horticulturae Article Developing Triploid Maples Ryan N. Contreras * and Tyler C. Hoskins Department of Horticulture, Oregon State University, 4017 Agricultural and Life Sciences, Corvallis, OR 97331, USA; [email protected] * Correspondence: [email protected]; Tel.: +541-737-5462 Received: 5 September 2020; Accepted: 19 October 2020; Published: 21 October 2020 Abstract: Maples are common street and shade trees throughout the temperate zone. They are widely used for their wide range of ornamental traits and adaptability, particularly to urban settings. Unfortunately, some species such as Acer tataricum ssp. ginnala (Amur maple) and A. platanoides (Norway maple) have escaped cultivation to become pests or in some cases threaten native flora. However, these species remain economically important and are still asked for by name. To ameliorate potential future ecological damage from additional escapes, we have been breeding for sterile forms using ploidy manipulation and backcrossing to develop triploids. We began with a series of experiments to develop tetraploids of Amur, Norway, and trident (A. buergerianum) maples. Treatment of seedlings at the cotyledon or first true leaf stage was successful in inducing tetraploids of each species. Mortality, cytochimeras, and tetraploids varied among species. After identifying tetraploids, they were field planted alongside diploid cultivars and seedlings, which served as pollinizers in open-pollination. Seedlings derived from open-pollinated tetraploids were generally found to be a high percentage triploids. Thus far, no Norway or trident maple triploids have flowered but after three years we observed five, 22, and 22 Amur maple triploids flowering over three respective years with no seedlings recovered to date. Further evaluation is required but our findings are encouraging that the triploids we have developed thus far will be sterile and provide new cultivars for nursery growers and land managers. Keywords: Acer buergerianum; Acer ginnala; Acer platanoides; Acer tataricum ssp. ginnala; ploidy manipulation; sterility 1. Introduction Maples (Acer sp.) are popular landscape trees often identified by commonly having palmately lobed leaves and characteristic schizocarps (joined samaras). However, many species differ from the archetype palmately lobed leaves and most species are not particularly desired for their flowers or fruit, though few exceptions exist (e.g., ‘Flame’ amur maple grown for its red schizocarps). In 2014, overall U.S. sales of maples exceeded $173 million, which accounted for 31% of the nearly $562 million of deciduous shade trees sold nationally [1]. Much of the nursery stock produced around the country is marketed to population centers in the upper Midwest and New England regions. In fact, Oregon, the leading shade tree production state in the U.S., ships approximately 80% of its nursery plant material out of state and historically has relied on regions in the eastern part of the U.S. as principle markets. With the nearly complete loss of elms and ash from the market due to Dutch elm disease [2] and emerald ash borer [3], respectively, the nursery and landscape industries are more reliant on maples to fill the urban canopy. However, many maples have the significant drawback of being weedy. Several species have escaped cultivation and become invasive to the point of being banned in some states, which has resulted in a significant decline of staple species in historically key markets such as New England. Horticulturae 2020, 6, 70; doi:10.3390/horticulturae6040070 www.mdpi.com/journal/horticulturae Horticulturae 2020, 6, 70 2 of 15 Among the most widely used maples, Norway maple (A. platanoides) became popular in the U.S. in the mid- to late-1800s and continues to be widely used as a street and shade tree. Following previous devastating losses of elm trees from Dutch elm disease, large numbers of Norway maples were planted. Desirable forms include columnar growth habit, red leaves, and cultivars with improved fall color. Norway maples grow well under a wide range of conditions including sand to clay and Horticulturae 2019, 5, x FOR PEER REVIEW 2 of 17 acid to calcareous soils. It is hardy from USDA Zone 4 to 7 and tolerates hot, dry conditions better than sugar maple.Among Furthermore, the most widely Norway used maples, maple Norway is tolerant maple (A. of platanoides air pollution,) became including popular in ozonethe U.S. and sulfur dioxide, makingin the mid it ideal‐ to late for‐1800s use asand a streetcontinues tree. to However,be widely used a major as a street problem and withshade Norwaytree. Following maple is that it previous devastating losses of elm trees from Dutch elm disease, large numbers of Norway maples has becomewere naturalized planted. Desirable to the point forms of include invasiveness columnar in growth forests habit, of New red England.leaves, and Between cultivars 2009with and 2014, sales of Norwayimproved maple fall color. fell byNorway more maples than 5%grow from well under $14.4 a million wide range to $13.6 of conditions million including but many sand growers to have seen muchclay more and dramaticacid to calcareous declines soils. in It sales.is hardy Based from USDA on conversations Zone 4 to 7 and tolerates with leading hot, dry shade conditions tree growers, this downwardbetter than trend sugar continues. maple. Furthermore, Robinson Norway Nursery, maple an is tolerant Oregon of shadeair pollution, tree grower,including estimatesozone their and sulfur dioxide, making it ideal for use as a street tree. However, a major problem with Norway market is downmaple is 90% that from it has a become decade naturalized ago. Their to the data point indicates of invasiveness they sold in forests approximately of New England. 