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Jen Colcol Marzu, Elizabeth Straley, and Michael J. Havey

introgression of the PR resistance on chromosome 4 into diverse onion populations. Table 3. Sequences of single nucleotide polymorphisms (SNP) on chromosome 4 of onion mapping within the 1.5 logarithm of odds (LOD) confidence interval for pink root-resistance in

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Smita Barkataky, Robert C. Ebel, Kelly T. Morgan, and Keri Dansereau

for 72 h and weighed to determine dry weight (DW). RWC was calculated as RWC = [(FW – DW)/(TW – DW)] × 100. Estimation of root resistance. Root resistance (R root ) was estimated for both experiments using the same approach. Water movement in the soil

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Lesley A. Judd, Brian E. Jackson, William C. Fonteno, and Jean-Christophe Domec

resistances to hydraulic movement in vascular plants occur in the roots ( Tataranni et al., 2012 ). Root resistance to water flow determines the water status of the shoot, even under well-watered conditions ( Markhart and Smit, 1990 ), and imposes the greatest

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Mark Rieger and Antonio Motisi

Estimates of root hydraulic conductivity (Lp) were obtained on intact peach (Prunus persica × P. davidiana `Nemaguard') and sour orange (Citrus aurantium L.) rootstock over a broad range of transpiration rates. Within a species, Lp was lower when estimated using the Ohm's law analog than the reciprocal of the slope of the linear regression between transpiration (E) and stem xylem water potential (Ψ). Nonzero y-intercepts in linear regressions of Ψ vs. E resulted in the lack of agreement between Lp estimates. Removal of the root system caused xylem Ψ to rapidly approach zero in both species when E ≈ 0, suggesting that factors responsible for nonzero y intercepts resided within roots. Lp was 2.2 and 3.5 times lower for sour orange than peach when calculated by the Ohm's law and regression methods, respectively.

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Mark Rieger

Growth, gas exchange, root hydraulic conductivity, and drought response of seedling and rooted cuttings of Lovell and Nemaguard peach [Prunus persica (L.) Batsch], and Carrizo (Poncirus trifoliata × Citrus sinensis) and sour orange (C. aurantium L.) citrus rootstocks were compared to determine the influence of propagation method on these characteristics. Rooted peach cuttings had a higher proportion of root biomass in fibrous roots (≤ mm in diameter) and lower root: shoot ratios than seedlings, although this did not occur in citrus. Net CO2 assimilation (A) was higher for peach seedlings than for cuttings, but similar for `Redhaven' (RH) scions on either seedling- or cutting-propagated rootstocks, suggesting that leaf-associated factors were responsible for differences. As in peach, A was higher for Carrizo seedlings than for cuttings, but A was not affected by propagation method in sour orange. Peach seedlings maintained higher A than cuttings as water potentials declined during short-term soil drying, although in citrus this occurred only for Carrizo. RH scions on either root type exhibited similar declines in A as soil dried, indicating the lack of a rootstock effect. Root hydraulic conductivity (Lp) was similar between seedlings and cuttings of all cultivars when expressed on a length basis. Leaf conductance and osmotic adjustment were similar for RH scions on seedling- or cutting-propogated rootstocks during 45 days of drought stress, indicating the lack of a rootstock effect on long-term stress response.

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Ockyung H. Bark, Michael J. Havey, and Joe N. Corgan

The edible Alliums are economically important world-wide. The bulb onion (Allium cepa) is the most widely grown. The Japanese bunching onion (A. fistulosum) has many desirable characters, e.g., resistance to pink root, Thrips, smut, maggot, and Botrytis. Transfer of pink root resistance from A. fistulosum into A. cepa has been attempted for over 60 years. However, sterility of the F1 hybrid is a barrier and there is little evidence of gene introgression during backcrossing to A. cepa. Dr. Corgan has made crosses between A. fistulosum as the seed parent and A. cepa. He backcrossed the F1 hybrids to A. cepa and generated BC2 progenies which showed excellent pink root resistance. RFLPs in the chloroplast genome showed all BC2 progenies had either the normal or sterile cytoplasm of A. cepa. This may be due to not strictly maternal inheritance of the chloroplast DNA or a seed mixture during backcrossing. Other interspecific hybrids and their BC1 progenies had the cytoplasm of A. fistulosum. Nuclear RFLPs show hybrid patterns in the F1 plants. BC1 progenies possess some A. fistulosum markers as evidence of DNA introgression from A. fistulosum into the backcross progenies.

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D.M. Glenn and R. Scorza

In reciprocal grafts of tall (`Elberta' and `Loring') and dwarf (`Empress' and `Juseito') peach (Prunus persica Batsch.) phenotypes, we measured dry-matter partitioning, resistance to root system water flow, and phytohormone content of xylem exudate. Scion characteristics determined the phenotype and growth characteristics of the tree irrespective of the rootstock. Tall phenotypes had higher dry weight and lower root resistance to water flow than dwarf phenotypes. Cytokinin-like activity and auxin levels in xylem sap were higher in dwarf than in tall phenotypes; whereas gibberellin-like activity was unaffected by either rootstock or scion. The scion of peach influenced phytohormone levels and resistance to water flow in the root system in addition to root and shoot growth.

