Ptelea trifoliata L. (Rutaceae) has been used as an ornamental shrub since the 18th century and has gained international favor in England and Scotland; however, it remains a rare choice among landscapers (Harvey, 1981; Lancaster, 1995). Two cultivars exist, ‘Glauca’, which has dark green leaves, and ‘Aurea’, which has golden leaves (Dirr, 1998). Horticultural attention has focused on P. trifoliata; however, the genus comprises two other species, Ptelea crenulata Greene and Ptelea aptera Parry (Yang and Applequist, 2015). Uncommonly used plants have the potential to add species diversity to landscapes, which can stabilize ecosystems (Tilman et al., 2006).
Ptelea trifoliata and P. crenulata are understory, shade-tolerant shrubs measuring 2 to 5 m in height with trifoliate leaves (Dirr, 1998; McMinn, 1989). The fruits are compressed samaras that are broadly winged, indehiscent, and light brown to cream in color (Dirr, 1998). In the center of the fibrous, papery pericarp are two cells that may contain two dark-colored seeds (USDA Forest Service, 1948). The small greenish-white pistillate and the creamy-white staminate flowers form inflorescences that emit a strong, unique fragrance (Knuth, 1908; Kurz, 1997; McMinn, 1989; Nokes, 2001). Almost entirely dioecious, P. trifoliata blooms in late May to June, and P. crenulata blooms in April (Ambrose et al., 1985; Dirr, 1998).
Ptelea trifoliata comprises five subspecies that occur in much of North America from Ontario to Florida and as far west as Nebraska and Arizona (Bailey, 1962; Yang and Applequist, 2015). Ptelea crenulata is found in California and P. aptera is found in Baja California (Yang and Applequist, 2015). In Canada, 102 species of insects from 40 families visit P. trifoliata and are thought to be pollinators (Ambrose et al., 1985). Leaves are eaten by larvae of swallowtail butterflies and grasshoppers (Scriber and Dowell, 1991; Sword and Dopman, 1999). The larvae of the moths Agonopterix pteleae Barnes and Busck and Agonopterix costimacula Clarke feed exclusively on P. trifoliata in Illinois (Harrison and Berenbaum, 2005).
Several reports of the propagation of P. trifoliata from seeds are available (Dreesen and Harrington, 1997; McLeod and Murphy, 1977; USDA Forest Service, 1948). With 3 to 4 months of cold stratification, 28% germination was reported (USDA Forest Service, 1948). Other researchers who stratified seeds for 18 weeks found differences in germination based on samara size and color (Dreesen and Harrington, 1997). The highest germination (50%) occurred among the large, green-colored samaras that had the proximal end removed (Dreesen and Harrington, 1997). Higher germination (68%) was reported when seeds were excised from pericarp and stratified for 181 d (McLeod and Murphy, 1977). Samara leachate inhibited germination, implying a chemical inhibitor (McLeod and Murphy, 1977). Despite the previous attention to seed germination in P. trifoliata, the need for a more precise procedure exists, and there is no information about the germination of seeds of P. crenulata. Soaking seeds in water with dissolved polyethylene glycol (PEG), called osmopriming, has been used to increase the germination rates of various crops (Farooq et al., 2005; Naglreiter et al., 2005; Salehzade et al., 2009). PEG decreases the water potential difference between dry seeds and surrounding moisture. PEG is physiologically inert because its large molecular mass prevents it from entering cell membranes (Naglreiter et al., 2005). Rapid imbibition can lead to reduced cellular respiration and germination (Powell and Matthews, 1978); in theory, PEG should facilitate more gradual imbibition.
Our objective was to optimize the speed, uniformity, and completeness of germination of P. trifoliata and P. crenulata with stratification and pericarp removal. During our first experiment, the effects of stratification duration were compared with seeds that had been separated from their pericarps. During our second experiment, the effects of two methods of pericarp removal were compared with use of intact, whole samaras because it is not clear how McLeod and Murphy (1977) removed pericarps. Both our removal methods involved soaking samaras to soften the pericarp, but one was soaked in a PEG solution to determine if rapid imbibition had any effect on seed vitality.
AmbroseJ.D.KevanP.G.GadawskiR.M.1985Hop tree (Ptelea trifoliata) in Canada: Population and reproductive biology of a rare speciesCan. J. Bot.6319281935
DirrM.A.1998Manual of woody landscape plants: Their identification ornamental characteristics culture propagation and uses. Stipes Publ. Champaign IL
DreesenD.R.HarringtonJ.T.1997Propagation of native plants for restoration projects southwestern U.S. - preliminary investigationsU.S. Dept. Agr. For. Serv.7788
FarooqM.BasraS.M.A.SaleemB.A.NafeesM.ChishtiS.A.2005Enhancement of tomato seed germination and seedling vigor by osmopriming. PakistanJ. Agr. Sci.4234503506
KnuthP.1908Handbook of flower pollination. Clarendon Press Oxford
KurzD.1997Shrubs and woody vines of Missouri. Conservation Commission of the State of Missouri Jefferson City MO
McMinnH.E.1989An illustrated manual of California shrubs. J.W. Stacey Inc. San Francisco CA
MichelB.E.WigginsO.K.OutlawW.H.1983A guide to establishing water potential of aqueous two-phase solutions (polyethylene glycol plus dextran) by amendment with mannitolPlant Physiol.726065
MillerA.PetersJ.2010AOSA/SCST tetrazolium testing handbook. Tetrazolium subcommittee of the Assoc. of Official Seed Analysts and the Soc. of Commercial Seed Tech. Fort Collins CO
NaglreiterC.ReichenauerT.G.GoodmanB.A.Bolhar-NordenkampfH.R.2005Free radical generation in Pinus sylvestris and Larix decidua seeds primed with polyethylene glycol or potassium salt solutionsPlant Physiol. Biochem.43117123
NokesJ.2001How to grow native plants of Texas and the Southwest. Univ. of Texas Press Austin TX
SalehzadeH.ShishvanM.I.GhiyasiM.ForouzinF.SiyahjaniA.A.2009Effect of seed priming on germination and seedling growth of wheat (Triticum aestivum L.)Res. J. Biol. Sci.4629631
ScriberJ.M.DowellR.V.1991Host plant suitability and a test of the feeding specialization hypothesis using Papilio cresphontes (Lepidoptera: Papilionidae)Great Lakes Entomol.241503506
SwordG.A.DopmanE.B.1999Developmental specialization and geographic structure of host plant use in a polyphagous grasshopper, Scistocerca emarginata (=lineata) (Orthoptera: Acrididae)Oecologia120437445
USDA Forest Service1948Woody-plant seed manual. U.S. Dep. Agr. Misc. Publ. Washington DC
YangE.S.M.ApplequistW.L.2015Phenetic analyses and revised classification of the Ptelea trifoliata complex (Rutaceae)West. N. Amer. Nat.75406428