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- Author or Editor: Ryan N. Contreras x
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Platycladus orientalis (L.) Franco (syn. Thuja orientalis L.), Thuja occidentalis L., and T. plicata D. Don. are conifers often used in the landscape. Most of the available cultivars of these species share the character of having foliage that turns an off-color during winter as a result of photoinhibition. Tetraploids of the related japanese-cedar [Cryptomeria japonica (L. f.) D. Don.] have exhibited greener color retention than diploids during winter and a recent report described a simple technique to double its chromosomes. The technique used to double the chromosome number of C. japonica was applied to the three species mentioned to determine if it would be effective for inducing polyploidy and, if so, optimal duration of treatment. Seedlings were treated at the cotyledon stage for 0 (control), 10, 20, or 30 days with an aqueous solution containing 150 μM oryzalin + 0.1% Tween® 20 using a standard household spray bottle that created a fine mist. No tetraploids were observed for any species in control treatments, indicating all recovered tetraploids resulted from applying oryzalin. Tetraploids were observed for all other treatments except T. plicata at 30 days. Efficacy ranged from 0% to 27.1% of transplanted seedlings being tetraploid. There was a quadratic relationship between duration of treatment and percent tetraploids in T. occidentalis and T. plicata and a linear relationship for P. orientalis. Based on regression analysis, the optimal duration of treatment was 20.5 days for T. occidentalis and 13.9 days for T. plicata. The highest percent tetraploids recovered for P. orientalis was at 30 days and it is unclear if increasing duration beyond this would continue increasing percent tetraploids recovered. Morphology was not useful in early identification of tetraploids for any species.
Wide hybridization can potentially lead to the combination of diverse traits, but these hybrids are often sterile as is the case with the inter-subgeneric hybrid Rhododendron `Fragrant Affinity'. Induction of polyploidy can restore chromosome homology and fertility in wide hybrids. In this study we successfully developed an allopolyploid form of R. `Fragrant Affinity' using oryzalin as a mitotic inhibitor and chromosome doubling agent. Approximate genome size (2C), determined using flow cytometry, was 1.6 pg for the diploid and 3.2 pg for the allotetraploid. Pollen viability, determined by staining and germination tests, was 4% and 0%, respectively for the diploid and 68% and 45%, respectively for the allotetraploid. No seeds were produced when the diploid R. `Fragrant Affinity' was crossed with pollen from viable diploid and tetraploid parents. The allotetraploid produced viable seeds and seedlings when crossed with viable pollen from either diploid or tetraploid parents, including self pollination, demonstrating restored fertility. Additional crosses were successfully completed using the allotetraploid as part of an ongoing breeding program to develop new fragrant, cold hardy, evergreen rhododendron.
Prunus lusitanica (2n = 8x) and Prunus laurocerasus (2n = 22x) are evergreen woody shrubs commonly used in landscapes across the United States and Europe. To reduce the difference in ploidy between these species and with the expectation of successful hybridization, an experiment was performed to double the chromosome number of P. lusitanica. Colchicine was applied at 0%, 0.2%, 0.4%, and 0.8% (w/v), and 125 µM oryzalin as a viscous liquid to the apical meristem of open-pollinated P. lusitanica seedlings. Solutions were semisolidified using 0.55% agar (w/v). Cellular penetration was increased by adding 1% dimethyl sulfoxide (v/v) in all groups except oryzalin. As a result, three chromosome doubled (2n = 16x) plants, one 2n = 12x plant, and 14 cytochimeras (2n = 8x + 16x) were recovered. Application of 125 µM oryzalin had a meristem-survival rate of 17%, statistically lower than all other treatments. The oryzalin treatment also produced the highest number of altered ploidy seedlings. Oryzalin at 125 µM was the most effective chromosome doubling agent in this experiment. Phenotypic examination indicated that chromosome doubled (2n = 16x) plants displayed shorter stems, thicker leaves, and fewer but larger guard cells than the untreated controls.
