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Nele De Belie, Ian C. Hallett, F. Roger Harker, and Josse De Baerdemaeker

The tensile properties of european pear (Pyrus communis L. `Beurre Bosc') and asian pear (Pyrus pyrifolia Nakai `Choguro') were examined using a microscope-mounted apparatus that allowed direct observation and recording of cell and tissue changes during testing. To manipulate turgor potential, tissue slices from fruit of different firmness (ripeness) were incubated in sucrose solutions of differing water potential. Solution water potentials were adjusted for individual fruit, and varied between -2.5 and 1 MPa from the water potential of the expressed juice. Fruit firmness declined from 100 to 20 N and from 60 to 25 N during ripening of european and asian pears, respectively. For both european and asian pears the relationship between fruit firmness and tensile strength of tissue soaked in isotonic solutions was sigmoidal, with the major mechanism of tissue failure being cell wall failure and cell fracture at high firmness and intercellular debonding at low firmness. In the intermediate zone, where fruit firmness and tissue tensile strength decreased simultaneously, a mixture of cell wall rupture and intercellular debonding could be observed. Tissue and cell extension at maximum force both declined similarly as fruit softened. Tensile strength of tissue from firm pears (>50 N firmness, >0.8 N tensile strength) decreased by as much as 0.6 N during incubation in solutions that were more concentrated than the cell sap (hypertonic solutions). When similar tissue slices were incubated in solutions that were less concentrated than the cell sap (hypotonic solutions), the tensile strength increased by up to 0.4 N. This is interpreted as stress-hardening of the cell wall in response to an increase in cell turgor. Tensile strength of tissue from soft pears was not affected by osmotic changes, as the mechanism of tissue failure is cell-to-cell debonding rather than cell wall failure.

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Kim E. Hummer

The pear, Pyrus L., originated in prehistoric times. Records of its cultivation date back 3000 years both in Europe, with the ancient Romans and Greeks, and in Asia, with the Chinese. Pear culture was significant in France and England by the 16th century. The European golden age of pear improvement occurred from the 17th through the 19th centuries. The pear genetic resource collection for the United States Department of Agriculture, Agricultural Research Service, National Plant Germplasm System is maintained at Corvallis, Ore. This collection preserves more than 2000 diverse pear accessions, represents 26 species, and includes more than 410 heirloom cultivars. At least 10 of the cultivars have obscure origins from the ancient Roman, Greek, or Chinese cultures. Another dozen are at least 400 years old, and more than 250 were introduced during the European golden age. Another 120 “antique” cultivars of the collection were introduced during the first half of the 1900s. The “big four” economically important Pyrus communis L. cultivars in the United States, `Bartlett', which originated in 1777; `Anjou', late 1700s; `Bosc', 1807; and `Comice', 1845; are also represented. Origin and background information for these heirloom clones is web accessible through the Germplasm Resource Information Network (GRIN) database. Although many ancient pear genotypes have been lost, the Repository staff continues to search for significant heirloom cultivars that are not yet represented. Besides having direct value in crop improvement, these plants are a significant part of our human heritage. Their preservation is a sacred trust.

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Richard L. Bell and L. Claire Stuart

Four genotypes of pear (Pyrus spp.) of East European origin, a susceptible control, `Bartlett' (P. communis L.), and a moderately resistant control, NY 10352 (P. ussuriensis Maxim. × P. communis B C1 hybrid), were artificially infested with pear psylla (Cacopsyll a pyricol a Foerster) nymphs in the laboratory. Ten neonate first instars were placed on each of the two youngest leaves of four small trees per genotype. On PI 506381 and PI 506382, wild seedlings of P. nivalis Jacq., all nymphs died within 5 days. Mortality and development of nymphs on PI 502173, a wild P. communis seedling, was similar to that observed on `Bartlett', with 43% and 45% of the nymphs surviving to adulthood, respectively. On `Karamanlika' (PI 502165) and NY 10352, 15% of the nymphs developed into adults. Increased mortality and delayed development of nymphs was associated with feeding inhibition. The mode of host plant resistance to pear psylla nymphs in these accessions of East European pear is, therefore, similar to that previously characterized for NY 10352, in which the resistance is derived from germplasm of East Asian origin.

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Hla Aung and Frank B. Matta

One year old Asian pear scion cultivars were grafted onto Pyrus calleryana rootstock utilizing two grafting methods (whip grafting and splice side grafting). Percentage survival of grafted scions was 78 and 96 via the splice side graft and the whip graft, respectfully. Shoot length and caliper 80 days after grafting did not vary between cultivars. `Yakumo' and 'Chojuro' produced a greater number of branches as compared to the remaining cultivars. `Yakumo', `Chojuro', `Seuri' and `Hosui' produced the least amount of shoot growth. Branching angle was greatest for `Seigyoku', `Chojuro' and `Yakumo' with 60, 70, and 55 degrees, respectfully. As indicated by leaf area, `Seuri' and `Hosui' produced large leaves and `Yakumo' and `Chojuro' produced small leaves.

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Hla Aung and Frank B. Matta

One year old Asian pear scion cultivars were grafted onto Pyrus calleryana rootstock utilizing two grafting methods (whip grafting and splice side grafting). Percentage survival of grafted scions was 78 and 96 via the splice side graft and the whip graft, respectfully. Shoot length and caliper 80 days after grafting did not vary between cultivars. `Yakumo' and 'Chojuro' produced a greater number of branches as compared to the remaining cultivars. `Yakumo', `Chojuro', `Seuri' and `Hosui' produced the least amount of shoot growth. Branching angle was greatest for `Seigyoku', `Chojuro' and `Yakumo' with 60, 70, and 55 degrees, respectfully. As indicated by leaf area, `Seuri' and `Hosui' produced large leaves and `Yakumo' and `Chojuro' produced small leaves.

