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Jaejoon Kim and David J. Wolyn

Winterhardiness is an important trait for asparagus (Asparagus officinalis) cultivars grown in temperate climates. Several biochemical and physiological parameters are correlated with the acquisition of freezing tolerance during cold acclimation in the fall and could be used as indirect measures for selection in a breeding program. Genetic variation was assessed before and after fall acclimation in August and November, respectively, for freezing tolerance attributes in 18 asparagus hybrids and 24 clones, which included male, supermale, and female genotypes. Fern chlorophyll and rhizome sucrose concentrations and storage root and rhizome percentage water decreased, whereas the concentrations of storage root proline, glucose and sucrose, and rhizome proline and high-molecular-weight fructan increased during the fall. Germplasm did not differ in August but significant variation was observed in November for most parameters, indicating genotype-specific responses to fall acclimation and the acquisition of traits associated with freezing tolerance. Narrow-sense heritability estimates were significant for fern chlorophyll, storage root proline, and rhizome glucose and sucrose concentrations. With significant genetic variation and heritability, breeding to improve freezing tolerance could be possible with indirect selection measures.

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Mahmoud Panjtandoust and David J. Wolyn

Winterhardiness in asparagus (Asparagus officinalis) may be related to proper cold acclimation and induction of freezing tolerance in the fall, levels and maintenance of freezing tolerance in the winter, and the timing of deacclimation in the spring. Premature deacclimation and the inability to reacclimate could result in crown damage from spring freeze-thaw cycles. A field experiment was conducted, replicated over 2 years, to determine how three cultivars with varying adaptation to southern Ontario deacclimate in the spring by assessing LT50 (the temperature at which 50% of plants die) and biochemical and physiological parameters associated with freezing tolerance. ‘UC 157’ (UC), the least-adapted cultivar, deacclimated after soil temperatures rose above freezing; LT50 values increased linearly over time and were unaffected by fluctuations in soil temperature. ‘Jersey Giant’ (JG), a cultivar with moderate adaptation, rapidly deacclimated with increased soil temperature but appeared to partially reacclimate as temperatures decreased. For ‘Guelph Millennium’ (GM), the most-adapted cultivar, LT50 values did not change, maintaining the greatest levels of freezing tolerance during the spring sampling period. Although LT50 values did not differ among cultivars on the first spring sampling date, ranking for freezing tolerance at the final sampling in each year was GM>JG>UC, which is consistent with adaptation. Rhizome traits were most associated with freezing tolerance and included high concentrations of low-molecular-weight fructans (LFs), glucose, and proline and low percentage water and sucrose concentration. Overall, data suggest that the timing of deacclimation and loss of freezing tolerance in the spring may significantly affect winterhardiness; cultivars that lose freezing tolerance early and cannot reacclimate could suffer most from late spring freeze-thaw cycles.

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David J. Wolyn and Xiaorong Feng

Asparagus (Asparagus officinalis L.) anthers from flowers of field-grown plants were cultured for five genotypes, four incubation temperatures, and three sampling dates. Treatments were evaluated for total and embryogenic callus production. Incubating anthers at 35C was optimal for initiating embryogenic callus for three genotypes. Another line performed best and equally well at 29 and 32C, while one was recalcitrant to embryogenic callus formation at the temperatures evaluated. For all genotypes, almost half of the anthers produced callus for at least one temperature treatment, hut the percentage of these calli that was embryogenic ranged from 0% to 50%. Sampling date affected response only for specific genotype-temperature combinations. Embryo recovery ranged from six to 14 per callus. For the four responsive genotypes, 77% to 100% of plantlets was haploid. Culturing anthers at several temperatures ranging from 29 to 35C, with repeated samplings of flowers from the field, likely will allow recovery of haploid embryos from many selections. This result will expand the germplasm base to develop all-male asparagus hybrids.

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Erik J. Landry and David J. Wolyn

Adequate winterhardiness is crucial for yield stability of asparagus (Asparagus officinalis) cultivars in southern Ontario, Canada, and could be influenced by pattern of the fall fern senescence. Fern of cultivar Guelph Millennium (GM) turns yellow or senesces by mid-October, before that of cultivar Jersey Giant (JG), which often remains green until a killing frost. Early fern senescence could be a signal for cold acclimation competency and consequently winterhardiness, explaining the superior stand longevity and yield observed for GM compared with JG. A field experiment was conducted from mid-August to November to measure physiological parameters related to cold acclimation in fern, rhizome, and storage roots. During fall, fern chlorophyll concentration, rhizome nitrogen concentration, percent water of the crown, and storage root LT50 (temperature at which 50% cell death occurs) decreased. Cultivars did not differ for storage root percent water; however, values were smaller (greater dehydration) for GM than JG in the rhizome. At the end of the sampling period, GM had higher and lower concentrations of rhizome low-molecular-weight, non-structural carbohydrates and sucrose, respectively, than JG, which could support a hypothesis of greater winterhardiness in GM. Storage root LT50 values of –19 °C and the lack of cultivar differences for this trait, in conjunction with differences between GM and JG for rhizome traits thought to be important for freezing tolerance, suggest characteristics of the rhizome in conjunction with timing of fern senescence may be important in cold acclimation of asparagus.