Sweetbox (Sarcococca confusa) is an evergreen shrub valued for attractive foliage, winter fragrance, black fruit, as well as shade and drought tolerance. The high degree of apomixis in Sarcococca limits the effectiveness of conventional breeding practices. However, mutation breeding may be a valuable tool to induce variation in seedling crops. As such, a study was conducted to expose seeds of S. confusa to ethyl methanesulfonate (EMS) at varying concentrations and exposure durations, and to evaluate the effects on seed germination, growth, and relative frequency of polyembryony. In 2010, seeds of S. confusa were treated with 0%, 0.2%, 0.4%, 0.8%, and 1.2% EMS for 24 and 48 hours. Seeds were sown and the relative germination and occurrence of polyembryony were recorded. Seedlings were later evaluated for size and phenotypic variation in the subsequent growing seasons. Percent germination was found to decrease with increasing EMS rates in the 24- and 48-hour treatments. The occurrence of polyembryony also decreased with increasing EMS rates in the 24-hour treatment but was observed to be greatest at 0.2% EMS. No significant differences in plant size index (SI) were found after four growing seasons. When mature, seeds were collected from any plants that bore fruit in each treatment and were then sown. The percentage of plants that bore fruit in each treatment decreased with increasing EMS rates, although the germination rate of these seeds did not differ. A chlorophyll mutant and several dwarf forms were identified in this population as well. They have been propagated asexually for future evaluation as new cultivars. This study demonstrates the utility of chemical mutagenesis to induce phenotypic variation in S. confusa while reducing the rate of polyembryony.
Cape hyacinth (Galtonia candicans) is a geophytic herbaceous perennial from South Africa. It produces large inflorescences of pendulous white flowers during mid to late summer, followed by capsules filled with copious amounts of seed. The species has potential as a low-water-use landscape plant, but lodging and excessive seed production, which pose a risk of escape or invasion, are issues that should be addressed before marketing. Ethyl methanesulfonate (EMS) is a chemical mutagen known to induce usable mutations including dwarfing and sterility. We exposed seeds of cape hyacinth to increasing concentrations of EMS (0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1%). Increased concentrations of EMS resulted in a linear decrease in seed germination when not exposed to a presoak treatment in water before exposure to EMS. No seedlings survived or were viable to field plant at 0.6%, 0.8%, or 1%. Resulting plants were field planted in 2013 and evaluated during 2014 and 2015. In both years, the inflorescence height at first flower, average seed number per capsule, and percent lodging were reduced in EMS-treated plants compared with controls. In 2015, pollen staining was evaluated and was reduced from 83% in control to less than 3% in the 0.4% treatment. Our study demonstrated that EMS is a viable option to reduce height and decrease seed set in cape hyacinth.
Genome size estimates and chromosome number information can be useful for studying the evolution or taxonomy of a group and also can be useful for plant breeders in predicting cross-compatibility. Callicarpa L. is a group of ≈140 species with nearly worldwide distribution. There are no estimates of genome size in the literature and the information on chromosome numbers is limited. Genome size estimates based on flow cytometry are reported here for 16 accessions of Callicarpa comprising 14 species in addition to chromosome counts on six species. Chromosome counts were conducted by staining meristematic cells of roots tips using modified carbol fuchsin. Holoploid genome size estimates ranged from 1.34 pg to 3.48 pg with a mean of 1.74 pg. Two tetraploids (2n = 4x = 68; C. salicifolia P'ei & W. Z. Fang and C. macrophylla Vahl GEN09-0081) were identified based on holoploid genome size and confirmed by chromosome counts. There was little variation among species for monoploid genome size. 1Cx-values ranged from 0.67 pg to 0.88 pg with a mean of 0.77 pg. Chromosome counts for six species revealed a base chromosome number of x = 17. Callicarpa chejuensis Y. H. Chung & H. Kim, C. japonica Thunb. ‘Leucocarpa’, C. longissima Merr., and C. rubella Lindl. were confirmed as diploids (2n = 2x = 34). Cytology supported flow cytometry data that C. salicifolia and C. macrophylla GEN09-0081 were tetraploids. The two accessions of C. macrophylla included in the study were found to be of different ploidy levels. The presence of two ploidy levels among and within species indicates that polyploidization events have occurred in the genus.