water-impermeable coats (Baskin and Baskin, 2001; Pe´rez et al., 2008). Therefore, physical and combinational (physical + physiological) dormancy are not present in palm seeds (Baskin and Baskin, 2001). A distinguishing feature of some palm seeds is their small, late-maturing Fig. 2. Loulu palm (Pritchardia remota) seedcoat and operculum. Seed coats (sc) embryo (Uhl and Dransfield, 1987). stained with phloroglucinol (A) or toluidene blue (B) display lignification of the These underdeveloped embryos re- cells and a prominent lignin layer (ll). The endosperm (es) is composed of thick- quire further growth outside of the walled cells. Seed coat sections are 10 mM (microns). Thick sections (50 mM) of the seed and endocarp before germination lateral (C) and central (D) part of the operculum. Note that the central part of the is complete; hence, they are consid- operculum is subtended by a layer of endosperm one to two cell layers thick, while the lateral portion has thicker underlying endosperm layers. The operculum is an ered morphologically dormant (Baskin extension of the seedcoat. Opes = operculum endosperm, op = operculum, sc = and Baskin, 2004). Analysis of data seedcoat (testa), and es = endosperm. Arrows in (D) denote zone where operculum presented by Wood and Pritchard breaks free from the seedcoat during displacement by the growing embryo. (2003) showed that embryos of bot- Magnification ·40 in (A) and ·63 in (B); 1 mm = 0.0394 inch. tle palm (Hyophorbe lagenicaulis)are • October–December 2009 19(4) 683 WORKSHOP testa, and endocarp. Opercula pro- vide mechanical resistance to embryo elongation in african oil palm (Hus- sey, 1958). In de-operculated seeds, embryos commenced elongation after 4 d at 30 °C. However, intact seeds failed to germinate for 6 months. Similarly, embryo elongation in nee- dle palm (Rhapidophyllum hystrix) was controlled by the

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