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Landscape Plant Development Center's Breeding Project - Status.

Harold Pellett

Contact Information:
Landscape Plant Development Center
PO Box 39, Chanhassen, MN 55317
Phone: (952) 443-1504.

A paper from the Proceedings of the 12th Metropolitan Tree Improvement Alliance Conference (METRIA 12), Landscape Plant Symposium: Plant Development And Utilization, held in Asheville, NC, May 23-25, 2002, co-sponsored by the North Carolina State University, North Carolina Division of Forest Resources, USDA Forest Service Southern Region, North Carolina Landscape Association, North Carolina Association of Nurserymen, The Landscape Plant Development Center, The North American Branch of The Maple Society, and The International Ornamental Crabapple Society.

History and Organization of the Center

The Landscape Plant Development Center is a relatively young organization. It was established in 1990 with a mission of developing superior landscape plants with emphasis on plants that are more tolerant of biological and environmental stresses. The Center was started because there is a need for greater diversity of landscape plants that are tolerant of environmental and biological stresses. Unfortunately, many landscape plants that are presently available are not well adapted to withstand the harsh conditions of the man-modified environments found on many landscape sites. The Center is a non-profit corporation with a Board of Directors from all over the United States. The Center relies completely on grants and donations for the support of its research efforts. It operates as a cooperative effort of researchers located at many different institutions across North America, Europe and Asia. Headquarters for the Center is at the Minnesota Landscape Arboretum.

We utilize the following general approach for our breeding projects: The cooperative nature of the Center, allows us to use the plant collections of participating institutions to do hybridization. This gives us access to a very broad range of diverse germplasm. First generation hybrids (F1's) are grown at a location with a very favorable climate. When a plant with good tolerance to a given environmental stress is crossed to another plant that may not be very tolerant but that possesses other desirable qualities, the first generation hybrids are intermediate in tolerance to the stress between that of the two parents. Thus, if these hybrids were grown in a severe climate the F1 population may not survive. However when the next generation of plants is produced by intercrossing the F1 siblings, some of the progeny will be as stress tolerant as the most tolerant of the original parents and a few plants may even have greater tolerance than the tolerant original parent. We can therefore select those individuals that combine the desired qualities from the less tolerant parent with the tolerance of the hardier parent. Second generation hybrids are planted in many different geographic regions by our cooperators. Superior plants will be selected from these populations that are well adapted to the climatic conditions of the region in which they are selected. Thanks to a generous donation of land by the J. Frank Schmidt Family Charitable Trust, we now have a research station in Oregon to grow our first generation hybrid populations and produce the second-generation plants. We constructed a machine shed-office building, installed an automated drip irrigation system, acquired field maintenance equipment and started research activities at the station in 1999.

Projects in Progress

Breeding of Ornamental Pears

The first breeding activity that we started was to develop small landscape pear trees. Many people have asked, why pears? Aren't there already too many varieties of pears and aren't they over planted? It's true that there are many varieties of Callery pear and they are perhaps used too much in areas where they are adapted. However none of the varieties are reliably winter hardy in northern areas and there are many Pyrus species that are little known that have some very valuable traits. Many of them also have some severe faults that limit their landscape potential in their present form. Collectively the many species of Pyrus possess tolerances to most of the environmental stresses of concern and also have many different plant forms and foliage qualities. Great potential exists for recombining the desirable characteristics through a controlled breeding program to provide a great diversity of plant types, forms, foliage characteristics and climatic adaptation. We decided to start with pears for several reasons. Number one: there is a need for more small trees. Plants in the genus Pyrus are quite tolerant to heavy soils, which is a common problem in landscape situations all over the world. Next, there are many different species with various qualities that could be very valuable in landscape plants. Finally, there is a good collection of pear species at the USDA Clonal Repository in Corvallis, Oregon that we were able to utilize in starting this breeding effort. That collection was developed primarily as a repository for germplasm for use in fruit breeding activities but the species available serve our purpose equally as well. Along with the species pears, the Corvallis collection also includes some interspecific F1 hybrids between some of the species that we used. Dr. Mel Westwood made those crosses many years ago. He was interested in them for their potential use for understocks for fruiting pears. In 1991 and 1992, we made interspecific crosses using 10 different species of Pyrus in many of the possible combinations. The goal is to develop small statured trees with small fruits for landscape use.
The Callery pear was one of the species that we used. Within the Callery group, there are several different cultivars available such as 'Chanticleer', a more upright narrow form. The cultivar 'Autumn Blaze' has a wider form and good fall color. It has been the hardiest of the Callery pears at the University of Minnesota Landscape Arboretum. We grew some for several years but they perished following a winter when temperatures dropped to -33o F in midwinter. In our laboratory cold hardiness testing, it tests out hardy to -31F or -32F.

