Walnut (Juglans regia L.), a deciduous tree in the Juglandaceae family, is a forest tree species that has a long cultivation history and wide distribution in China because of its economic importance (Pei and Lu, 2011). China accounts for more than 50% of the world’s total production of walnuts that are cultivated in more than 30 countries. In China, walnut breeding programs began throughout the country in the mid to late 1960s. In 1990, 32 early fruiting elite walnut cultivars that were public favorites were appraised and selected by the former Ministry of Forestry. Soon after, ‘Xiangling’, ‘Wen 185’, and series cultivars Liaoning and Zhonglin became the major walnut cultivars in production (NAWI, 2019).
In China, Shanxi Province produces the best-selling walnuts, including ‘Fenzhou’ and ‘Zuoquan Mian’ walnuts. The province has a long history of walnut cultivation and is China’s major walnut producer (Lv et al., 1993).
Since the 1970s, the Shanxi Academy of Forestry has conducted walnut breeding programs and introduced cultivars of foreign origin. More than 10 walnut cultivars have been bred from ecological adaptability trials of cultivars from the United States and Romania.
In recent years, however, late frost or below-freezing temperatures have occurred frequently in early spring in most areas in northern China. In 2006, 2009, 2013, 2015, and 2018, late-frost damage reduced walnut production in the northern region and caused harvest failure in some areas. For example, late frost in 2006 occurred in large areas of the Xinjiang Kashgar region, and in the central and southern parts of Shanxi Province it caused freezing damage to walnut trees and decimated production in select areas (Liu et al., 2011; Wang et al., 2007). The damage caused by late frost in 2013 resulted in significant reductions in walnut production in Shaanxi Province, particularly in Weibei region, where walnut yield loss was more than 60% (Fan et al., 2014). In 2018, a very serious late frost occurred nationwide that caused devastating production losses in most areas (Wang et al., 2020).
To alleviate loss caused by late-frost damage, the Shanxi Academy of Forestry conducted breeding programs for walnut cultivars with resistance to late frost. In Dec. 2019, the Shanxi Provincial Cultivar Certification Committee approved and certified ‘Xiaohe 1’ as an excellent walnut cultivar with resistance to late-frost temperatures as low as −6 °C. ‘Xiaohe 1’, with the assigned cultivar number Jin S-SC-JR-004-2019, has high yield, a high fruit set rate after secondary emergence of accessory and other buds, and strong late-frost resistance. The average single-fruit weight of ‘Xiaohe 1’is 12.0 to 15.5 g, the shell thickness is 1.22 mm, the suture line is tight, and the kernel yield is 59.54%. The kernel color is light, the quality is good, and the flavor is sweet.
Fan, J., Zhang, J., Xu, X., Chen, X., Chen, J., Wang, D. & Feng, J. 2014 An investigation on frozen damage of walnut trees in Shaanxi Province J. Northwest For. Univ. 29 5 2047 2051 doi: 10.3969/j.issn.1001-7461.2014.05.22
Liu, D., Zhang, B., Peng, S., Zhu, H. & Sun, H. 2011 Observation on phenological period and varieties choice in avoiding the late frost damage of early-fruiting walnut cultivars. North. Hort. (24):14–17
Lv, Z., Wang, G. & Gao, Z. 1993 High quality and high yield cultivation techniques of new walnut cultivars. Shanxi University Federation Press, Taiyuan
National Innovation Alliance of Walnut Industry (NAWI) 2019 White paper on the national innovation alliance of the walnut industry - walnut technology in China over the past 70 years. NAWI, Aksu
Pei, D. & Lu, X.Z. 2011 Walnut germplasm resources in China. China Forestry Publishing House, Beijing
Wang, Y., Chen, Y., Wang, H., Zhu, F. & Hu, Y. 2020 Research progress on late frost injury of Walnut in China Agr. Technol. 40 11 2047 2051 doi: 10.19754/j.nyyjs.20200615015
Wang, Y., Han, Y., Tian, J., Cheng, E., Wang, J. & Tian, X. 2007 Investigation of freezing injury of walnut in early spring in Shanxi Province in 2006. Chin. Fruit (1):53–54, doi: 10.3969/j.issn.1000-8047.2007.01.025
Zhao, B., Liu, F., Gong, Y., Li, D., Chang, Y. & Wang, Y. 2020 ‘Liaoning 1’ walnut cultivar HortScience 55 3 2047 2051 doi: 10.21273/hortsci14801-19