Bunge, a member of the Sapindaceae family, is a deciduous shrub or tree that is primarily distributed in north China. Xanthoceras sorbifolium is considered to be an ideal oil-rich seed tree species that has significant ecological, economic, and medicinal value (Ao, 2012; Liu et al., 2017). It can grow well in cold, barren environments, and even in saline-alkali land. It is also a valuable ornamental landscaping plant because of its beautiful form and luxuriant, colorful flowers. It has a long blooming period and flowers later than apricot (Armeniaca vulgaris Lam.) and peach (Prunus persica L.), therefore, it can fill the gap of blossom appreciation in spring (Wang et al., 2020; Yang, 2019; Zhou, 2020). Because of its strong stress resistance, multiple valuable traits, and outstanding landscape performance, X. sorbifolium has been the subject of extensive breeding efforts, and significant progress has been made in cultivation of ornamental cultivars such as Sen Miao, Yan Hua, and Du Xiu.

‘Yan Liu’ was selected from abundant morphological variations in X. sorbifolium and cultivated by grafting. The leaves of ‘Yan Liu’ are narrow and long, similar to those of willow. Compared with other X. sorbifolium cultivars, Yan Liu exhibits extremely large leaf spacing. Cultivars with traits similar to those of Yan Liu have not been reported to date. This novel cultivar combines adaptability and ease of cultivation with a unique appearance, and its study has important implications for landscape planting and enrichment of ornamental X. sorbifolium germplasm resources.

Origin

In 2018, the maternal parent plant of ‘Yan Liu’ was discovered in an orchard in Chaoyang City, Liaoning Province, China, because of its unique leaves and petals. The variety was cultivated by grafting. The branches of the maternal parent plant were collected and grafted onto 1-year-old X. sorbifolium rootstocks in July 2018. The second generation was propagated in Mar. 2019, and the scions were collected from the first generation of grafted plants. The survival rate of the two consecutive grafts was more than 95%. To date, several generations have been propagated. After years of observation, all grafted plants showed stable morphological traits consistent with those of the maternal parent tree. The traits were genetically stable, indicating that ‘Yan Liu’ was successfully cultivated. ‘Yan’ was chosen to represent Dr. Yan Ao, the discoverer of the novel cultivar, and ‘Liu’ is the pronunciation of willow in Chinese, representing the willow-like shape of its leaves. Furthermore, the seeds of the cultivar also can be used to produce oil that is highly valuable for cooking, medicine, and biofuel.

Plant Characteristics

Tree.

‘Yan Liu’ could grow into a deciduous shrub or tree with a height of 5 to 8 m according to the site environmental conditions. The annual branches are green (L* = 76, a* = –13, b* = 48).

Flowers.

‘Yan Liu’ has racemose inflorescences, and the flowers bloom before or at the same time as the leaves sprout. Each flower has five petals, one pistil, eight stamens, and a torus with five golden (L* = 78, a* = 22, b* = 75) hornlike appendages. The petals are long lanceolate and slightly curled. The base and apex of the petals are acuminate (Fig. 1A). Compared with the similar cultivar ‘Sen Miao’ (Fig. 1B), whose petals are explanate and obovate, the petals of ‘Yan Liu’ are slenderer and lighter in color. The petal is ≈3 cm long and 1 cm wide. The petal color changes gradually as the flowers mature. The bases of the petals of ‘Yan Liu’ are yellow-green (L* = 67, a* = –2, b* = 57) in the early bloom period and light red (L* = 33, a* = 28, b* = 20) in the full bloom period. The edges of the petals are white (L* = 100, a* = –1, b* = 2) throughout the bloom period. ‘Yan Liu’ begins to bloom at the beginning of May, and the flowering period lasts ≈20 d.

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Comparison of the petals of ‘Yan Liu’ (A) and ‘Sen Miao’ (B).