Another species that has a lot of potential to offer in the breeding program is P. betulifolia. This species has not been used widely in landscaping but there is at least one cultivar, 'Dancer', that is now available in the nursery trade and has good potential for use where it is winter hardy. As I look at this plant, it reminds me of the quaking aspen. The upper part of the leaf is green with silver underneath. When the leaves flutter in the wind, it gives the same effect. Another desirable quality of this plant for landscape use is the small fruit.

Most plants of the species P. eleagrifolia show little promise from a landscape standpoint. They have a coarse, somewhat spiny habit of growth, and not a very nice plant form. The nice features about it are the silver foliage, and tolerances to different adverse conditions. It is native in the Mid East and therefore has a good heat tolerance and possibly drought tolerance.

There is a lot of confusion in the trade between the subspecies P.calleryana fauriei (sometimes listed as cultivar 'Fauriei') and the true species P. fauriei. The species P. fauriei is more of a large shrub growing 8-10 feet high and 12-15 feet wide in Minnesota. Seedlings have shown excellent fall color ranging from golden-yellow to orange and reddish-purple. It has been reliably hardy in the Minneapolis/St. Paul area, although severe winters in the past have caused slight amounts of tip kill. There is a lot of potential for it in large areas for use as a background screen. It could possibly be grafted on a standard to produce a better tree form. Other desirable features that it possesses are small stature and small fruit.

The snow pear, Pyrus nivalis, probably does not have a lot of potential for direct use in landscaping. Its fruit is larger than desired, more than an inch in diameter. The plant has a slightly irregular growth form. Yet, it has very thick, glossy foliage and probable tolerances to heat and drought based on its native areas.

Pyrus salicifolia has a weeping form with silver foliage similar to that of Russian olive. The fruit is one inch in diameter, again, larger than desired. We had one plant of Pyrus salicifolia `Silver Frost' at the Minnesota Landscape that did quite well for several years. It tolerated a lot more cold than I ever expected it to stand. It survived the -33F two winters ago. Unfortunately, last year it contracted fireblight and was completely killed in one season.

Pyrus ussuriensis, Ussurian pear, is definitely the hardiest of the pears. It has a slightly larger flower than the Callery pear and is even showier. It grows a little larger than the Callery pear, around 40 feet tall or higher under ideal growing conditions. The fruit size is undesirable. It is used in our breeding program for its hardiness and big, showy flowers.

The possibility of recombining the good qualities of these plants is our goal. Dr. Rita Hummel has been growing the plants from successful crosses at the Washington State University Research and Extension Center-Puyallup. Second generation hybrids exhibit tremendous diversity in plant form, and foliage characteristics and have great potential for tolerances to different environmental stresses. Approximately 4,000 second-generation hybrid plants were planted in a nursery at our Oregon station in 1999 and 2000. Heavy selection was made in October and November 2000 to reduce the population by discarding plants with poor foliage characteristics or that lacked vigor. The number of plants was reduced to approximately 1,000. These plants were dug and shipped in the spring of 2001 to research cooperators at 10 different sites across North America. They are now growing in Georgia, Pennsylvania, New York, Kentucky, North Carolina, Texas, Arizona, Michigan, Minnesota, Iowa, and a number of selected plants have been replanted in wider spacing at the station in Oregon. The plants will be evaluated in these different locations and superior plants will be selected. An additional crop of F2 seedlings were grown in 2001 and screened for resistance to fireblight. Survivors will be field planted at the Oregon station this coming spring. Seed to grow additional populations was collected from plants with different hybrid backgrounds at the Research station at Puyallup, Washington. These are now germinating at the North Willamette Research and Extension Center. Some promising selections were propagated in 2000 and 2001 for further evaluation and have been replanted in Oregon.

close-up of branches pyrus hybrid selection #4
leaves and fruit

Selection #4 is a promising selection for possible introduction

One selection (#4) looks very promising. It has survived -35 F. in January in Minnesota and is growing very well in Georgia. It has a compact growth habit, silvery cast to the new foliage, small fruit and excellent resistance to fireblight based both on field evaluation in Minnesota and in controlled screening by Andy Bell and Tom Ranney in North Carolina. We are seriously considering introducing it. Budwood was made available in 2000 to several nurseries that have been major supporters of the Center for evaluation and to provide propagation material should we decide to proceed with introduction.

Breeding of Acer & Carpinus

The second project underway is development of small trees in the genus Acer. This project was started by Dr. Wiegrefe while working for the Center. She describes the progress of this project in a separate presentation at this symposium. A similar cooperative project with Dr. Wiegrefe is underway with interspecific hybridization of Carpinus.

