Walnut (Juglans regia L.) is an important woody oil tree species in the Juglandaceae family (Vahdati et al. 2019; Wu et al. 2020). It is one of the oldest tree foods, which world widely cultivated throughout temperate regions for both its nuts and timber (Hassani et al. 2020; Ji et al. 2021; Vahdati et al. 2023). In China, walnut has been cultivated for more than 2000 years (Pei and Lu 2011). China has a harvested area of walnut up to 356,656 hm2 with a total output of 1,400,000 tons, which is ranked first globally (FAO 2024). Walnut genetic resources from China had been introduced to many countries such as USA and widely used in the walnut breeding program (Akça et al. 2020). A large number of walnut cultivars have been developed by Chinese breeders since the 1960s, and new cultivars began to be introduced into China from abroad starting in the 1980s (Chen et al. 2014). Currently, more than 200 walnut cultivars have been developed and cultivated in China (Ma et al. 2019). In particular, most walnut cultivars in northern China originated from walnuts grown in Xinjiang Province, which owns abundant walnut germplasm resources and produces 22% of the country’s walnuts. With the purpose of adapting to diverse market demands, walnut breeders continuously carry out further cultivar breeding works.
Origin
In 2013, a large-scale general survey of walnut resources was held in Xinjiang Province, People’s Republic of China. ‘Xin Xiong’ was a new cultivar selected from indigenous individuals located at Hotan County (lat. 37°7′34.219″N, long. 79°49′31.91″E). This area is part of a warm, temperate zone with an extremely arid desert climate. The type of the soil is yellow sandy loam. Scions were collected and grafted onto six seedlings in Jiamu Experimental Station of Xinjiang Academy of Forestry. The secondary flowers have appeared since 2015. The growth habits and nut traits were investigated during 2018–21. Consequently, in 2023, the new cultivar Xin Xiong was designated by the State Forestry Administration of the People’s Republic of China (Beijing, China) as certificate No. 20230770.
Methods
All the characteristics of ‘Xin Xiong’ were classified and described according to The National Standard of the People’s Republic of China: Guidelines for the Conduct of Tests for Distinctness, Uniformity and Stability—Juglans (Juglans L.) (Pei and Lu 2011) and “UPOV Guidelines for the Conduct of Tests for Distinctness, Uniformity and Stability - Walnut (Juglans regia L.)” (International Union for the Protection of New Varieties of Plants 2023). Thirty nuts were randomly selected from each cultivar for nut characteristic analysis. DNA was extracted from young leaves, and the microsatellite marker or simple sequence repeat (SSR) analysis was conducted as reported by Chen et al. (2014). Moreover, the relative susceptibility of the new cultivar to common diseases such as blight and bark canker were also evaluated as described by Hassani et al. (2020).
Description and Performance
Taking the cultivar Zha 200 (J. regia) as a reference, we evaluated the new Xin Xiong cultivar for tree growth characteristics as well as nut traits on six grafted trees (5 years after grafting, seedlings as rootstocks).
‘Xin Xiong’ is characterized by its long secondary male inflorescence, which occurs in early May, similar to ‘Zha 200’. These secondary flowers are usually male, whereas those of ‘Zha 200’ are usually mixed. The inflorescence length of ‘Xin Xiong’ measured up to 80 cm, whereas it is shorter than 60 cm for ‘Zha 200’. Precocious walnut genotypes are beneficial in the production and usually have the characteristic of secondary flowering (Mohseniazar et al. 2021). Secondary flowers are also taken as excellent materials for studying the induction, initiation, and differentiation of walnut flower organs (Hassankhah et al. 2018, 2020). Additionally, this large number of long inflorescences is of great ornamental value.
