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Andrew Riseman and Richard Craig

Research in Penn State's Exacum breeding program has revealed genotypic variation for the development of zinc deficiency, which may indicate the presence of zinc efficiency factors. Through preliminary experiments, we have identified both genetic families and individual genotypes that can be classified as either zinc-efficient or zinc-inefficient. Chi-square contingency analyses indicate significant differences (P < 0.001) in segregation patterns for zinc deficiency among hybrid families. Segregation patterns within families ranged from 100% of the progeny developing zinc deficiency to 100% of the progeny remaining healthy. Two genotypes contrasting in zinc efficiency have been identified and used in experiments designed to investigate physiological factors related to zinc efficiency. The zinc-efficient genotype has a significantly higher ability to decrease solution pH (P < 0.01), significantly higher root cation exchange capacity (P < 0.007), significantly lower root/shoot ratio (P < 0.001), significantly lower water loss/cm2 leaf (P < 0.03), and significantly higher fresh weight/dry weight ratio (P < 0.001). Research on zinc uptake rates is currently being conducted utilizing the efficient and inefficient genotypes. Based on all of our research, we conclude that 1) a strong genetic effect is involved in the zinc nutritional status of interspecific Exacum hybrids and 2) a number of physiological traits differ between zinc-efficient and zinc-inefficient genotypes.

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Andrew Riseman and Richard Craig

The morphology of interspecific Exacum hybrids was investigated in order to determine transmission patterns of several horticultural traits and to confirm interspecific hybrid status of plants. The evaluations utilized 557 accessions from interspecific hybridization of the following species: Exacum. macranthum, E. pallidum, E. pedunculatum, E. trinervium ssp. ritigalensis, and E. trinervium ssp. trinervium. Transmission patterns are proposed for stem shape (three phenotypes- clylindrical, winged-cylindrical and qudrangular; no dominance), petal shape (two phenotypes-rounded and acuminate; rounded dominant to acuminate) and flower form (two phenotypes- imbricate and separate; imbricate dominant to separate) Examination of the progenies confirm interspecific hybridization by the appearance of combinations of traits from the parental species and/or the observation of unique forms not previously observed.

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Andrew Riseman and Siva Chennareddy

Protocols for in vitro propagation are reported for interspecific hybrids of Exacum L., derived from Sri Lankan taxa. Four genotypes were used to evaluate the effects of MS (Murashige and Skoog) and WP (Woody Plant) media supplemented with 2-iP, BA, or KIN during establishment and multiplication phases. In addition, rhizogenesis and associated root characteristics were evaluated using MS medium supplemented with NAA or IBA. Overall, either 2-iP or BA was significantly more effective than kinetin in establishment and shoot proliferation with significant genotype × treatment interactions present. Maximum multiplication rates were achieved in the following genotype-hormone combinations: E-6, 2 mg·L-1 BA (4.5 per explant); E-23, E-32, and E-37, 2 mg·L-1 2-iP (3.5, 2.5, and 3.6 per explant, respectively). In vitro rhizogenesis was greatest in liquid MS medium supplemented with 1 mg·L-1 NAA while significantly reduced rooting was observed with IBA supplements. Our results demonstrate that micropropagation of Sri Lankan Exacum hybrids is possible and that adequate multiplication and rooting percentages can be achieved. However, the high level of genetic variation identified requires genotype-specific media modifications. Chemical names used: benzyladenine (BA); 2-isopentenyladenine (2-iP); indole-3-butyric acid (IBA); α-naphthaleneacetic acid (NAA); kinetin (KIN).

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David Noshad, Andrew Riseman and Zamir Punja

Many daphne cultivars are susceptible to fungal root pathogens and require frequent fungicide applications during production. To identify taxon differences to disease susceptibility, we evaluated 32 Daphne species and cultivars for resistance to the soil-borne pathogen, Thielaviopsis basicola (Berk. and Broome) Ferr., by both in vitro- and in vivo-based methods. Disease-free plant roots were inoculated with the pathogen through topical application of a spore suspension and observed weekly for disease development/progression. Significant variation for disease severity among the taxa evaluated was determined using a plant disease index. Plant reactions ranged from highly resistant, e.g., D. tangutica and D. retusa, to highly susceptible, e.g., D. cneorum. In addition, a high correlation was found between the in vitro and in vivo techniques for the seven selected species, indicating that they are comparable. However, the in vitro assay provided results in significantly less time than the in vivo assay.

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Andrew Riseman, Richard Craig and Jonathan P. Lynch

Interspecific hybrids of exacum (Exacum L.) display variable responses to zinc nutrition. Our research compared two genotypes with contrasting zinc efficiency phenotypes in terms of root cation exchange capacity (CEC), whole plant 65Zn uptake, and the effects of Cu+2 and Mg+2 on 65Zn uptake and partitioning to shoot tissues. Results show that the zinc efficient and inefficient genotypes had significantly different root CEC [27.2 and 16.9 cmol(+)·kg-1 root dry weight (DW), respectively] and whole plant 65Zn uptake rates (0.048 and 0.026 μmol·h-1·g-1 DW, respectively). In equimolar concentrations to Zn+2, Cu+2 reduced Zn+2 uptake by approximately 50% in both genotypes while supplemental Mg+2 enhanced Zn+2 uptake. In addition, Mg+2 facilitated a larger proportion of absorbed 65Zn to the upper shoot of the efficient genotype. We conclude zinc is absorbed through a specific Zn+2/Cu+2 transporter and that zinc efficiency in exacum is based on a combination of apoplastic and symplastic traits. In addition, a secondary Mg+2 × Zn+2 interaction may contribute to the zinc efficiency phenotype.

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Andrew Riseman, V.A. Sumanasinghe, Douglas Justice and Richard Craig

We propose the name Exacum Styer Group for an interspecific population derived from several Sri Lankan Exacum taxa. Confirmation of hybrid status was determined by the appearance of either unique trait combinations or intermediate forms of traits originally represented by individual native taxa. Through 12 sexual generations, the proposed cultivar-group continues to exhibit these unique traits and now forms a cohesive fertile population.