Development of Thornless Varieties of Barberry

Plants of thornless green Japanese barberry and several red-leafed cultivars were acquired and planted at the Oregon station. These were established in 2000 for use as parents to develop thornless red-leafed varieties. The plants were interplanted to promote insect pollination between the different varieties. We had hoped that these would flower profusely in 2001 but unfortunately, only a few flowers were produced but we did collect a small amount of open pollinated seed, which is now being stratified.

Development of Sterile Cultivars

We are pursuing three approaches to develop sterile cultivars. The first method is to produce triploid plants by first doubling the chromosome number and then crossing the produced tetraploids with normal diploid plants. We have verified tetraploid plants of Spiraea bumalda and open pollinated seed were collected this past fall from plants growing next to diploid plants. We also have plants of Honeysuckle, Crabapple and Lilac that have been grown from microshoots treated with colchicine and oryzaline to produce tetraploids. We treated seedlings of crabapple, Acer ginnala, Acer platanoides, and Lonicera tatarica hybrids with oryzaline this past winter in an attempt to induce tetraploidy. We are estimating ploidy level of the treated plants by measuring stomata size. Further verification of the ones that appear to be polyploids will be done using flow cytometry techniques. We also initiated efforts to induce mutations by use of Ethyl Methane Sulfonate, a mutagenic agent. Sterility and dwarfness are often induced by mutagenic agents. We will expand mutation-breeding approaches during the coming year. The third approach that we are pursuing is to induce sterility by genetic transformation. We are cooperating with Dr. Alan Smith, Dept. of Horticultural Science at the University of Minnesota in that effort. The first step of this research is to develop sterility genes. Dr. Smith has applied the knowledge gained from the molecular analysis of tomato flower development toward the objective of producing sterility genes, which, when introduced into plants will cause male and female sterility. The sterility genes cause sterility by disrupting specific cells within either the stamen (male reproductive organ) or the pistil (female reproductive organ). Alan is initially testing the effectiveness of the genes that he is developing by introducing them into cultivars of petunia, tobacco and Arabidopsis. We are currently establishing tissue cultures of different woody plant species so that we can try the system on species where sterility would be desirous. Grants from the Horticultural Research Institute and the J. Frank Schmidt Family Charitable Foundation have made it possible for us to initiate this effort. The Minnesota Nursery and Landscape Association has recently received a $100,000 USDA block grant for us that will enable us to greatly expand our effort over the next couple of years.

Biotechnology Approach to Develop Dwarf Shrub Cultivars

We are also pursuing use of biotechnology approaches to develop dwarf shrub cultivars. Along with attempts to induce sterility, we will use similar mutation breeding and gene transfer techniques to develop dwarf shrub cultivars. We are currently attempting to establish tissue cultures of desired shrub species and cultivars. Establishing plants in tissue culture and manipulating conditions to induce adventitious shoot formation is critical to all three approaches that we are using to develop both sterile and dwarf cultivars. Tissues of desired plant species are collected as new buds are just emerging. They are sterilized, rinsed and placed on sterile media to grow. When new shoots emerge they are subdivided until sufficient quantities of cultures are obtained to initiate experiments to develop a protocol for inducing adventitious shoot formation. Different conditions of media, absence or presence of light, etc. are tried to determine the best combination of conditions and media components.

Development of Freestanding Cultivars of Clematis

Thanks to grants from the Perennial Plant Association and the St. Paul Garden Club, we are conducting a breeding program to develop freestanding bush forms of Clematis. This effort was initiated in the fall of 1999 by acquiring plants of Clematis integrifolia, C. recta, C. fruticosa, and C. heracleifolia. Plants of C. hexapetala, grown from seed collected in the wild in the Russian Far East, are also being used in this breeding effort. As the plants came into flower in the summer of 2000, various combinations of the species were crossed. Seed from the successful crosses were collected as they matured and stratified for 90 days. Seedlings were started in the greenhouse during the winter of 2000-2001. Much of the first season of breeding was devoted to learning breeding techniques specific to these species of Clematis. We find that pollen is often shed while the flowers are still in the balloon stage and that considerable self pollination apparently occurs. Results from our first season of hybridization are very encouraging.

image of hybrid between C. integrifolia and C. hexapetala
Hybrid between C. integrifolia and C. hexapetala

We have quite a few plants that are definitely hybrids between C. integrifolia x C. recta and C. integrifolia x C. hexapetala. These plants flowered during their first season. The flowers are blue in color and up facing unlike the nodding flowers of C. integrifolia. There are differences in shades of blue, flower size and petal shape between different plants of the same cross. Flowers open with good blue color but fade as they mature especially in hot weather. Individual flowers are maintained on the plants for a considerable period. Flowers from these hybrids produce very limited pollen and do not seem to self. Crosses on the hybrids did not produce viable seed from open or controlled pollinations. Some of the flowers from C. integrifolia x C. hexapetala plants have four petals while others have five and six petals. If the plants prove to have good vigor, I believe that selections can be made from both of these crosses that will be worthy of introduction. Many of the young plants are now starting to branch more profusely.

