Nursery Crop Science
Breeding for Non-invasive Landscape Plants
Richard T. Olsen and Thomas G. Ranney
Mountain Horticultural Crops Research and Extenstion Center
Department of Horticultural Science
455 Research Drive, Fletcher NC 28732-9244
Invasion Biology
One of the more pervasive topics in biology today is that of invasive species. This has consumed researchers across all fields of biology, from evolutionary biologists and ecologists to botanists and horticulturists. The arguments, debates, articles, research, facts and myths, have all led to finger pointing, botanical cries of outrage, doomsday extinctions, and further floral hysteria. The path of reality and compromise between players is a continuing battle that has brought warring factions to the tables, while at other times has led to covert lobbying and legislation. Through all the mayhem and hype, a constant that has prevailed is the fact that some introduced species (via anthropomorphic means, both incidental and purposeful) have established themselves in new habitats and ecosystems, impacting both local and global biodiversity.
These plants have been assessed various names: alien species, adventive species, non-indigenous species, naturalized exotic species, and invasive species. Regardless of semantics, the impact of plant introductions in terms of agricultural crops, landscape plants, and biological diversity, has unfortunately been both negative and positive.
A driving force in the strength of the nursery industry here in North Carolina is the introduction of new nursery crops, either through traditional selection, breeding and propagation, targeted plant collection expeditions or serendipitous discovery. New plants have a profound impact on expanding new markets, niches and profits for the ornamental nursery industry, as well as improving the aesthetics of our urban environments. However, there is a negative aspect to introducing plants (or any organism) to bio-geographic regions beyond their natural ranges. The majority of plant introductions have had little impact on natural systems in new areas of cultivation. However, some species are able to persist, and a rarer few, are able to reproduce and integrate into their adopted floras.
Role of the Nursery Industry
Introduced species that are capable of surviving and reproducing outside of cultivation to effectively establish in the local flora often are defined as “invasive species”. Approximately 85% of the invasive plant species in the United States were introduced for either ornamental or landscape use (Reichard and White, 2001). This is alarming when one considers that horticulturists often select traits that risk assessment models would consider intrinsic to invasive plants. These traits would include rapid growth, ease of propagation, broad environmental fitness, and early and abundant flowering and fruiting (White and Schwartz, 1998). Furthermore, many plants which are sometimes considered invasive, or have the potential to be invasive in the Southeast, are being sold by the nursery industry. Examples of popular ornamental plants that have made invasive plant lists in the Southeastern United States include Japanese barberry (Berberis thunbergii DC), butterfly bush (Buddleja davidii Franch.), bittersweet (Celastrus orbiculatus Thunb.), thorny elaeagnus (Elaeagnus pungens Thunb.), winged euonymus [Euonymus alatus (Thunb.) Sieb.], Chinese privet (Ligustrum sinense Lour.) and miscanthus (Miscanthus sinensis Anderss.) (Website addresses cited below have complete invasive species lists for individual southeastern states).
Options and Solutions
Exotic plant pest councils and other non-governmental working groups (e.g. academics, land managers, wildlife biologists) have been instrumental in raising public and governmental awareness of the invasive plant issue. Development of risk assessment models for limiting introduction of potentially invasive plant materials to new countries has met with limited success. Early models were based on screening plants introduced for whole continents, such as North America (Reichard and Hamilton, 1997) and Australia (Pheloung, 1995) that have vastly different climatic and eco-regions. These theoretical models were demonstrated to be successful at preventing greater than 80% of known invasives and prevented nearly 50% of known non-invasive species from being grown. Newer, regional based models have shown greater accuracy in accepting those species now known to be non-invasive, while preventing the introduction of invasive species (Frappier and Eckert, 2003; Widrlechner et al., 2004). Concurrently, exotic plant pest councils have been developing “black lists” of plant species that are known to be invasive species in their respective regions. It is often easy for interested parties to reach a consensus on the worst of the invaders [i.e. kudzu, Pueraria montana (Lour.) Merr.]. However, heated debates ensue on classifying species with limited, regional or anecdotal data regarding integration into undisturbed habitats and ecosystems, especially when those species in contention are important woody crops to the ornamental nursery industry.
Interspecific hybrid plant species were unlikely to be considered invasive in Reichard and Hamilton’s (1997) risk assessment model, due to their high propensity for being sterile. White and Schwarz (1998) recommended that botanical gardens adopt a program of developing or promoting sterile exotic plant materials. Also, this would apply to universities with active ornamental breeding programs and arboreta with plant introduction programs. Due to asexual propagation of most horticultural crops, breeding is often unnecessary in the development of novel plants. Desirable mutations that occur during the development or growing of plants can be propagated, evaluated, and subsequently introduced. Many ornamental plants already in general cultivation are improved forms of species that need no further enhancement from breeding, other than inducing sterility. Therefore, a method must be devised that can be used to induce sterility in those plants that are already in the nursery industry and unlikely to be removed from production due to their popularity, such as selections of Buddleja, Berberis, Elaeagnus, and Euonymus cited above.
Breeding Triploids
Development of seedless forms of many of our nursery crops is an ideal solution that would allow us to continue using these plants for their aesthetic and environmental benefits, while greatly reducing any potential for invasiveness. One approach is the development of seedless triploids. One breeds a diploid plant (two sets of chromosomes) with a tetraploid (four sets of chromosomes) to create a triploid (three sets of chromosomes). Although triploids typically grow and function normally, they have an inherent reproductive barrier in that three sets of chromosomes (two from the tetraploid parent, one from the diploid parent) do not divide evenly during meiosis, yielding unequal segregation of the chromosomes (aneuploids) or complete meiotic failure. Also, triploids can be developed by regenerating plants from the seed’s endosperm tissue, typically triploid due to the double fertilization process of higher plants. However, this can be very difficult for many woody plants that do not have established tissue culture protocols for regeneration of plants from callus or cell lines.
