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  • Author or Editor: S. S. Korban x
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Modem taxonomists classify the genus Malus into 25 to 30 species and several subspecies (9, 25). Most species of this genus intercross freely, since there are no evident physiological and genetic barriers. Moreover, collections of Malus found in arboreta are classified inadequately and usually are grown from seed which is generally a result of interspecific or intraspecific hybrids (9).

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Abstract

Progenies of ‘Golden Delicious’ apple (Malus domestica Borkh.) crossed with 12 Malus species and hybrids were screened for resistance to powdery mildew incited by Podosphaera leucotricha (Ell. & Ev.) Salm. under greenhouse conditions. Noninfected seedlings were recovered from Morton Arboretum (M.A.) 8 (an unknown interspecific hybrid), M. zumi calocarpa Rend., M. sargenti Rehd. 843 × self, and M. baccata jackii Rehd. Evidence for a single heterozygous dominant gene conferring resistance in M.A.8 and M. sargenti was confirmed by field tests of resistant seedlings.

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Abstract

A historical analysis of apple nomenclature leads to the conclusion that the legitimate epithet for the cultivated apple is Malus Xdomestica Borkh.

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Established shoot cultures of three apple genotypes, `Dayton', `McIntosh', and `Golden Delicious' were subcultured into culture tubes containing a modified MS medium and maintained in a dark chamber at 1.0±0.5°C for periods of 3, 6, 9, and 12 months. Following each cold storage period, culture tubes of each of the three genotypes were transferred to a growth room and maintained under 16 h of light (60 uEs-1m-2) and 21°C. The overall morphological condition of each shoot was then recorded. After 4 weeks of growth, both number and length (in cm) of proliferating shoots were recorded. In general, shoots subjected to 3 or 6 months of cold storage remained green however most cultures did not initiate any new shoots. Cultures subjected to 9 or 12 months of cold treatment were etiolated however new axillary shoots were observed. The proliferation rate after 4 weeks of growth under standard growth conditions were variable among the different genotypes. The implications of using long term cold storage of apple shoot cultures will be discussed.

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Established shoot cultures of three apple genotypes, `Dayton', `McIntosh', and `Golden Delicious' were subcultured into culture tubes containing a modified MS medium and maintained in a dark chamber at 1.0±0.5°C for periods of 3, 6, 9, and 12 months. Following each cold storage period, culture tubes of each of the three genotypes were transferred to a growth room and maintained under 16 h of light (60 uEs-1m-2) and 21°C. The overall morphological condition of each shoot was then recorded. After 4 weeks of growth, both number and length (in cm) of proliferating shoots were recorded. In general, shoots subjected to 3 or 6 months of cold storage remained green however most cultures did not initiate any new shoots. Cultures subjected to 9 or 12 months of cold treatment were etiolated however new axillary shoots were observed. The proliferation rate after 4 weeks of growth under standard growth conditions were variable among the different genotypes. The implications of using long term cold storage of apple shoot cultures will be discussed.

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Leaf segments of Prunus persica L. (peach) collected from greenhouse-grown plants and from micropropagated shoots were cultured on a basal medium containing half-strength Murashige and Skoog (MS), Staba vitamins, sucrose (30 g/1) and agar (6.5 g/l); medium adjusted to pH 5.6. The influence of 6 different growth regulators at 3 concentrations (5, 10, 15 μM) were investigated using leaf explants from proliferating shoots of 'Elberta Queen' peach. With thidiazuron (TDZ), compact and multiple green calli were obtained; with benzyladenine and zeatin, lower numbers of small sized calli were obtained; with kinetin, no callus development was observed. Among auxin treatments, both Dicamba and 2,4-D resulted in friable white and yellow calli. Most of the calli produced in all treatments were formed along the cut margins of the explants. In an another experiment, leaf explants of' Bellaire' (greenhouse) and `Elberta Queen' (in vitro shoots) were used to determine the influence of a large scale concentration of TDZ (3 to 23 |iM). Explants from greenhouse and in vitro leaves resulted in higher levels of callus development at TDZ concentrations of 8-13 μM. Higher TDZ levels resulted in necrosis of leaf explants. The-influence of different carbon sources on callogenesis was investigated. We observed more green and compact calli with glucose than with sucrose and fructose at 100 mM. The influence of the glucose at 10 different concentrations (30 to 300 mM) was also investigated.

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Developing an efficient gene transfer system for apple (Malus ×domestica L.) remains a major objective in genetic engineering efforts of this fruit crop. Transient expression of the uidA gene coding for β-glucuronidase (GUS) and driven by the cauliflower mosaic virus 35S promoter (CaMV35S) has been induced in apple cotyledonary explants of mature seeds by tungsten particle bombardment using the Particle Inflow Gun (PIG). Several factors that affect transient expression of the GUS gene in apple cotyledons were investigated. The gene transfer efficiency was monitored by recording the number of blue spots observed on explants two days following bombardment. Precultivation of cotyledons for 18 hours before bombardment significantly increased the number of blue foci. Of the three different precipitation methods tested including water, 25% PEG, and 60% glycerol, the latter was the most effective for coating DNA onto tungsten particles. Washing DNA-coated tungsten particles with 70% ethanol and resuspending in 100% ethanol significantly enhanced gene delivery to cotyledons. The amount of particles used for each bombardment also influenced GUS expression. About 0.5 mg of particles per shot resulted in the highest number of blue foci. Using larger quantity of particles (i.e., 2 mg) drastically decreased GUS expression probably due to the toxicity of tungsten particles.

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DNA was extracted from leaves of various Malus genotypes and used to screen synthetic decamer oligonucleotide primers. Samples from the following two groups were bulked: 1) seven scab-susceptible apple cultivars, and 2) 15 scab-resistant apple genotypes derived by introgressive hybridization from the previous group of cultivars. A third sample consisted of DNA extracted from Malus floribunda Sieb. clone 821, the original source of apple scab resistance for all genotypes in the second group. A total of 59 primers from kits A, L, and R (Operon Technologies) were screened. Amplified fragments were obtained for 93% of the primers tested, while random amplified polymorphic DNA (RAPD) fragments were detected among samples for 76% of the primers. One primer, A15, amplified a unique band in both M. floribunda clone 821 and the bulked scab-resistant sample. This RAPD marker, designated OA15900, identifies an amplified, introgressed fragment that likely corresponds to a region of the genome that may serve as a modifier for the scab resistance gene block V, derived from M. floribunda clone 821.

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