25,000 Norway maples annuallyBetween between 2009 and 2014, 2000 sales and of 2006, Norway but maple today fell they by sellmore closer than 5% to from 3000. $14.4 It is million likely to that $13.6 a continued downwardmillion trend but will many be growers observed have unless seen much alternative more dramatic cultivars declines are in introduced. sales. Based on conversations Otherwith street leading trees shade that tree previously growers, this were downward dominant trend in continues. the market Robinson have Nursery, seen precipitous an Oregon declines shade tree grower, estimates their market is down 90% from a decade ago. Their data indicates they in sales duesold to approximately emerging pests. 25,000 Norway Ash (Fraxinus maples annuallyspp.) between was a leading 2000 and 2006, street but tree today until they recently; sell closer however, with the emergenceto 3000. It is likely of emerald that a continued ash borer downward as a major trend pest, will be the observed viability unless of alternative ash as a commercial cultivars are crop has been reduced.introduced. Sales declined from nearly $15 million in 2009 to less than $10 million in 2014 [1] and the pest continuesOther to street devastate trees that the previously Midwestern were dominant urban treein the canopy, market have thus seen one precipitous would expect declines ash sales to in sales due to emerging pests. Ash (Fraxinus spp.) was a leading street tree until recently; however, continue towith fall. the Since emergence currently of emerald there ash is notborer a as viable a major source pest, the of viability resistance of ash in as the a commercial U.S., it is imperativecrop we have alternativeshas been reduced. in the nursery Sales declined and landscapefrom nearly $15 industry million toin 2009 replace to less street than $10 trees million in American in 2014 [1] cities. Ecologicaland the and pest economiccontinues to harm devastate occurs the Midwestern when ornamental urban tree canopy, plants thus escape one would cultivation expect ash and displace native flora,sales which to continue requires to fall. costly Since actioncurrently on there the is part not ofa viable land source managers of resistance [4]. Furthermore, in the U.S., it is legislation imperative we have alternatives in the nursery and landscape industry to replace street trees in preventingAmerican nurseries cities. from producing and selling these species causes economic harm to growers. The fruit (wingedEcological samaras and oreconomic schizocarps) harm occurs that when are so ornamental well recognized plants escape in the cultivation landscape and aredisplace also notorious for introducingnative flora, unwanted which requires maple costly seedlings action on into the part both of the land urban managers and [4]. natural Furthermore, landscapes. legislation Currently, there are threepreventing maples nurseries species from [Acer producing tataricum and sellingssp. ginnala these species(Amur causes maple), economicA. platanoides harm to growers., A. pseudoplatanus The fruit (winged samaras or schizocarps) that are so well recognized in the landscape are also notorious (sycamorefor maple)] introducing listed unwanted as noxious maple weeds seedlings in into Connecticut both the urban and and Massachusetts. natural landscapes. Of theseCurrently, three species, two are bannedthere are from three the maples state ofspecies Massachusetts [Acer tataricum and ssp. one ginnala is banned (Amur in maple), Connecticut A. platanoides [5]. While, A. not yet listed in otherpseudoplatanus regions (sycamore of the country, maple)] listed these as species noxious weeds are widely in Connecticut distributed and Massachusetts. and have Of naturalized these over large areasthree (Figure species,1). two Norway are banned maple from isthe extremely state of Massachusetts shade tolerant and one andis banned its phenology in Connecticut is [5]. such that it While not yet listed in other regions of the country, these species are widely distributed and have releases itsnaturalized seeds at over an appropriate large areas (Figure time 1). for Norway survival maple (early is extremely fall) and shade natural tolerant cold and its stratification. phenology This is in contrastis to such red that maple it releases (A. rubrum its seeds), forat an example, appropriate that time has for mature survival schizocarps (early fall) and during natural summer cold when summer droughtstratification.