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Yi Zhang and Imed Dami

The purpose of this study was to develop a protocol to increase freezing tolerance of field-grown ‘Chambourcin’ grapevines (Vitis spp.) using exogenous abscisic acid (ABA). The specific objectives were to determine the optimum concentration and timing for ABA foliar application in ‘Chambourcin’ and to evaluate morphological and physiological changes that lead to increased freezing tolerance in response to foliar ABA application. ‘Chambourcin’ grapevines were treated with a foliar ABA application of concentrations of 0, 100, 200, 300, 400, 500, 600, 700, and 800 mg·L−1 at 50% fruit set stage to evaluate ABA phytotoxicity under field conditions and identify the optimum concentration. In a subsequent experiment, ‘Chambourcin’ grapevines were treated with 400 and 600 mg·L−1 of ABA at different stages of development corresponding to 50% fruit set, 21 days after 50% fruit set, 50% veraison, 20, 30, 40, and 55 days postveraison. ABA concentrations of 700 and 800 mg·L−1 were phytotoxic and caused significant damage to leaves and flowers. Optimum concentrations of ABA did not affect yield components or basic fruit chemical composition, yet it promoted anthocyanin accumulation at harvest. Furthermore, ABA advanced bud dormancy, decreased bud water content, and eventually increased freezing tolerance under simulated freezing tests. The increased freezing tolerance of ABA-treated vines was confirmed by bud injury assessment after a natural freezing event in Jan. 2011. It was also determined that ABA was most effective when applied with an optimum concentration of 400 mg·L−1 20 to 30 days postveraison. It is concluded that exogenous ABA enhanced dormancy and increased freezing tolerance; thus, it has the potential to protect grape cultivars from freezing injury.

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Yi Zhang, Tracey Mechlin, and Imed Dami

The purpose of this study was to investigate the influence of foliar application of abscisic acid (ABA) on grapevine dormancy, specifically to: 1) determine the optimum foliar application concentration of ABA and 2) evaluate the morphological and physiological changes of greenhouse-grown grapevines in response to exogenous ABA application. Vitis vinifera ‘Cabernet Franc’ and Vitis spp. ‘Chambourcin’ with different leaf ages (40, 50, 80, 100, 110, and 120 days) were subjected to foliar ABA application at different concentrations (0, 100, 200, 400, 600, 800, 1600, and 3200 mg·L−1) and to a cold-acclimated regime. Concentrations of 800 mg·L−1 or higher were phytotoxic and the optimum concentrations were between 400 and 600 mg·L−1. Optimum concentrations of ABA inhibited shoot growth and advanced growth cessation, periderm formation, and leaf senescence, which led to advanced dormancy in both cultivars. In this study, it was concluded that exogenous ABA induced endodormancy because single cuttings (not paradormant) under favorable growing conditions (not ecodormant) were used. Furthermore, grapevine response to ABA was influenced by leaf age and cold treatment. ABA was effective in inhibiting shoot growth and increasing periderm formation in the young vines with 40- to 50-day old leaves and the old grapevines with 80- to 120-day old leaves. However, ABA was effective in inducing early shoot cessation, leaf senescence and abscission, and dormancy in old vines with 100- to 120-day old leaves only. The advanced morphological and physiological changes induced by exogenous ABA mimicked those triggered by environmental cues during the cold acclimation process. It was suggested that advancing the cold acclimation process using foliar ABA application may be beneficial for long-season grape cultivars grown in regions with short growing seasons and early fall frost events.

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Yi Tan, Baisha Li, Yi Wang, Ting Wu, Zhenhai Han, and Xinzhong Zhang

Agrobacterium-mediated genetic transformation is commonly used in dicotyledon plants such as apples. The regeneration ability of the recipient is an important factor in transformation efficiency. Here, the variations in bud regeneration rate (BRR) and the number of adventitious buds (NAB) formed per explant in Malus germplasm accessions with phenological stage were estimated. Both BRR and NAB of explants at the dormancy broken and spring sprouting stages were significantly higher than those at the autumn sprouting stage. The genetic diversity and inheritance of BRR and NAB were evaluated using 153 Malus germplasm accessions and 78 hybrid trees of Jonathan × Golden Delicious. Malus sieversii 31, Liberty, and Smoothee exhibited significantly high BRR (98.33%, 98.33%, and 93.33%, respectively) and a large NAB without vitrification. BRR and NAB linearly correlated with each other but not with callus formation rate. The broad sense heritability of the regeneration rate was 92.16%. The three Malus accessions that had high regeneration ability, and some of their sexual descendants, might be outstanding genetic resources for future genetic transformation.