Citation: HortScience 57, 7; 10.21273/HORTSCI16616-22

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Leaves.

The imparipinnate leaves of ‘Yan Liu’ are green (L* = 35, a* = –17, b* = 17) and slightly curled (Fig. 2A). The narrowly lanceolate leaves (≈5.1 cm long and 1.0 cm wide) resemble willow leaves. By contrast, the leaves of the similar cultivar Sen Miao are mostly broad-lanceolate or subovate (Fig. 2B). A comparison of the characteristics of ‘Yan Liu’ and ‘Sen Miao’ is shown in Table 1. The average ratio of leaf length to width of ‘Yan Liu’ (5.0) is greater than that of ‘Sen Miao’ (3.1). The average leaf spacing is ≈2.7 cm, twice that of ‘Sen Miao’. The leaves sprout in early April and fall in the middle of October.

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Comparison of the leaves of ‘Yan Liu’ (A) and ‘Sen Miao’ (B).

Citation: HortScience 57, 7; 10.21273/HORTSCI16616-22

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Table 1.

Characteristics of ‘Yan Liu’ and ‘Sen Miao’.

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Fruits and seeds.

The fruit is a capsule with three carpels. The fruit is globose with a transverse diameter of ≈4 to 6 cm and a longitudinal diameter of ≈4 to 8 cm. The pericarp is green (L* = 61, a* = −35, b* = 43) and becomes yellowish-brown (L* = 40, a* = 8, b* = 35) at maturity. The seeds are spherical, black (L* = 17, a* = 1, b* = 3), and glossy. The new cultivar has the character of high-yielding. The seed yield of most 10-year-old trees can reach ≈1500 kg/ha with ideal field management.

Cultivation

‘Yan Liu’ is highly adaptable, deep-rooted, and sun-loving, showing tolerance to cold and drought. It grows well on deep, well-drained soils with salinity below 0.5% and alkalinity below 8.8, especially soils with depth ≥50 cm and slope ≤30°. ‘Yan Liu’ is mainly distributed from 28°34' to 47°20' N and from 73°20' to 120°25' E. It can be planted in China from the west to Xinjiang, the northeast to Liaoning, the north to Inner Mongolia, and the south to Henan. It can be propagated in warm temperate climate zones with altitudes between 52 m and 2260 m, annual average temperatures from 3 to 18 °C, and annual average rainfall greater than 100 mm.

‘Yan Liu’ is mainly cultivated by bud or branch grafting in spring or summer. One-year-old robust X. sorbifolium seedlings can be used as rootstock, and vigorous branches with strong development and healthy, plump buds as scions. Furthermore, cutting is also suitable for the propagation of ‘Yan Liu’. After grafting and cutting, plants should be fertilized and watered. It is suggested that plant transplanting be carried out in early spring, and the roots should be protected. The planting density ranges from 1050 to 1650 plants per hectare, depending on soil fertility, the slope of forest land, and the land preparation method.

Watering is recommended three times a year: before flowering, during fruiting, and before the soil freezes. Timely drainage during the rainy season is important for avoiding root rot. Fertilization is suggested three times a year. Nitrogen fertilizer should be applied before flowering and fruiting, and potassium fertilizer should be applied during fruit expansion and ripening (Ao et al., 2018). The type of fertilizer depends on the soil conditions. Watering should be performed after fertilization, and plants should be fertilized after harvest in July or August to promote the accumulation of organic nutrients.

Plants should be pruned and trimmed during the dormancy period, focusing on overcrowded, overlapping, and weak branches, as well as branches with pests and diseases. At the same time, attention should be paid to the control of pests and diseases, including root rot, sooty blotch, wilt disease, Serica orientalis, and Psyllidae (Liu, 2022).

Availability

Small samples of ‘Yan Liu’ are available commercially from Beijing Forestry University. Requests for scions for research purposes may be addressed to Dr. Yan Ao (E-mail: aoyan316@163.com).