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Wol-Soo Kim* and Jin-Ho Choi

The stone cells events in the process of lignifications of plant tissues in flesh of Asian pear have been growing as a depressing factor of fruit quality. Therefore, these studies were carried out to search the effect of stone cells on fruit quality, to investigate the anatomical characteristics, such as formative period and distribution of stone cell, to seek forming causes, and to determine the effects of drought stress and calcium foliar application on the formation of stone cell. Fruit quality as contents of the stone cells, such as texture profile, reducing sugars, firmness, and fruit size, were determined. During the growing season of 2002 and 2003, samples for anatomical investigations were taken periodically in Pyrus pyriforia cv. Niitaka, Pyrus communis cv. Bartlett and Pyrus ussiriansis cv. Yari. The morphology of stone cell in the fruit flesh was observed by using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM).

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Maryna Serdani, Robert A. Spotts, Jill M. Calabro, Joseph D. Postman, and Annie P. Qu

Powdery mildew (PM) occurs worldwide and is prevalent on susceptible cultivars wherever pears are grown, causing economic losses due to russeted fruit and an increased need for fungicides. A core subset of the Pyrus germplasm collection at the USDA National Clonal Germplasm Repository in Corvallis, Ore., was evaluated for resistance to Podosphaera leucotricha, the causal agent of PM, using greenhouse and field inoculations of potted trees. The core collection consists of about 200 cultivars and species selections, representing most of the genetic diversity of pears and includes 31 Asian cultivars (ASN), 122 European cultivars (EUR), 9 EUR × ASN hybrids and 46 pear species selections. Three trees of each core accession were grafted on seedling rootstocks. In 2001–02, trees were artificially inoculated in a greenhouse, grown under conditions conducive for PM, and evaluated for symptoms. The same trees were subsequently evaluated for PM symptoms from natural field infections during 2003 and 2004. In the greenhouse, 95% of EUR and 38% of ASN were infected with PM. Average PM incidence (percent of leaves infected) in the greenhouse (8% for ASN and 30% for EUR) was much higher than incidence in the field (2% for ASN and 5% for EUR) during 2003. Symptoms were also more severe in the greenhouse, with 46% of ASN and 83% of EUR with PM symptoms having a mean PM incidence of >10%. In the field, 42% and 22% of EUR and 23% and 13% of ASN were infected with P. leucotricha in 2003 and 2004, respectively. Field infection was very low during both years, with percentage leaves infected in ASN and species selections significantly different from EUR. In the field, 6% of ASN with PM symptoms had a mean PM incidence >10% during both years, while 15% and 2% of EUR accessions with PM symptoms had a mean PM incidence >10% in 2003 and 2004 respectively. These results should be very useful to pear breeding programs to develop improved PM resistant cultivars in the future, by using accessions with consistent low PM ratings.

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Shogo Matsumoto and Kentaro Kitahara

A polymerase chain reaction (PCR)-based method for identifying the S-alleles in the Asian pear [Pyrus pyrifolia (Burm) Nak.] was applied to apple (Malus ×domestica Borkh.) cultivars. With minor modifications in one of the primers, the fragments from S-genes (S-RNases) with introns were amplified from total DNA of apple cultivars possessing S2-, S3-, S5-, S7-(=Sd-), S9-(=Sc-), Sf- and Sg-allele genotypes. S-genes within S24-(=Sh-) and S26-alleles were also amplified. The PCR amplification step of this method appears to be useful for preliminary investigation of apple S-genotypes, especially for species or cultivars of unknown origin or history. Using the primers, which are a part of a new S-allele, the Se-allele encoding Se-RNase with an intron in the Se-allele was amplified. We cloned the cDNA of Se-RNase, and developed a PCR-restriction fragment length polymorphism (RFLP) analysis method for Se-allele identification. S-allele genotypes of seven apple cultivars were investigated.

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Nihal C. Rajapakse, Nigel H. Banks, Errol W. Hewett, and Donald J. Cleland

Steady-state oxygen diffusion in flesh of apples (Malus domestics Borkh. cvs. Braeburn and Cox's Orange Pippin), Asian pears (Pyrus serotina Rehder. cvs. Hosui and Kosui), and nectarines [Prunus persica (L.) Batsch. cvs. Red Gold and Sunglo] was studied using a nondestructive method at 20C. Fruit flesh was found to exert a significant resistance to O2 diffusion resulting in measurable O2 gradients between tissues immediately beneath the skin and those at the fruit center for all these fruits. The magnitude of these O2 gradients varied between crops and cultivars and depended on the respiration rate and on effective O2 diffusivity in fruit flesh (De). Values of Dc varied with the cultivar and were broadly consistent with intercellular space volume. The range of De values obtained suggested that 02 diffusion in fruit flesh takes place in a combination of series and parallel modes in the intercellular space and fluid/solid matrix of the flesh. The results imply that O2 diffusivity in flesh tissues must be taken into consideration in the determination of critical external O2 level in controlled/modified atmosphere (CA/MA) storage.

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Beiquan Mou and Guangyao Wang

Asia covers a wide range of geographic areas and climates and is one of the most horticulturally dynamic regions in the world. There is a great biological diversity of plants in this region. It also has a long history of agriculture and many