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  • Author or Editor: Lisa Alexander x
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Hydrangea macrophylla (Thunb.) Ser., florist’s or bigleaf hydrangea, is the most economically important member of the Hydrangea genus, which accounted for over $120,000,000 in U.S. nursery sales in 2014. Both diploid and triploid H. macrophylla cultivars exist and there is some evidence that triploidy leads to larger plant and floral structures. The diploid cultivar, H. macrophylla ‘Trophee’, was previously shown to have a bimodal pollen size distribution which may be indicative of unreduced gametes. We used H. macrophylla ‘Trophee’ as a parent in a series of crosses with other diploid H. macrophylla cultivars. The objective of this study was to evaluate reciprocal full-sibling H. macrophylla families for ploidy and phenotype, determine the impact of ploidy on phenotype, and determine the efficacy of unreduced gamete breeding. Diploids and triploids were found in the offspring pool with mean 2C genome sizes of 4.5 and 6.7 pg, respectively. All offspring from crosses with ‘Trophee’ as the female parent were diploid as expected. The full-sibling family with ‘Trophee’ as the male parent contained 94% triploids, supporting the hypothesis that the bimodal pollen size distribution of ‘Trophee’ reflects the presence of unreduced male gametes. Triploids had fewer, wider inflorescences than diploids. The stems of triploids were 16% thicker and their leaves were 20% larger than those of diploid full and half-siblings. Triploids had significantly larger stomata (9.0 μm 2) than diploids (5.9 μm 2). These results establish a link between ploidy and phenotype in plants of similar genetic background and support the efficacy of unreduced gametes in polyploidy breeding.

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Production and use of sweet olive (Osmanthus armatus), fragrant tea olive (O. fragrans), holly tea olive (O. heterophyllus), and fortune’s osmanthus (O. xfortunei) as a landscape plant is currently limited to U.S. Department of Agriculture (USDA) Hardiness Zones 7 to 10, and nursery growers wish to extend the range of these species into colder climates. To provide recommendations to growers and landscapers and inform breeding efforts for cold-hardiness improvement, a replicated trial was conducted in a USDA Hardiness Zone 6b/7a transition zone. Fifteen cultivars and two unnamed accessions representing four species were evaluated for growth, stem necrosis, and flowering in a pot-in-pot production system from 2015 to 2017. One-half of the plants in each cultivar were moved to winter protection each November and returned to the field each May. There were significant differences in growth and cold-hardiness among cultivars. Percent increase in the growth index after three growing seasons for winter-exposed accessions of sweet olive, fortune’s osmanthus, fragrant tea olive, and holly tea olive averaged 867%, 1175%, 155%, and 6361%, respectively. Percent stem necrosis in May 2017 for sweet olive, fortune’s osmanthus, fragrant tea olive, and holly tea olive averaged 1.1%, 2.7%, 44.8%, and 20.2%, respectively. The most cold-tolerant accessions based on stem necrosis and growth index of winter-exposed plants were ‘Kaori Hime’, ‘Hariyama’, ‘Shien’, ‘Head-Lee Fastigate’, and ‘Rotundifulius’ holly tea olive, ‘San Jose’ fortune’s osmanthus, and ‘Longwood’ sweet olive. Of these cultivars, Kaori Hime, San Jose, and Longwood flowered under winter-exposed conditions. All fragrant tea olive cultivars were damaged by winter exposure. ‘Fodingzhu’ was the only fragrant tea olive cultivar that flowered each year under winter-exposed conditions. Evaluation and breeding efforts are continuing to extend the range for production and growth of this genus.

Open Access

Hydrangea macrophylla (bigleaf hydrangea) is one of the most important floral and nursery crops worldwide. However, breeding of new bigleaf hydrangea cultivars has been hampered by a long breeding cycle and lack of genetic resources. This study investigated the genetic diversity and population structure of 82 bigleaf hydrangea cultivars using single-nucleotide polymorphisms (SNPs) originated from genotyping-by-sequencing. A total of 5803 high-quality SNPs were discovered in a bigleaf hydrangea cultivar panel. A phylogenetic analysis and analysis of molecular variance based on discovered SNPs concluded the taxonomic classification of H. macrophylla ssp. serrata as a subspecies of H. macrophylla. Principal component analysis confirmed ‘Preziosa’ as a hybrid between H. macrophylla ssp. macrophylla and H. macrophylla ssp. serrata. In addition, the cultivar Lady in Red was also found to be a hybrid between the two subspecies. The population structure analysis identified three groups among the 82 cultivars. All H. macrophylla ssp. serrata cultivars belonged to one group, and two groups were revealed within H. macrophylla ssp. macrophylla. The separation within H. macrophylla ssp. macrophylla indicated a second gene pool due to breeding efforts that have targeted similar breeding goals for bigleaf hydrangea. The discovered SNPs and the phylogenetic results will facilitate further exploitation and understanding of phylogenetic relationships of bigleaf hydrangea and will serve as a reference for hydrangea breeding improvements.

