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Qin Chen and Hai Y. Li

An improved method is described for the isolation of potato metaphase chromosomes for karyotypic and cytogenetic studies. Root tips from diploid Mexican species, Solanum pinnatisectum (2n = 2x = 24) and tetraploid cultivated S. tuberosum (2n = 4x = 48) were given four different pretreatments. The synthetic pyrethroid, Ambush, was the most stable and effective pretreatment reagent, providing the highest percentage of mitotic chromosomes at metaphase and the best spread of countable chromosomes for cytogenetic studies. Compared with an Ambush pretreatment at concentrations of 100-400 ppm, 1 to 10 ppm Ambush produced more easily distinguished chromosomes, which can be useful for comprehensive observation and karyotype analysis in both 2x and 4x potato species. This improved technique for examining mitotic chromosomes will be helpful in describing karyotypes, characterization of new hybrids, and identifying chromosome structural changes that are important in breeding schemes.

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Kai-Ge Zhao, Ming-Qin Zhou, Long-Qing Chen, Donglin Zhang, and Gituru Wahiti Robert

Chimonanthus praecox (wintersweet) is endemic to China. It has been cultivated there for more than 1000 years as a garden, potted, and cut-flower plant. Many cultivars have been developed during its long history of cultivation, and recently many germplasms were collected in Wuhan and Nanjing, China. The identification and genetic relationship of these resources were studied based mainly on morphological traits. In the current study, intersimple sequence repeat markers (ISSR) and random amplified polymorphic DNA markers (RAPD) were used for the first time to investigate 72 wintersweet clones from the two regions. Eleven ISSR primers amplified 115 bands, 90 (78.26%) of which were polymorphic. Nineteen RAPD primers amplified 165 bands, 105 (63.63%) of which were polymorphic. Either ISSR or RAPD markers were sufficient to distinguish all the clones surveyed. A Dendrogram based on Jaccard's similarity coefficients indicated that the distribution pattern of the 72 clones was coherent with their geographical origins. Most of the genetic variation (85.68% with ISSR data; 86.75% with RAPD data) occurred among clones within each region. However, the difference between Wuhan and Nanjing groups is statistically significant (ΦST = 0.143, P < 0.001, with ISSR data; ΦST = 0.132, P < 0.001, with RAPD data). Morphological variation and classification of wintersweet cultivars were also discussed compared with the genetic relationship based on ISSR and RAPD markers. This is the first report of the partitioning of genetic variability within and between different cultivated wintersweet regions, and it provides useful baseline data for optimizing sampling strategies in breeding. These results are important for future genetic improvement, identification, and conservation of Chimonanthus praecox germplasm.

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Hui-Qin Chen, Katherine L. Dekkers, Lihua Cao, Jacqueline K. Burns, L. W. Timmer, and Kuang-Ren Chung

Postbloom fruit drop (PFD) of citrus is incited by the fungus Colletotrichum acutatum J. H. Simmonds and may result in young fruit drop and severe yield losses. Previous studies suggested that imbalance of growth regulators such as auxin, ethylene, and jasmonic acid (JA) plays an essential role in young fruit abscission. In this work, we determined the factors associated with fungal-induced fruit drop by testing compounds inhibitory to hormonal transport or biosynthesis. As assessed on sweet orange (Citrus sinensis Osbeck) and grapefruit (C. paradisi Macf.) for 4 years, we found that many auxin transport and action inhibitors such as 2,3,5-triiodobenzolic acid (TIBA), 2-(4-chlorophenoxy)-2-methyl-propionic acid (clofibrate), or quercetin and JA biosynthesis inhibitors such as salicylic acid (SA) and aspirin (methyl-SA) applied 7 d after C. acutatum infection resulted in higher percentages of young fruit retention compared with the water controls. The commercial products ReZist and Actigard, widely used as systemic acquired resistance (SAR) agents, also improved fruit retention. Furthermore, application of gibberellic acid (GA3) on sweet orange, regardless of C. acutatum infection, significantly increased fruit retention. These commercial products may be very useful in managing this destructive disease of citrus in the field.

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Yalong Qin, Yiming Chen, Weibing Zhuang, Xiaochun Shu, Fengjiao Zhang, Tao Wang, Hui Xu, Bofeng Zhu, and Zhong Wang

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Canbin Chen, Pengyang Wu, Fangfang Xie, Luyang Sun, Yemiao Xing, Qingzhu Hua, Zhike Zhang, Jianye Chen, Jietang Zhao, Guibing Hu, Yonghua Qin, Jiaqiang Li, and Yaoxiong Ye

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Min Zhang, Xiuxin Deng, Changping Qin, Chunli Chen, Hongyan Zhang, Qing Liu, Zhiyong Hu, Linlin Guo, Wenhua Song, Yong Tan, and Shengcai Liao

‘Zaohong’ navel orange [Citrus sinensis (L.) Osbeck + C. unshiu Marc.], a new strain of citrus from a graft chimera, was discovered in China. It was diploid and arose at the junction where a ‘Robertson’ navel orange scion was top-worked onto a Satsuma mandarin (C. unshiu). Some characteristics determined by the L1 cell layer, such as juice sacs of fruit and stoma length, were similar to those of Satsuma mandarin, while others, including leaf index, fruit shape, navel, and color and aroma of the rind, were determined by the L2 cell layer, were similar to ‘Robertson’ navel orange. High-performance liquid chromatography analysis of the carotenoid extracts of the flesh of ‘Zaohong’ navel orange indicated that it had the carotenoids profile of Satsuma mandarin with β-cryptoxanthin as the predominant component in the juice sacs in mature fruit. Simple sequence repeats (SSR) and chloroplast simple sequence repeats (cpSSR) analysis showed that both nuclear and chloroplast genomes of ‘Zaohong’ navel orange were composed of both donor plants. On the basis of these facts, ‘Zaohong’ navel orange was found to be a periclinal chimera consisting of L1 derived from Satsuma mandarin and L2/L3 from ‘Robertson’ navel orange. It combined the valuable traits of both donor plants, matured ≈1 month earlier than the present navel orange cultivars, and therefore had good potential in citrus fresh market.