‘Xin Xiong’ has moderate tree vigor, spreading growth habit (‘Zha 200’ is semiupright), and dense, yellowish-green annual branches (Fig. 1). Narrow elliptic yellowish-white lenticels are irregularly distributed on its shoots. It has a light gray bark color. Each compound leaf has five to seven leaflets with entire margin and pinnate vein. These leaflets are medium green and medium elliptic, whereas that of ‘Zha 200’ are dark green and broad elliptic. Leaf buds are round. The nut shape is ovate with smooth shell surface, which is similar to ‘Zha 200’. Its raised suture is tight in combination, so it is not easy to crack. The inner pleat wall is mid-developed, and half kernels are not difficult to be taken out from nuts. The average single nut weight for ‘Xin Xiong’ is 14.0 g, whereas ‘Zha 200’ weighs 12.2 g. Kernels are relatively small and comprise 45% of the nut weight. The shell thickness averaged at 1.9 mm, which is much thicker than ‘Zha 200’ (Table 1). The kernel is under a relatively dark color and intense astringency (Fig. 1). This cultivar has high and stable yields. We predict that it has great potential future product value for processing and landscaping use. According to the characteristics observed from 2018 to 2021, the botanical characteristics and biological properties within the clonal group generated via grafting were compatible and showed no degradation or differentiation.
Walnut cultivar Xin Xiong. (A) Eight-year-old ‘Xin Xiong’ tree. (B) Secondary flowers of ‘Xin Xiong’. (C) Nut of ‘Xin Xiong’.
Citation: HortScience 59, 9; 10.21273/HORTSCI17849-24
Phenological, nut and kernel characteristics of cultivar Xin Xiong compared with the reference cultivar Zha 200.
In the Wensu County area of Xinjiang Province (lat. 41°15′56.693″N, long. 80°32′26.383″E), budbreak of ‘Xin Xiong’ begins in late March. Leaf expansion and female flower opening take place in early April. In particular, they gradually wither in late July, last ≈20 d longer than ‘Zha 200’. The fruits ripen from early to mid-September. Crown leaf shedding starts in mid-November. ‘Xin Xiong’ has relatively high drought tolerance and is suitable for cultivation in the oasis at the margin of Tarim Basin and regions with similar climate. This cultivar is moderately resistant to bark canker and stalk rot.
Taking Xin Xiong, Zha 2000, and four other widely used cultivars in Xinjiang province (Xinxin 2, Wen 185, Zha 343, and Xin Feng) as material, molecular characterization was conducted using the SSR method. The primer combination CAF440 employed in the analysis could be observed two alleles, 158 and 166 bp. Further, Xin Xiong presented 158/158, which was unique among the six tested walnut cultivars (Fig. 2).
Simple sequence repeat fingerprinting patterns for walnut cultivar Xin Xiong compared with five other cultivars tenerated by the primer combination CAF440 (sequence: F-CCCGAGGGATCATCTTCAGC; R-GGTTGGTGAAAAGGACACGC).
Citation: HortScience 59, 9; 10.21273/HORTSCI17849-24
The current market has higher requirements for diversity among walnut cultivars. The distinctive features described here make Xin Xiong a multipurpose, high-value-added cultivar with broad market prospects. Moreover, besides ‘Xin Xiong’, there are rich walnut genetic resources in the oasis at the margin of Tarim Basin, China. These genetic resources are important for future breeding approaches of diverse cultivars.
Availability
Propagation rights of ‘Xin Xiong’ are owned by Xinjiang Academy of Forestry. Information about plant material and research on ‘Xin Xiong’ can be available from the author.
References Cited
Akça A, Yuldaşulu YB, Murad E, Vahdati K. 2020. Exploring of walnut genetic resources in Kazakhstan and evaluation of promising selections. Int J Hort Sci Technol. 7(2):93–102. https://doi.org/10.22059/ijhst.2020.299930.352.
Chen L, Ma Q, Chen Y, Wang B, Pei D. 2014. Identification of major walnut cultivars grown in China based on nut phenotypes and SSR markers. Sci Hort. 168:240–248. https://doi.org/10.1016/j.scienta.2014.02.004.