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Hydrangeas are popular landscape plants that are widely grown in many parts of the world. The objective of this study was to evaluate the salinity tolerance of three novel Dichroa ×hydrangea hybrids [Dichroa febrifuga ‘Yamaguchi Hardy’ × Hydrangea macrophylla ‘Hamburg’ (YH × Hamburg), Dichroa febrifuga ‘Yellow Wings’ ×Hydrangea macrophylla ‘Nigra’ (YW × Nigra), and Dichroa febrifuga ‘Yellow Wings’ ×Hydrangea macrophylla ‘Oakhill’ (YW × Oakhill)]. A 52-day greenhouse study was conducted by irrigating container-grown plants with nutrient solution at an electrical conductivity (EC) of 1.1 dS·m−1 (control) or saline solution at an EC of 5.0 dS·m−1 (EC 5) or 10.0 dS·m−1 (EC 10). At harvest, YH × Hamburg and YW × Nigra in EC 5 and EC 10 still exhibited good quality with average visual scores greater than 4.1 (0 = dead; 5 = excellent). For YW × Oakhill, moderate foliar salt damage was observed with an average visual score of 2.9 in EC 5 and 2.2 in EC 10. Compared with control, the shoot dry weight of YH × Hamburg, YW × Nigra, and YW × Oakhill in EC 5 reduced by 35%, 35%, and 55%, respectively, whereas that in EC 10 decreased by 58%, 58%, and 67%, respectively. Elevated salinity also decreased plant height, leaf area, and leaf greenness [Soil Plant Analysis Development (SPAD) readings]; chlorophyll fluorescence (Fv/Fm); performance index (PI); and net photosynthetic rate (Pn). All these responses might result from excess accumulation of sodium (Na+) and chloride (Cl) ions in hydrangea leaves. In this study, compared with control, leaf Na+ concentration of YH × Hamburg, YW × Nigra, and YW × Oakhill increased 11, 36, and 14 times, respectively, in EC 5, and 31, 53, and 18 times, respectively, in EC 10. Compared with control, leaf Cl concentration increased 4, 9, and 7 times in EC 5, and 10, 11, and 8 times in EC 10 for YH × Hamburg, YW × Nigra, and YW × Oakhill, respectively. Leaf nitrogen (N), phosphorous (P), potassium (K+), and iron (Fe3+) concentrations decreased at elevated salinity levels but did not cause any nutrient deficiency. In summary, the three Dichroa ×hydrangea hybrids exhibited different salinity tolerance: YH × Hamburg and YW × Nigra were more tolerant than YW × Oakhill. Salt-tolerant hydrangea hybrids should be chosen for landscape use if soil and/or irrigation water are salty.

Open Access

Botrytis cinerea is one of the problematic and notorious postharvest pathogens of bigleaf hydrangea (Hydrangea macrophylla) cut flowers. It causes flower blight, leaf blight, and stem rot, reducing the ornamental value (such as longevity, color, and texture) of flowers, ultimately making them unsalable. The objective of this study was to identify effective conventional fungicides and biorational products for botrytis blight management on bigleaf hydrangea cut flowers that can be easily and readily adopted by growers of ornamentals. Preventive preharvest whole-plant spray and postharvest dip treatment applications were used in this study. For the whole-plant spray applications, bigleaf hydrangea plants were sprayed with treatment solution 3 days before harvesting flowers. For the dip applications, cut flowers were dipped in treatment solutions after harvest. For both application types, flowers were inoculated with B. cinerea spores once treatment solutions dried. Flowers were stored in cold storage for 3 days and then displayed in conditions similar to retail stores. Botrytis blight disease severity, marketability of flower (postharvest vase life), phytotoxicity, and application residue were assessed in the study. Treatments showed variable efficacy in managing postharvest B. cinerea infection in bigleaf hydrangea cut flowers. Preventive preharvest whole-plant spray and postharvest dip applications of isofetamid and fluxapyroxad + pyraclostrobin significantly reduced the postharvest botrytis blight disease severity and area under disease progress curve (AUDPC) compared with the positive control (nontreated, inoculated with B. cinerea). When applied as a postharvest dip, the fungicide fludioxonil and biofungicide Aureobasidium pullulans strains DSM 14940 and DSM 14941 effectively lowered the disease severity and disease progress (AUDPC). These effective treatments also maintained a significantly longer postharvest vase life of bigleaf hydrangea cut flowers compared with the nontreated, inoculated control. The longer vase life may be attributed to lowered botrytis blight disease severity and the resultant proper physiological functioning of flowers.