In horticulture, the principle of sterile triploid plants has been used in the development of seedless fruit crops. Seedless watermelons are triploid plants where embryos are lacking or develop poorly, but the fruit still reaches maturity. Also, bananas are seedless, another sterile, triploid horticultural crop. There are few examples where plant breeders have taken advantage of sterile triploids in ornamental plants. The United States National Arboretum historically has had numerous plant breeding projects, including the development of improved cultivars of landscape rose-of-Sharon (Hibiscus syriacus L.). This included development of sterile cultivars, since landscape plants had been observed to seed freely in garden situations (Egolf, 1970). Four triploid cultivars are now available in the nursery trade: ‘Aphrodite’, ‘Diana’, ‘Helene’ and ‘Minerva’ (Egolf, 1970; 1981; 1986; 1988). Triploids are not always completely seedless (e.g. apples). However, even in the infrequent cases when a triploid plant can produce seeds, seedlings generally exhibit poor viability.
Our research group (includes Dr. Zenaida Viloria, Dr. Darren H. Touchell, Mr. Tom Eaker, Mr. Joel Mowrey, and Mr. Nathan Lynch) are currently working on developing non-invasive triploids of the species listed in Table 1. We have identified tetraploids for most of these species, and have developed triploids of Campsis, Hypericum, Ligustrum, and Pyrus successfully. This approach involved controlled breeding and provided additional opportunities for plant improvement. Simultaneously to breeding for seedlessness, we are working to enhance pest resistance, environmental adaptability, and commercial potential of these crops.
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Table 1. Targeted taxa for developing triploids.
Scientific Name Common Name
Acer tartaricum L. subsp. ginnala (Maxim.) Wesm. Amur maple
Acer platanoides L. Norway maple
Albizia julibrissin Durazz. silk-tree or mimosa
Campsis´tagliabuana (Vis.) Rehd. trumpetcreeper
Cytissus scoparius (L.) Link Scotch broom
Euonymus alatus (Thunb.)Sieb. burning bush
Elaeagnus L. spp. elaeagnus
Hedera helix L. English ivy
Hypericum androsaemum L. tutsan St. Johnswort
Koelreutaria paniculata Laxm. panicled goldenraintree
Ligustrum spp. privet
Miscanthus sinensis Anderss. maiden or eulalia grass
Pyrus calleryana Decne. callery pear
Ulmus parvifolia Jacq. lacebark elm
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References
Egolf, D.R. 1970. Hibiscus syriacus ‘Diana’, a new cultivar [Malvaceae]. Baileya
17:75-78.
Egolf, D.R. 1981. ‘Helene’ Rose of Sharon (Althea). HortScience 16:226-227.
Egolf, D.R. 1986. ‘Minerva’ Rose of Sharon (Althea). HortScience 21:1463-1464.
Egolf, D.R. 1988. ‘Aphrodite’ Rose of Sharon (Althea). HortScience 23:223.
Frappier, B. and R.T. Eckert. 2003. Utilizing the USDA PLANTS database to predict
exotic woody plant invasiveness in New Hampshire. Forest Ecol. and Mgt.
185:207-215.
Pheloung, P.C. 1995. Determining weed potential of new plant introductions to
Australia. A report on the development of a weed risk assessment system
commissioned and endorsed by the Australian Weeds Committee and the Plant
Industries Committee. Agricultural Protection Board, Western Australia. 143 p.
Pimentel D., L. Lach, R. Zuniga, and D. Morrison. 2000. Environmental and economic
costs of nonindigenous species in the United States. BioScience 50:53-65.
Reichard, S.H. and C.W. Hamilton. 1997. Predicting invasions of woody plants
introduced into North America. Conserv. Biol. 11:193-203.
Reichard, S.H. and P.White. 2001. Horticulture as a pathway of invasive plant
introductions in the United States. BioScience 51:103-113.
Widrlechner, M.P., J.R. Thompson, J.K. Iles, and P.M. Dixon. 2004. Models for
predicting the risk of naturalization of non-native woody plants in Iowa. J.
Environ. Hort. 22:23-31.
White, P. S. and A.E. Schwarz. 1998. Where do we go from here? The challenges of
risk assessment for invasive plants. Weed Tech. 12:744-751.
Invasive Plant Lists by State
Georgia http://www.gaeppc.org/weeds/
Kentucky http://www.se-eppc.org/states/KY/KYlists.html
Florida http://www.fleppc.org/Plantlist/list.htm
South Carolina http://www.se-eppc.org/states/southcarolina.cfm
Tennessee http://www.tneppc.org/Invasive_Exotic_Plant_List/The_List.htm
Virginia http://www.dcr.state.va.us/dnh/invlist.pdf
Websites for Invasive Species Information
NC Exotic Plant Pest Council http://www.se-eppc.org/states/northcarolina.cfm
Southeast Exotic Plant Pest Council http://www.se-eppc.org/
U.S. Government http://www.invasivespecies.gov/
Center for Invasive Plant Mgt. http://www.weedcenter.org/inv_plant_info/weed_org.htm
Conference on Invasive Plants and Horticulture, Chicago Botanical Garden, 2001 http://www.centerforplantconservation.org/invasives/mbgN.html