Clematis heracleifolia has been somewhat frustrating and confusing in our breeding efforts. They produce an abundance of pollen that sheds while the flowers are still very tightly closed. Emasculating these tightly closed flowers is difficult as the pedicel is very fragile. Because of the difficulty in emasculating the flowers without breaking them off and since we did not know that the plants self pollinate while we were making crosses in 2000, we made a number of pollinations on plants that were emasculated as larger buds but after the pollen had started to shed. We had good seed set from crosses with C. fruticosa, C. hexapetala, and C. integrifolia. Plants grown from these crosses appear to be self-pollinated based on foliage appearance although there is considerable variation in plant form. Some plants stay in a rosette for a much longer period while others start stretching while quite young. These plants flowered in late 2001 and flower appearance is much more diverse than that found on the original plants of C. heracleifolia that we used as parents.

We continued the breeding effort in 2001. We made a large number of crosses in attempts to hybridize the species with many different large flowered vine cultivars. Seedlings from those crosses are now growing. Although the plants are still very small it appears that many of the seedlings are selfs rather than hybrids but a few plants appear to be hybrids. We have made some selections of the hybrids between C. integrifolia x C. hexapetala and C. integrifolia x C. recta that we are propagating by tissue culture.

Development of rootstocks

Another research area that we are interested in starting is selection of clonal rootstocks for their potential to impart better tolerance to soil related stresses or to influence growth characteristics of the top. There is a lot of potential in this area. Dwarfing rootstocks are used a lot in the fruit industry for size control. Selected rootstocks that result in better tolerance to certain soil related stresses such as drought, high soil pH, poorly drained and compacted soils and low soil fertility would be very beneficial. Some of our Pyrus progeny may have potential for use as dwarfing rootstocks. We will evaluate them for this potential.

Initiation of Additional Breeding Activities

Following a visit to Estonia in 2000, we entered into a cooperative breeding effort with the Tallinn Botanic Garden. Plants of Syringa, Malus, and Forsythia grown from seed, collected from plants growing in plantings with many different cultivars at the Minnesota Landscape Arboretum, are now growing at the Tallinn Botanic Garden in Estonia. Unfortunately Dr. Heiki Tamm, our contact person has taken a position as Director at another arboretum. We hope to be able to continue this effort with a broader cooperative arrangement.
Some breeding efforts were started with other shrub groups in 2001. These include efforts to develop yellow flowered cultivars of Weigela, golden foliage and compact plant forms of red-twigged dogwood, and compact forms of Physocarpus with bronze leaf color. We hope to expand efforts with shrubs in the future with emphasis on developing dwarf, more compact varieties and also emphasis on developing and selecting superior cultivars of native plant species. Plants of different genera to use as parents are being acquired and planted at our station in Oregon. We are also expanding our cooperative efforts with Dr. Rita Hummel at the Washington State University - Puyallup. They are propagating some of our seedling populations from successful crosses for us in their greenhouses. We are currently developing a cooperative breeding program with Cornell Plantations and the Dept. of Horticulture at Cornell University. I traveled to Ithaca New York in late April to explore and initiate that cooperative effort. Cornell Plantations have recruited several volunteers to help establish that effort and we will hire a scientist to carry out the day-to-day activities of the breeding program in the future. In cooperative Dr. Robert Schutzki, Michigan State University, we are planning to initiate activities to select and develop superior cultivars of native plant species. Bob has received a block grant from the Michigan Dept. of Agriculture to collect native plants in the great lakes region to begin that effort.

Potential for Success

There is tremendous potential for developing superior landscape plants of many different plant genera. The Center's cooperative approach provides an efficient and effective approach for developing superior new cultivars that are well adapted to the stresses encountered in many different geographic regions. However breeding of new plants is a time consuming and costly process. The success of the Center is very dependant on the level of support that it receives. Broad support from the nursery industry and avid gardeners is a must if the Center is to reach its full potential. I personally strongly believe in the potential of the Center and will continue to donate my time and expertise to see the Center succeed as long as my current good health permits. As support increases, the Center is able to expand the projects underway and initiate breeding efforts with more plant groups. My personal goal is to see the Center progress to the point that it can become self-sustaining indefinitely. It needs to gain a sufficient level of support so that it can hire a full time director to continue the effort and to also support additional research personnel.

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