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An Overview of Environmental Stress Research Activities at the Minnesota Landscape Arboretum

Steve McNamara and Harold Pellett

Department of Horticultural Science
University of Minnesota Landscape Arboretum
3675 Arboretum Boulevard , Chanhassen, Minnesota 55317

Research on environmental stress resistance of woody plants is being conducted at the University of Minnesota Landscape Arboretum to facilitate the development of stress resistant landscape plants. This report contains an overview of current activities investigating plant responses to four abiotic stress factors: 1) cold temperatures, 2) water deficits, 3) waterlogged soils, and 4) alkaline soils. General approaches to studying the impact of these stresses on plant growth and development will be described along with some of the results obtained to date.

Introduction

Plants in the landscape are frequently exposed to environmental conditions that can induce physiological stress and adversely affect their growth and appearance. Over the last ten years, we have been studying how woody plant species respond to several environmental stress factors. As a component of the Woody Plant Breeding Program led by Dr. Harold Pellett, the objective of this research is to attain information about plant stress adaptations that will facilitate development of techniques for identifying stress resistant genotypes. Germplasm so identified can ultimately be used for breeding and development of stress-resistant landscape plants.

Following is a description of some of our research activities on plant responses to cold temperatures, water deficits, waterlogged soils, and high soil pH.

Cold Hardiness

The Minnesota Landscape Arboretum is located in USDA hardiness zone 4a where the average annual minimum temperature is between –25 and –30 F. Consequently, low temperature tolerance is a critical criterion used for selecting plants suitable for our area. In the last decade, we’ve evaluated the cold hardiness of multiple cultivars of several woody landscape species including crabapples (6), forsythia (4), ornamental pears (5), and weigela (7). A programmable freezer is used to determine the hardiness level of flowers or shoots of cultivars of interest on multiple sampling dates beginning in the fall. The resultant hardiness profiles allow us to compare maximum midwinter hardiness levels, and acclimation and deacclimation characteristics of the cultivars. Cultivar hardiness information is useful for nursery and landscape professionals selecting plants materials for production and use in northern climates. Plant breeders can also use this information when choosing parent materials for breeding hardy new varieties.

As part of our effort to expand the range of hardy plant material suitable for northern climates, we’ve begun screening seedling populations of nonnative tree species that historically have not been hardy in Minnesota. Eastern redbud (Cercis canadensis) is an example of a plant species that is well out of its native range in Minnesota, but that is now grown there from seed derived from several exceptionally hardy trees imported to the area many years ago and identified through natural selection. We believe that similar genetic variation in cold hardiness likely exists in other nonindigenous species and that cold hardy genotypes of these species could be identified with appropriate screening and selection.

To this end, we’ve been acquiring seeds or young plants of desirable species for testing. Ideally, the material acquired originates near the northern end of the species’ native range. Because seedlings of some species are particularly vulnerable to cold injury during their first few years of life (8), seedlings are grown in containers and protected from cold for several years prior to screening. In the fall of the 3rd growing season, the containerized plants are placed in in-ground beds and mulched to protect their root systems from cold injury. Protective covers are placed over the beds to prevent an accumulation of insulating snow cover and the plants are then exposed to ambient outdoor winter temperatures. In the event that temperatures are not sufficiently cold to provide an adequate test [-25 F (-32 C) or colder], we can treat the plants artificially using a portable, insulated box that fits over the plant beds and lowers the air temperature slowly by injecting liquid nitrogen at a controlled rate. However, use of this system is only practical from an economic standpoint if ambient outdoor temperatures are already quite cold.

Seedlings are visually evaluated for cold injury the following spring when plants resume active growth. The species tested to date have differed markedly in their ability to withstand cold temperatures. Some of the species currently under evaluation are Acer pseudosieboldianum, Cornus florida, Cornus mas,Liquidambar styraciflua, Taxodium distichum, and Ulmus parvifolia. The best-performing seedlings are being propagated and we will be conducting additional tests on these plants during the next few years to determine whether our earliest selections possess sufficient cold hardiness to survive in our climate.

Water Stress

Because water availability is limited in many landscape settings, development of trees better able to withstand water deficits is another long-term objective of our breeding program. We’ve been evaluating the responses of a number of tree species to imposed water deficits to gain a better understanding of the physiological and morphological characteristics that contribute to resistance. Studies are conducted on plants grown in containers and in 2 raised sand-filled beds inside our greenhouse. One advantage to using the sand beds is that because the system does not physically restrict root development to the same extent as containers, differences among taxa in rooting depth are more easily detected.

One of the species we’ve been studying for several years is Acer truncatum, Purpleblow or Shantung maple. Purpleblow maple is an attractive, small, round-headed tree native to northern China, Japan, Korea, and Manchuria. Recent reports (2, 3) indicate that Purpleblow maple is quite drought tolerant. This species also hybridizes readily with Acer platanoides (Norway maple). We’ve been comparing the responses of Purpleblow and Norway maple seedlings to imposed water deficits in containers and the sand beds. Our results to date indicate that Purpleblow maple is more resistant to water deficits than Norway maple. Nonirrigated Purpleblow maple seedlings in the sand bench sustained higher rates of photosynthesis (Fig. 1) and higher leaf water potentials (data not shown) than nonirrigated Norway maple seedlings throughout the 24-day treatment period. Roots of Purpleblow maple seedlings tended to be less fibrous, but longer, than those of Norway maple seedlings which may have enabled the Purpleblow maples to avoid water stress by providing access to moisture deeper in the sand bed. Based on desiccation rates of excised leaves, Purpleblow maple appears to possess greater cuticular resistance to water loss than Norway maple, which may help sustain higher leaf water potentials during periods of limited moisture availability. Thus, Purpleblow maple may prove to be valuable for development of stress resistant shade trees through hybridization with Norway maple. Several attractive hybrid selections from our breeding program have exhibited excellent cold hardiness to date. We hope to further investigate their environmental adaptability in the future.

figure 1
Fig. 1. Photosynthesis of irrigated and nonirrigated seedlings of Acer platanoides and Acer truncatum seedlings growing in raised sand beds in the greenhouse.

