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Michael E. Compton and D.J. Gray

Cotyledon explants of four watermelon [Citrullus lanatus (Thunb.) Mataum. & Nakai] breeding lines (F92U8, SP90-1, SP90-2, and SP90-4) were prepared from mature seed or from 2-, 4-, 6-, 8-, or 10-day-old seedlings. Explants were incubated on shoot regeneration medium for 8 weeks followed by 4 weeks on shoot elongation medium. The four genotypes differed in their ability to produce shoots at each explant age. The highest frequency with which F92U8 (66%) and SP90-2 (60%) explants produced shoots was for 2-day-old seedlings. Fewer explants formed shoots when established from mature seed or seedlings older than 2 days. In contrast, the percentage of SP90-4 explants that produced shoots was highest when cotyledons were obtained from 4-day-old seedlings (40%), but the response was less than the optimum for F92U8 and SP90-2. SP90-1 cotyledon explants exhibited the poorest response of the four breeding lines (<11% produced shoots), with little difference in response among the explant ages tested. The number of shoots per responding explant also depended on the age of the explant source. Explants from 2- to 4-day-old seedlings produced the most shoots. Fewer shoots formed on cotyledons from mature seed or seedlings older than 4 days.

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Victor Medina-Urrutia, Karla Fabiola, Lopez Madera, Patricia Serrano, G. Ananthakrishnan, Jude W. Grosser and Wenwu Guo

No presently available rootstock combines all the available rootstock attributes necessary for efficient long-term citriculture (production and harvesting) of Mexican limes and other commercially important scions. In the present study, somatic hybridization techniques were used to combine the widely adapted Amblycarpa mandarin (also known as Nasnaran mandarin) with six different trifoliate/trifoliate hybrid selections: Benton, Carrizo, and C-35 citranges; Flying Dragon and Rubidoux trifoliate oranges; and a somatic hybrid of sour orange + Flying Dragon. The ultimate goal of this research is to generate polyploid somatic hybrids that express the complementary horticultural and disease resistance attributes of the corresponding parents, and have direct potential as improved tree-size controlling rootstocks. Somatic hybrids from all six parental combinations were confirmed by a combination of leaf morphology, flow cytometry, and randomly amplified polymorphic DNA (RAPD) (for nuclear hybridity) and cleaved amplified polymorphic sequence (CAPS) analyses (for mtDNA and cpDNA). This is the first report of citrus somatic hybridization using Amblycarpa mandarin. Unexpected hexaploid somatic hybrid plants were recovered from the fusion of Amblycarpa mandarin + C-35 citrange. Hexaploid hybrids should be very dwarfing and may have potential for producing potted ornamental citrus. Resulting somatic hybrid plants from all six combinations have been propagated by tissue culture and/or rooted cuttings and are being prepared for commercial field evaluation for their potential as improved rootstocks for Mexican lime and other important scions.

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Mohammad Majdi, Ghasem Karimzadeh, Mohammad A. Malboobi, Reza Omidbaigi and Ghader Mirzaghaderi

some medicinal plants ( Adaniya and Shira, 2001 ; Gonzalez and Weathers, 2003 ), it has not been observed for feverfew. The development of tetraploid feverfew may play an important role in the generation of triploid lines. The latter may have higher

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Hamidou F. Sakhanokho and M. Nurul Islam-Faridi

tetraploid (2 n = 4 x = 48) nature of the suspected polyploid plant by comparing control diploid (2 n = 2 x = 24) plants with the suspected tetraploid (2 n = 4 x = 48) cytotype using flow cytometry analysis, stomatal dimensions, chloroplast numbers, and

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Zhitong Li and John M. Ruter

characteristics. For example, the induced octaploid Hibiscus acetosella ‘Panama Red’ and tetraploid japanese privet remain more compact ( Contreras et al., 2009 ; Fetouh et al., 2016 ), whereas the induced tetraploid Humulus lupulus develops thinner and

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Jude W. Grosser, Divya Kainth and Manjul Dutt

from interploidal hybridization between a tetraploid and a diploid ( Esen and Soost, 1972 ; Grosser and Gmitter, 2011 ). Interploidal hybridization is the most common and efficient way for breeders to generate triploid cultivars. However, triploid

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Ryan N. Contreras, John M. Ruter and Brian M. Schwartz

) reported that chloroplasts in sun-exposed leaves were transformed into rhodoxanthin-containing chromoplasts during winter, resulting in the brown–red color. In Japanese forestry nurseries, tetraploid forms of japanese-cedar have been identified based on