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Asma Ziauddin, Mingsheng Peng and David J. Wolyn

Clear visualization of asparagus (Asparagus officinalis L.) microspore nuclei with common stains such as acetocarmine or DAPI is difficult, hindering cytological analyses. The addition of saturated aqueous ferric chloride solution to Carnoy's I fixative (30 μL·mL-1) resulted in clear visualization of nuclei. A distinct nucleus was observed in uninucleate cells and the vegetative and generative nuclei were clearly visible in binucleate microspores. This method can be used reliably for determination of asparagus microspore developmental stage. Chemical name used: 4′,6-diamidino-2-phenylindole-2HCL (DAPI).

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Katrina J.M. Hodgson-Kratky and David J. Wolyn

Russian dandelion (Taraxacum kok-saghyz) is a candidate species for introducing natural rubber production into North America, and its domestication is currently underway to create an economically viable crop. Development of hybrid cultivars is essential to maximize the yield potential of the species, and cytoplasmic male sterility (CMS) is an important breeding tool that can facilitate this process. Male sterility was discovered in two full-sib russian dandelion families, and additional crosses were performed to create F2, F3, and backcross generations to assess inheritance of the phenotype. A sterility-inducing cytoplasm (S) that could be restored to fertility by a dominant allele at a single nuclear restorer of fertility (Rf) gene and a fertility-inducing cytoplasm (F) were identified. Characteristics of the CMS system were 1) (S)rfrf × (F)rfrf crosses produced only sterile progeny, 2) crosses where the female parents were (F) produced only fertile progeny, and 3) segregation was observed only when the maternal parent had (S). Sterility was not stable in all plants; some with sterility-inducing cytoplasm produced small amounts of pollen or formed both sterile and fertile flowers simultaneously. On the basis of seasonal differences in the frequency of partially sterile plants, sterility was stable at low temperatures and unstable at high temperatures. This germplasm can be of central importance for development of stable CMS lines for hybrid production.

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Katrina J.M. Hodgson-Kratky and David J. Wolyn

Russian dandelion [Taraxacum kok-saghyz (TKS)] is a latex-producing, temperate species that has the potential to be grown as a source of natural rubber in North America. Flowering habit varies within the species; winter-type plants require a cold period or vernalization to flower, whereas spring-type plants flower without this treatment. Because flowering habit is correlated with rubber yield, understanding the genetic factors governing the trait would be useful for breeding. The objective of this research was to determine the inheritance of vernalization requirement in TKS. Winter-type and spring-type plants were intercrossed to create the F1, F2, and backcross generations and progeny segregation ratios were analyzed. A genetic model with three major genes is proposed, where a dominant allele at locus A, in combination with homozygous recessive alleles at either or both of two loci, B and C, confers winter type, whereas spring type is conferred by homozygous recessive alleles at A, regardless of genotype at B or C, or dominant alleles at A, B, and C.

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Katrina J.M. Hodgson-Kratky, Olivier M. Stoffyn and David J. Wolyn

Russian dandelion [Taraxacum kok-saghyz (TKS)] is a promising alternative to the Para rubber tree (Hevea brasiliensis) as a source of natural rubber; however, rubber yields must be improved for this undomesticated species to become a profitable new crop. Half-sib family recurrent selection was conducted for four cycles to increase rubber yield, estimated as the product of rubber percentage, and root dry weight per plant. Two distinct populations were developed for adaptation to sand and loam soil types. Rubber percentage increased from 4.17% for the cycle 0 (C0) population to 6.40% for the C4-loam population. Rubber yield also increased from 0.15 to 0.22 g/plant after four selection cycles. Although phenotypic variation was observed, selection had no effect on root weight; all populations averaged 3.70 g/plant. Rubber yield and percentage and root dry weight were not increased after four selection cycles on sandy soils, likely because of poor adaptation and high environmental variation. Year and soil type affected rubber yields; however, rubber percentage was more stable than root dry weight. Overall, russian dandelion can be improved for rubber yield, and further studies with increased error control should be considered to enhance root dry weight.

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Katrina J.M. Hodgson-Kratky, Olivier M. Stoffyn and David J. Wolyn

Russian dandelion [Taraxacum kok-saghyz (TKS)] is a promising candidate for introducing natural rubber production into North America. Seeds normally germinate in a humid microenvironment, such as the thatch layer of a lawn or under a canopy of grass; however, 5% to 15% establishment is often observed on bare soil, presumably due to water stress. Phenotypic selection and half-sib family recurrent selection were conducted for three cycles to improve germination in vitro, under low osmotic potential (Ψs), using a polyethylene glycol (PEG) solution. Populations were then tested for establishment on bare soil in the greenhouse and field. Germination under water stress in vitro increased from 5.8% for the cycle 0 (C0) population to 40.8% and 47.8% for the C3-phenotypic and C3-half-sib family populations, respectively. Soil establishment in the greenhouse and field was improved up to two- and 4-fold, respectively, compared with the C0, in two of four greenhouse experiments and three of eight field experiments. Overall, recurrent selection for germination under water stress in vitro has potential to improve establishment in the field and can be incorporated into current breeding programs to support the overall goal of creating cultivars with high-rubber yield.