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Andrew C. Ludwig, John F. Hubstenberger, Gregory C. Phillips, and G. Morris Southward

Callus cultures were established from intraspecific lines of Allium cepa L., interspecific F1 progeny of A. cepa crossed to A. fistulosum L. and to A. galanthum L., advanced generations of A. fistulosum x A. cepa backcrossed to A. cepa, and lines of A. fistulosum and A. galanthum. These genotypes had been identified as susceptible, resistant, or partially resistant tester lines based on prior seedling and field nursery screenings using the pink-root pathogen Pyrenochaeta terrestris (Hansen) Gorenz, Walker and Larson. Tester line calli were challenged in vitro with culture filtrates of the fungal pathogen and were assessed by visible damage ratings expressed as the percentage of pigmentation in response to the filtrate. The degrees of callus sensitivity to the filtrate observed in vitro corresponded well with the in vivo tester line classifications. These results eliminated the possible confounding influence of using various species of Allium for in vitro screening. Our results indicated the suitability of the in vitro screening approach for the possible identification of useful segregants or somaclonal variants possessing pink-root resistance. However, in vivo pathogenicity may involve mechanisms in addition to sensitivity to the putative toxins present in the filtrate.

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Christopher S. Cramer, Jose L. Mendoza, and Joe N. Corgan

Current emphasis of onion breeding programs has been to develop male-sterile, maintainer, and pollinator inbred lines for the production of hybrid cultivars. Five short-day, male-sterile inbred lines from the New Mexico State Univ. Onion Breeding Program were crossed to four short-day, pollinator inbred lines in all possible combinations. In addition, six intermediate-day male-sterile inbred lines were crossed with seven intermediate-day, pollinator inbred lines in all possible combinations. The resulting hybrid lines from all crosses were evaluated for maturity, bolting resistance, pink root resistance, Fusarium basal rot resistance, percentage of marketable bulbs, marketable yield, average bulb weight, and percentage of bulbs with single centers. The average performance among male-sterile and among pollinator lines within each group was determined by averaging over hybrid lines that pertained to the respective male-sterile or pollinator line. Among the short-day inbred lines, NMSU 97-28-2 and NMSU 97-109-2 as female parents produced the best hybrid lines, while NMSU 96-17-1 and NMSU 96-51-1 as male parents produced the best hybrid lines. The best hybrid combinations among the short-day parents were NMSU 97-28-2 × 96-17-1 and 97-46-2 × 96-51-1. Among the intermediate-day inbred lines, NMSU 96-196-2 and 96-300-2 as female parents produced the best hybrid lines, while NMSU 96-280-1, NMSU 96-274-1 and 96-395-1 as male parents produced the best hybrid lines. Some of the best intermediate-day hybrid combinations included NMSU 96-300-2 × 96-335-1 and NMSU 96-300-2 × 96-274-1.

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Stephanie J. Walker and Paul W. Bosland

The inheritance of resistance to Phytophthora capsici Leonian root rot and foliar blight was compared in two different Capsicum annuum L. var. annuum pod types. The seedling was screened for phytophthora root rot, while a genetically identical stem cutting was screened for phytophthora foliar blight to determine if the same gene(s) confer resistance to both disease syndromes. The susceptible parents were `Keystone Resistant Giant #3' (`Keystone'), a bell pepper type, and `Early Jalapeño', while `Criollo de Morelos-334' was the resistant parent. Resistance was observed in both F1 populations screened for phytophthora root and foliar infection indicating dominance for resistance. Reciprocal effects were not detected. To determine if the same gene(s) conferred root rot and foliar resistance, root rot screening results were matched to the corresponding foliar blight stem cutting reaction. The segregation of resistance in the F2 generations was dependent on the susceptible parent. In the F2 generation derived from `Early Jalapeño', root rot resistance and foliar blight resistance segregated in a 9:3:3:1 (root resistant/foliar resistant: root resistant/foliar susceptible: root susceptible/foliar resistant: root susceptible/foliar susceptible) ratio. One independent, dominant gene was necessary for root rot resistance, and a different independent, dominant gene was needed for foliar blight resistance. In the F2 generation derived from `Keystone', root rot and foliar blight resistance segregated in a 7:2:2:5 (root resistant/foliar resistant: root resistant/foliar susceptible: root susceptible/foliar resistant: root susceptible/foliar susceptible) ratio. This segregation ratio is expected when one dominant gene is required for root resistance, and a different dominant gene is required for foliar resistance. In addition to these two genes, at least one dominant allele of a third gene must be present for expression of root rot and foliar blight resistance.