Food and Agricultural Organizaiton of the United Nations. 2024. FAO statistical yearbooks—world food and agriculture. FAO, Rome, Italy. https://doi.org/www.fao.org/faostat/. [ accessed March 22, 2024].
Hassani D, Mozaffari MR, Soleimani A, Dastjerdi R, Rezaee R, Keshavarzi M, Vahdati K, Fahadan A, Atefi J. 2020. Four new Persian walnut cultivars of Iran: Persia, Caspian, Chaldoran, and Alvand. HortScience. 55(7):1162–1163. https://doi.org/10.21273/HORTSCI15044-20.
Hassani D, Sarikhani S, Dastjerdi R, Mahmoudi R, Soleimani A, Vahdati K. 2020. Situation and recent trends on cultivation and breeding of Persian walnut in Iran. Sci Hort. 270:109369. https://doi.org/10.1016/j.scienta.2020.109369.
Hassankhah A, Rahemi M, Ramshini H, Sarikhani S, Vahdati K. 2020. Flowering in Persian walnut: Patterns of gene expression during flower development. BMC Plant Biol. 20(1):136. https://doi.org/10.1186/s12870-020-02372-w.
Hassankhah A, Rahemi M, Mozafari MR, Vahdati K. 2018. Flower development in walnut: Altering the flowering pattern by gibberellic acid application. Not Bot Horti Agrobo. 46(2):700–706. https://doi.org/10.15835/nbha46211183.
International Union for the Protection of New Varieties of Plants. 2023. Guidelines for the conduct of tests for distinctness, uniformity and stability—Walnut (Juglans regia L.). TG/125/7. UPOV, Geneva, Switzerland.
Ji FY, Ma QG, Zhang WT, Liu J, Feng Y, Zhao P, Song XB, Chen JX, Zhang JP, Wei X, Zhou Y, Chang YY, Zhang P, Huang XH, Qiu J, Pei D. 2021. A genome variation map provides insights into the genetics of walnut adaptation and agronomic traits. Genome Biol. 22(1):300. https://doi.org/10.1186/s13059-021-02517-6.
Ma QG, Yue JX, Song XB, Zhou Y, Pei D. 2019. Fruit scientific research in New China in the past 70 years: Walnut (in Chinese). J Fruit Sci. 36(10):1360–1368. https://doi.org/10.13925/j.cnki.gsxb.Z10.
Mohseniazar M, Vahdati K, Aliniaeifard S, Wang Y. 2021. Cloning and in silico characterization of GAI gene and its promoter region from dwarf/precocious and vigorous/non-precocious Persian walnut genotypes. Acta Hortic. 1315:313–318. https://doi.org/10.17660/ActaHortic.2021.1315.47.
Pei D, Lu X. 2011. Walnut germplasm resources in China. China Forestry Publishing House, Beijing, China.
Vahdati K, Sheikhi A, Arab MM, Sarikhani S, Habibi A, Ataee H. 2023. Cultivars and genetic improvement, p 402–405. In: Mir MM, Rehman MU, Iqbal U, Mir SA (eds). Temperate nuts. Springer, Singapore. https://doi.org/10.1007/978-981-19-9497-5_4.
Vahdati K, Arab MM, Sarikhani S, Sadat-Hosseini M, Leslie CA, Brown PJ. 2019. Advances in Persian walnut (Juglans regia L.) breeding strategies, p 6–13. In: Al-Khayri J, Jain S, Johnson D (eds). Advances in plant breeding strategies: nut and beverage crops. Springer, Cham. https://doi.org/10.1007/978-3-030-23112-5_11.
Wu J, He Q, Zhang C, Wang G. 2020. ‘Xiaohe 1’: A new walnut cultivar with high fruit setting rate of secondary bud germination after the harm of late frost. HortScience. 55(12):2047–2051. https://doi.org/10.21273/HORTSCI15340-20.