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Yi Kai, Yang Bin, Zhang Min, Gao Ainong, Zhang Jinger, Liu Zhi, Sha Shoufeng, and Xie Chongxin

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Meiling Zhang, Ming Chen, Zhen Wang, Ting Wu, Yi Wang, Xinzhong Zhang, and Zhenhai Han

Grafting has been widely used in orchard management because the rootstock can make the tree more tolerant to environmental stresses. Iron deficiency is one of the major limiting environmental factor in apple production worldwide. Systematic research has been made about iron-deficiency adaptive responses in the level of organs, cells, and subcells, whereas the interactions between Fe and other divalent cations in tissue level are little known. Synchrotron radiation X-ray fluorescence (SR-μXRF) was used to map the location of selected elements Fe, Zn, Mn, Ni, and Co in the longitudinal and latitudinal root samples of Malus xiaojinensis. Iron deficiency induced a significant increase in the relative contents of five micronutrients in epidermis and cortex. The ratio of element contents of roots under Fe-deficient condition and Fe-sufficient condition at same position increased obviously in the section of 1000- to 2000-μm distance from the root tip in xylem. Expression analysis of iron absorption- and transport-related genes in roots showed that MdNramp3 and MxCS1 increased significantly. These results indicated that iron deficiency promoted the long-distance transport of micronutrients in xylem, and MdNramp3 and MxCS1 might play an important role in this process. Importantly, this study directly provides visual divalent metals distribution in tissue level for an improved understanding of metal absorption process in apple rootstock.

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Renbing Zhang, Yong Xu, Ke Yi, Haiying Zhang, Ligong Liu, Guoyi Gong, and Amnon Levi

A genetic linkage map was constructed for watermelon using 117 recombinant inbred lines (RILs) (F2S7) descended from a cross between the high quality inbred line 97103 [Citrullus lanatus var. lanatus (Thunb.) Matsum. & Nakai] and the Fusarium wilt (races 0, 1, and 2) resistant U.S. Plant Introduction (PI) 296341 (C. lanatus var. citroides). The linkage map contains 87 randomly amplified polymorphic DNA (RAPD) markers, 13 inter simple sequence repeat (ISSR) markers, and four sequenced characterized amplified region (SCAR) markers. The map consists of 15 linkage groups. Among them are a large linkage group of 31 markers covering a mapping distance of 277.5 cM, six groups each with 4 to 12 markers covering a mapping distance of 51.7 to 172.2 cM, and eight small groups each with 2-5 markers covering a mapping distance of 7.9 to 46.4 cM. The map covers a total distance of 1027.5 cM with an average distance of 11.7 cM between two markers. The map is useful for the further development of quantitative trait loci (QTLs) affecting fruit qualities and for identification of genes conferring resistance to Fusarium wilt (races 0, 1 and 2). The present map can be used for further construction of a reference linkage map for watermelon based on an immortalized mapping population with progenies homozygous for most gene loci.

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Kai Zhao, Feng Zhang, Yi Yang, Yue Ma, Yuexue Liu, He Li, Hongyan Dai, and Zhihong Zhang

GA20-oxidase (GA20-ox) is a key enzyme involved in the biosynthesis of gibberellic acid (GA). To investigate its role in plant growth and development, we suppressed MdGA20-ox gene expression in apple (Malus domestica cv. Hanfu) plants by RNA interference (RNAi). After 20 weeks of growth in the greenhouse, significant phenotype differences were observed between transgenic lines and the nontransgenic control. Suppression of MdGA20-ox gene expression resulted in lower plant height, shorter internode length, and higher number of nodes compared with the nontransgenic control. The expression of MdGA20-ox in transgenic plants was significantly suppressed, and the active GA content in transgenic lines was lower than that in the nontransgenic control. These results demonstrated that the MdGA20-ox gene plays an important role in vegetative growth, and therefore it is possible to develop dwarfed or compact scion apple cultivars by MdGA20-ox gene silencing.

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Haiying Zhang, Jianguang Fan, Shaogui Guo, Yi Ren, Guoyi Gong, Jie Zhang, Yiqun Weng, Angela Davis, and Yong Xu