Flood Stress

At the opposite end of the moisture availability spectrum, we are evaluating the ability of woody taxa to withstand waterlogged soils. Simple inundation trials are conducted periodically in the greenhouse and can provide useful preliminary information about the relative ability of species to tolerate flooding. In a recent trial evaluating multiple birch species, we observed that Betula costata seedlings appeared to withstand 45 days of inundation nearly as well as Betula nigra (river birch), a flood tolerant species. The environmental adaptability of Betula costata merits further study. Red and Freeman maple progeny from trees growing in a local bog are also being screened in the greenhouse. The best-performing seedlings are being grown on for additional testing.

Flood stress resistance is also being evaluated outdoors in two excavated basins at the Horticultural Research Center formerly used for wild rice research. The basins, which are approximately 0.2 acres each in size, provide a valuable opportunity to evaluate flood stress resistance under natural, outdoor conditions. Each basin is equipped with irrigation and can be can be flooded in approximately 12-14 hours. Surface water can be drained off in 1-2 days by removal of a single drain cover in each basin. An assortment of tree taxa have been lined out and flooded in the basins during the last few years as we’ve learned how to best manage the site. Current plans call for establishing seedling production beds in the basins to optimize the efficiency of growing and screening large seedling populations for flood tolerance.

High Soil pH / Alkalinity

Many tree and shrub taxa develop severe foliar chlorosis when grown in alkaline soils due to the reduced availability of iron and /or manganese at high pH. Some species, however, are well adapted to such conditions (10), and intrageneric variation in alkalinity tolerance has been reported for a number of important landscape tree genera including Acer (1), Prunus (9), and Quercus (1). To date, the range of pH adaptability of most landscape tree species remains unexplored.

Several years ago, we began examining the feasibility of evaluating alkalinity resistance of woody landscape species using a media culture system similar to that described by Shi and Byrne (9). In this system, plants are grown in a perlite:vermiculite medium and irrigated frequently with a K2CO3-amended nutrient solution adjusted to either pH 7.3 or 8.3. Control treatment plants receive the same nutrient solution adjusted to pH 6.0 and containing no K2CO3. In experiments conducted thus far, this system has been effective at inducing foliar chlorosis in multiple species and cultivars of maples, birches, and roses. We also found a good correlation between visual ratings on a 5-point scale, SPAD-502 chlorophyll meter (Minolta Corp.) readings, and actual chlorophyll content of leaf extracts, indicating that visual or SPAD-502 meter evaluations could be used to rapidly evaluate the performance of large numbers of seedlings. Differences in alkalinity resistance have also been noted among some of the taxa tested. For example, based upon the severity of chlorosis induced by the pH 7.3 and 8.3 treatments, Betula alleghaniensis (yellow birch) has repeatedly been less severely affected than either Betula nigra (river birch) or B. lenta (sweet birch). Further work is needed to assess the range of genetic variation for alkalinity resistance extant in these species.

Future research at the Arboretum will continue to focus on characterization of the adaptability of woody plants to multiple stress factors. We are hopeful that exploitation of the knowledge gained ultimately will result in development of plants better equipped to survive in the landscape.

Literature Cited

  1. Dirr, M.A.. 1998. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses. 5th Ed. Stipes Publishing Company. Champaign, Illinois.
  2. Gu, Z.Y., J.J. Hu, J.L. Wen, and S.Q. Wang. 1999. A study on adaptability of maple to drought stress. J. Northwest Forestry College. 14: 1-6.
  3. Hu, J.J., Z.Y. Gu, J.L. Wen, and S.Q. Wang. 1999. Effect of water stress on membrane lipid peroxidation in maple. J. Northwest Forestry College. 14: 7-11.
  4. McNamara, S. and H. Pellett. 1993. Cold hardiness of Forsythia cultivars. J. Environ. Hort. 11: 35-38.
  5. McNamara, S. and H. Pellett. 1994. Cold hardiness of landscape pear taxa. J. Environ. Hort. 12: 227-230.
  6. McNamara, S. and H. Pellett. 1996. Cold hardiness of flowering crabapple cultivars. J. Environ. Hort. 14: 111-114.
  7. McNamara, S. and H. Pellett. 1998. Cold hardiness of weigela cultivars. J. Environ. Hort. 26(4): 238-242.
  8. McNamara, S. and H. Pellett. 2000. Cold hardiness of Phellodendron sachalinense Friedr. Schmidt seedlings increases with age. HortScience 35:304-305.
  9. Shi, Y. and D.H. Byrne. 1995. Tolerance of Prunus rootstocks to potassium carbonate-induced chlorosis. J. Amer. Soc. Hort. Sci. 120:283-285.
  10. Ware, G. 1990. Constraints to tree growth imposed by urban soil alkalinity. J. Arbor. 16: 35-38

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Format updated July 24, 2009