Open Access

Witchhazel (Hamamelis sp.) cultivars are now available in an array of forms and flower colors, including several native, pollinator-friendly cultivars. However, little is known about response of witchhazel cultivars to powdery mildew (Podosphaera biuncinata) or the growth and flowering characteristics of witchhazel cultivars in a nursery field production setting. To provide growth, flowering, and disease incidence data to nursery growers, a cultivar trial including 23 cultivars of witchhazel representing five species was planted Apr. 2016 in McMinnville, TN. Plant growth, flowering density, length of bloom, and foliar disease incidence were evaluated over three growing seasons between May 2016 and Oct. 2018. ‘Zuccariniana’ japanese witchhazel (H. japonica) and ‘Sunglow’ common witchhazel (H. virginiana) showed the greatest height increase during the trial, and ‘Sunglow’ also added the most width during the trial. Cultivars with negative height or width growth included Sweet Sunshine chinese witchhazel (H. mollis) and hybrid witchhazels (H. ×intermedia) Aphrodite, Twilight, and Barmstedt Gold. Ten of the 23 cultivars experienced winter injury in the form of stem necrosis. Root crown sprouts were observed for all cultivars at least once during the trial. ‘Wisely Supreme’ chinese witchhazel had the longest bloom period, followed by ‘Westerstede’ and ‘Twilight’ hybrid witchhazels, whereas ‘Quasimodo’ vernal witchhazel (H. vernalis) had the greatest density of flowers. The hybrid witchhazel cultivars Aphrodite, Nina, and Arnold Promise and the common witchhazel cultivars Green Thumb and Sunglow were resistant to powdery mildew under trial conditions in all 3 years. ‘Twilight’ and ‘Barmstedt Gold’ hybrid witchhazel, ‘Little Suzie’ common witchhazel, ‘Wisley Supreme’ chinese witchhazel, and ‘Shibamichi Red’ japanese witchhazel were moderately resistant to powdery mildew.

Open Access

Oakleaf hydrangea (Hydrangea quercifolia Bartr.) is an understory shrub native to the southeastern United States. Hydrangeas are popular ornamental landscape plants; however, little is known about the diversity in horticulturally important traits for oakleaf hydrangea. Information regarding the variation in important traits could guide future breeding efforts for the species. Seed was collected from 55 populations throughout the range of the species for the purpose of conducting a horticultural characterization of the species compared with select cultivars. Plant architecture was characterized as plant height, number of nodes, internode length, number of branches, and plant width. Plant architecture was measured for container-grown and field-grown plants in two locations (Minnesota and Tennessee). Tolerance to leaf spot (Xanthomonas campestris L.) was characterized for wild-collected seedlings and cultivars by measuring disease severity under exposure to ambient inoculum. Cold hardiness was characterized during two winters with a controlled freezing experiment. During the first winter, seedlings were tested in January; during the second winter, seedlings and cultivars were tested monthly from October through April. Plant architecture varied by environment, with plants growing larger in Tennessee than in Minnesota. The heights of container-grown and field-grown plants were correlated with the collection site latitude (r = −0.66), with populations from the northeastern extent of the range of the species being the most compact, and populations from Florida being the tallest. Leaf spot severity varied significantly among populations and cultivars and was also correlated with latitude for the seedlings (r = 0.70). Two populations in Florida were identified as sources of high tolerance to leaf spot, whereas ‘Flemygea’ and ‘Alice’ were identified as having moderate tolerance to leaf spot. Cold hardiness varied among populations and cultivars and among months of the winter. The overall maximum cold hardiness was observed in February [mean lethal temperature (LT50) = −33.7 °C], and several populations maintained an extreme level of cold hardiness into late winter. Midwinter cold hardiness also varied by latitude (r = −0.65), with northern populations showing higher levels of cold hardiness. These results indicate that certain wild oakleaf hydrangea populations will be useful for introgressing novel variation into breeding programs.

Open Access

Oakleaf hydrangea (Hydrangea quercifolia) is an understory shrub native to the southeastern United States. The species occupies a relatively small native range, and little is known about its demography, genetic diversity, or needs for conservation. Samples were collected from 188 plants in 73 locations throughout the species range and were genotyped using genotyping by sequencing. A structure analysis identified six genetic clusters that are geographically defined. Although these clusters are weakly differentiated, each has unique alleles. An environmental association analysis revealed that environmental variables explain 11.3% of genetic diversity, and population structure explains 13.5%. Further, 231 putative adaptive alleles were identified, most of which are correlated with precipitation-related variables, indicating that precipitation has an impact on genetic diversity in H. quercifolia. Many historically documented populations were found to be either extirpated or at risk of extirpation. The genetic clusters on the southern extent of the species range are relatively small and contain putative adaptive alleles at relatively high frequencies. These results highlight the importance of preserving representative germplasm from throughout the species range.

Open Access