Breeding, selection and evaluation of woody landscape plants has been an active project at the Univ. of Minnesota for many years. The goal of the project is to develop and or identify superior plants that are well adapted to the climatic conditions of Minnesota and other northern areas. About 20 cultivars of many species have been introduced to the nursery trade through this program in the past 20 years. These introductions result from selections made from populations arising from controlled crosses and from open-pollinated populations and native plant populations. One of the primary efforts has been development of the cold hardy, “lights series” of deciduous azaleas. These possess flower bud hardiness of from –35 to –40 °C. Other current breeding activities include efforts with Viburnum, Acer rubrum, Rosa, and intergeneric hybridization in the Pomoidaea subfamily of Rosaceae. An integral part of the project is development and use of techniques to screen for tolerances to various environmental stresses. Approaches used will be discussed and plants currently under evaluation will be described and illustrated with slides.
Steve McNamara and Harold Pellett
Seedlings of several landscape tree species frequently experience cold injury at temperatures that are noninjurious to older specimens of the same species. However, there are few published reports quantifying age-related differences in hardiness. In this study, the stem cold hardiness of a mature, 35-year-old female Sakhalin corktree was compared with that of half-sib seedling progeny of different ages. Ten-, 22-, and 34-month-old seedlings were hardy to -4 °C on 9 Oct., while the 35-year-old parent withstood -12 °C. Ten-month-old seedlings exhibited no further increase in hardiness on 26 Oct., whereas the 34-month-old seedlings and the mature parent were hardy to -16 °C. The 22-month-old seedlings were intermediate in hardiness on this date. The 10- and 22-month-old seedlings had died back to the snowline by late January, but the 34-month-old seedlings and the mature tree were uninjured. The corktree seedlings did not attain midwinter hardiness levels comparable to the adult tree until the winter following their fourth season of growth. The absence of flower buds on cold-tolerant 4- and 5-year-old seedlings suggests that physiological maturation is not a prerequisite for full expression of the cold acclimation capability of this species.
Peter H. Velguth and Harold Pellett
We evaluated flow cytometric measurement of nuclear DNA content to determine ploidy level in azalea. If ploidy level correlates with DNA content, ploidy level could be determined more readily than by direct chromosome counts and assist in planning crosses and evaluating progeny. Tested plants included azalea cultivars, materials from the azalea breeding project at the Univ. of Minnesota, and species from the Rhododendron Species Botanic Garden and the North Carolina Arboretum. Data compiled from DNA assays of practically all material analyzed fell into distinct groups consistent with their being either diploid, triploid, or tetraploid. Additionally, a known diploid plant of each of four diploid species, together with a natural or derived tetraploid plant of each of these species was obtained. Results showed that the four diploids had a similar DNA content compared to one another. DNA content of the tetraploids was also similar, and the tetraploid's DNA content was approximately twice that of the diploids, as expected. Unfortunately, success with direct chromosome counts in other material has proven elusive, currently precluding direct correlation of DNA amount with ploidy level across other species and cultivars. Although many cases exist in the literature where DNA content has a direct relationship to ploidy level, this does not always hold. Although the majority of plants tested fell into a diploid, triploid, or tetraploid grouping based on DNA content, further study is required to determine the exact relationship between ploidy level and DNA content in azalea.
Julie Padrutt, Harold Pellett, and Peter Ascher
Complete diallel matings were performed during two consecutive seasons in a full sibling population of Rhododendron prinophyllum, the pinkshell azalea. Examination of pollen tubes from collected and fixed styles revealed no differences in growth, rate of pollen tubes between selfs and outcrosses. Penetration of pollen tubes through the ovular micropyle region occurred 4 to 7 days after pollination, regardless of pollen source. Embryogenesis was studied in pistils collected from forced greenhouse plants of the same population. All ovules appeared to develop for a short period before senescing. Percent capsule set data from both years' diallel pollinations indicated that some active form of self-recognition and rejection was operating and that environmental stresses and resource allocation were also influential. Additional information gathered included ovule counts, seed count to capsule size correlations, and germination trials. These pointed to a reduction in reproductive success at each developmental stage. Self-incompatibility (SI), defined as inability to set seed following self-pollination, is clearly not applicable here. There are inherent difficulties in separating an active, late-acting self-recognition/rejection system from inbreeding depression, which is a passive accumulation of homozygous recessive lethal and sublethal genes.