Journal of the APS Vol 72 Number 3 July 2018

P runus

147

 These examples from Prunus provide a basis to study whether winter-hardy Prunus have invasive potential. Kolar and Lodge (2001) define the first stage in invasiveness as the transport of the species into a new en- vironment. Once present in the new environ- ment, a viable population establishes itself and becomes reproductive (Kolar and Lodge, 2001). Thus, seed germination and seedling establishment are important to understand in- vasiveness. The objective of our study was to determine winter-hardy Prunus seed germi- nation as it relates to invasive potential. Materials and Methods  Genotypes and Seed Collection. We ex- amined three fruit types of Prunus for ger- mination of open pollinated seed including 28 Prunus winter-hardy genotypes (Table 1). Fruit type was defined as apricot, tart cherry, or plum. Although there are two types of tart cherries, amarelle and morello genotypes (Brown et al. 1989), all tart cherries were classified under one category for the purpos- es of this experiment. In 2012, all apricot, tart cherry, and plum fruits were collected from trees at the University of Minnesota research plots in Excelsior, MN (44°52’06.4” N lat., -93°38’00.5” W long.) during weeks 25-26 and 31-34. Week number is defined as the number of weeks from the first week of the year beginning 1 Jan.  Experimental Design. For each genotype, 48 seeds were randomly chosen and divided into two groups of 24 each. One group was mechanically scarified with a hammer hard enough to crack the stony endocarp (pit); the endocarps were left in place when the seeds were sown. Three seeds per pot (11.43 x 11.43 cm Jumbo Junior pots, Belden Plastics, St. Paul, MN) were planted in BM2 germi- nation mix (Berger, Quebec Canada) for the greenhouse or pasteurized field soil (Wauke- gin silt loam) collected from the University of Minnesota St. Paul campus (44°59’17.8” N lat., -93°10’51.6” W long.) for the field. The pots, rather than individual seeds, were considered experimental units.

tion in Prunus . For P. americana , P. cerasus L., and P. persica Batsch., scarification was shown by Chen et al. (2007), Grisez et al. (2008) and Kristiansen and Jenson (2009) to increase both the percent and rate of germi- nation. In P. domestica L. and P. angustifolia Marsh., scarification did not alter germina- tion percentage or rate (Grisez et al. 2008; McMahon et al. 2015).  Physiological dormancy is overcome through a long period of moist, cold strati- fication (Baskin and Baskin, 1998; West- wood, 1993). However, in some Prunus spe- cies, moist and warm stratification increased seed germination (Baskin and Baskin, 1998; Chen et al. 2007; Grisez et al. 2008; West- wood, 1993). Prunus armeniaca L. requires 50 days of cold stratification whereas other species such as P. domestica and P. cera- sus require 90 or 90-150 days, respectively (Jauron, 2000; Grisez et al. 2008; Seeley and Damavandy, 1985). As stratification period lengthens, germination is often higher. For example, germination in P. persica begins after 56 days of cold stratification and con- tinues to until 84 days at an increasing rate (Martınez-Gómez and Dicenta, 2001).  The spread of invasive species is often the result of human activities including ag- riculture, horticulture, and forestry (Reichard and White, 2001; Vanhellemont et al. 2009). Many winter-hardy Prunus genotypes have been cultivated since the early 1900s (Ander- sen andWeir, 1967; Brooks and Olmo, 1997). Some Prunus species have escaped culti- vation and become invasive. For example, P. serotina Ehrh. , a species native to North America, has escaped cultivation in parts of Europe and become invasive (Deckers et al. 2005). Phartyal et al. (2009) estimated that 44% of mature seed of the invasive species P. serotina germinated in situ . Prunus ameri- cana has also demonstrated high invasive po- tential as it is adapted to a variety of habitats and is spread across a wide geographic range (Francis, 2004). Whether other Prunus spe- cies and genotypes will become invasive is not known.