Watermelon belongs to the genus Citrullus. There have been continuing interests in breeding of watermelon for economic benefits, but information on the scope and utilization of genetic variations in Citrullus is still limited. The present study was conducted in 2012–13, to evaluate the genetic diversity and population structure of the 1197 line watermelon collection maintained by the Beijing Vegetable Research Center (BVRC), which belongs to seven Citrullus species including Citrullus naudinianus, Citrullus colocynthis, Citrullus rehmii, Citrullus ecirrhosus, Citrullus amarus, Citrullus mucosospermus, and Cirullus lanatus subsp. vulgaris. Twenty-three highly informative microsatellite markers evenly distributed in the watermelon genome were used to assess genetic diversity in this collection. The markers detected on an average of 6.05 alleles per locus with the average value of polymorphism information content (PIC) at 0.49. A high level of gene diversity [Nei’s gene diversity index (Nei) = 0.56] and a low observed heterozygosity (H o = 0.10) were revealed within the collection. Structure analysis grouped the 1197 accessions into two main populations (Pop I and Pop II) and an admixture group. Pop I contained 450 accessions from C. lanatus subsp. vulgaris (446) and C. mucosospermus (4). Pop II comprised 465 accessions, 379 of which belonged to C. lanatus subsp. vulgaris and 86 to C. naudinianus (3), C. ecirrhosus (2), C. rehmii (2), C. colocynthis (11), C. amarus (58), and C. mucosospermus (10). The remaining 282 accessions were classified as an admixture group. The two main populations were further subdivided into four subgroups. The groupings were consistent with the estimation of F statistics (F st) and Nei’s genetic distances in collections. We confirmed the distinct genetic backgrounds between American and East Asian ecotypes. Subsequently, we defined a core set consisting of 130 accessions including 47 from Pop I, 68 from Pop II, and 15 from the Admixture group. This core set was able to capture all 133 alleles detected by 23 simple sequence repeats (SSRs) in 1197 accessions. These results will facilitate efficient use of genetic variations in Citrullus in watermelon breeding and help optimization of accessions in genomewide association studies.

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Rui Sun, Hui Li, Qiong Zhang, Dongmei Chen, Fengqiu Yang, Yongbo Zhao, Yi Wang, Yuepeng Han, Xinzhong Zhang, and Zhenhai Han

Flesh browning is an important negative trait for quality preservation of fresh-cut fruits. To obtain a better understanding of the inheritance and genetic control of flesh browning in apple, the phenotype of a hybrid population derived from ‘Jonathan’ × ‘Golden Delicious’ was studied for 2 successive years. The inheritance of the flesh browning trait was analyzed by the frequency distribution of the phenotypes. Flesh browning-associated major genes were then mapped by screening genome-wide simple sequence repeat (SSR) markers. Flesh browning is inherited quantitatively and showed a clear bimodal frequency distribution, indicating that the segregation of major genes is involved in the variation. The segregation ratio of light and heavy browning was 7:1 in 2010, 2011, and 2010 + 2011, suggesting that the inheritance of the trait in apple involves three segregated loci of major genes. The heritability of the major gene effect was 72.14% and 72.76% in 2010 and 2011, respectively. SSR markers were screened from 600 pairs of SSR primers located on 17 apple linkage groups (LGs). The three major genes were mapped on LG10, 15, and 17 on the apple genome, respectively, by linkage analysis of flesh browning phenotypes and the genotypes of SSR markers. Two quantitative trait loci (QTLs) for flesh browning were mapped on LG15 of ‘Jonathan’ and LG17 of ‘Golden Delicious’, respectively, which are the same linkage groups that two major genes mapped on.

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Jin-wei Zhang, Yi-xue Liu, Jin-ping Yu, Wei Zhang, Ya-qiong Xie, and Ning-ning Ge

In the early Spring of 2015 and 2016, weed infestation surveys were conducted in areas of cool-season turfgrass Festuca arundinacea Schreb. at 23 sites within Tianjin municipality in northern China. The weed community within turfgrass areas comprised 37 weed species belonging to 14 families. Perennial weeds accounted for 45.9% of the total community of weed species, whereas annual or biennial weeds accounted for 54.1%. Asteraceae was the dominant family (43.2%), and the percentage of broadleaved weeds was 94.6%. Statistical analyses of the weed dominance index (integrating weed relative height and relative coverage) and relative abundance (integrating weed relative density, frequency, and uniformity) showed that the 10 most common weed species during the early spring were Ixeris polycephala Cass., Taraxacum mongolicum Hand.-Mazz., Inula japonica Thunb., Hemistepta lyrata Bge., Trigonotis pedunclaris (Trev.) Benth., Calystegia hederacea Wall., Lepidium apetalum Willd., Plantago asiatica L., Cirsium segetum Bge., and Ixeris sonchifolia Hance. Ixeris polycephala Cass. and T. mongolicum Hand.-Mazz were the most dominant and harmful weed species. Partial correlation analysis (PACA) indicated that the deterioration rate (percentage of bare soil or coverage of plants other than turfgrass) of the turfgrass area was significantly and positively correlated with the total dominance index [(TDI), an index to evaluate the weed infestation severity)] and that the soil organic matter (SOM) and salinity factors were negatively correlated with the TDI. Factors such as soil nutrient conditions (the contents of N, P, and K and the total N), soil physical properties (density and clay content), soil moisture, soil temperature, and soil pH did not correlate significantly with the TDI. We conclude that the deterioration rate was the most important factor influencing weed infestation in the early spring and that SOM and soil salinity might also be important factors. The results of this study can help turfgrass researchers and managers identify the most harmful weed species and integrate management strategies in areas of cool-season turfgrass F. arundinacea Schreb. during early spring in the Tianjin region, China.