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Review

Research Progress on Cuttings of Malus Rootstock Resources in China

by
Dajiang Wang
1,†,
Guangyi Wang
1,†,
Simiao Sun
1,
Xiang Lu
1,2,
Zhao Liu
1,2,
Lin Wang
1,
Wen Tian
1,2,
Zichen Li
1,
Lianwen Li
1,
Yuan Gao
1,* and
Kun Wang
1,*
1
Research Institute of Pomology, Chinese Academy of Agricultural Sciences (CAAS), Key Laboratory of Horticulture Crops Germplasm Resources Utilization, Ministry of Agriculture and Rural Affairs of the People’s Republic of China, No. 98 Xinghai South Street, Xingcheng 125100, China
2
Xinjiang Production and Construction Corps Key Laboratory of Special Fruits and Vegetables Cultivation Physiology and Germplasm Resources Utilization, Agricultural College of Shihezi University, Shihezi 832003, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Horticulturae 2024, 10(3), 217; https://doi.org/10.3390/horticulturae10030217
Submission received: 26 January 2024 / Revised: 22 February 2024 / Accepted: 22 February 2024 / Published: 24 February 2024
(This article belongs to the Section Propagation and Seeds)
Browse Figure
Versions Notes

Abstract

:
Apple (Malus Mill.) is one of the most important fruits in China, and it boasts the world’s largest cultivation area and yield. It needs to be grafted onto rootstocks to maintain a variety of characteristics. China has many apple rootstock resources that exhibit high resistance and strong adaptability; for these reasons, they are highly suited to China’s complex and diverse natural environment. In China, apple rootstock breeding began in the 1970s, and now, several rootstocks, such as the ‘GM256’ and ‘SH’ series, are widely used. However, domestic rootstock resources and varieties are difficult to root. This affects the selection, utilization, and promotion of apple rootstocks. Cutting is an important method of rooting for apple rootstocks. This study discusses the main factors that affect rooting in rootstock cutting propagation; it also summarizes the rooting ability of different apple rootstocks and presents analyses of the demand for rootstocks in the major areas of apple production in China. We present the apple rootstock resources that are suitable for the soil and climate conditions of this production. We also call for research on the cutting roots of these specific apple rootstock resources to be expanded and strengthened. It is hoped that cutting rootstocks suitable for major areas of apple production may be more easily screened and bred successfully in China in the near future.

1. Introduction

Apple (Malus Mill.) is a species of plant in the Rosaceae family, belonging to the Malus genus [1]. China is the world’s largest apple producer, accounting for more than half of global production and planting areas, and it holds a pivotal position in the international market [2]. China is rich in germplasm resources and is a global diversity center of Malus plants. There are 35 species of Malus plants in the world, 27 of which are wild species and 21 of which are native to China [3].
Apples are clonal plants that need to be grafted onto rootstocks. There are two types of rootstocks commonly used in apple cultivation: seedling rootstocks and vegetative rootstocks. Seeds are used to propagate seedling rootstocks, which have poor uniformity and are difficult to popularize on a large scale. Vegetative reproduction is advantageous in that it allows the mother plant’s best characteristics, such as high uniformity, early fruit, high yield, and high quality, to be retained [4].
Vegetative reproduction frequently employs tissue culture, layering, and cutting. Tissue culture technology has a significant effect on maintaining, purifying, or revitalizing the superior traits of the mother. It is a small-scale technology, which means it makes excellent use of space and can, therefore, improve reproduction efficiency [5]. However, its high cost and poor applicability limit its widespread use. Layering propagation is a method of propagation that involves branches. The branches are buried in soil or a substrate to grow roots and form new plants. This technical method is easy to use and produces plants quickly, but its resulting propagation volume is small, making it difficult to apply to large-scale plant production. Cutting propagation has the advantages of a short propagation cycle, easy mass production, and low cost when compared to other vegetative propagation methods [6].
The growth and development of the adventitious roots of cuttings are critical to the success of cutting propagation. The emergence of adventitious roots is a complex biological process that is influenced by numerous factors. As a result, it is critical that we review the progress of current research into apple rootstocks and the factors influencing the rooting of their cuttings; in doing so, we might breed excellent rootstocks and develop self-rooting propagation technologies. This review covers the following topics in the following order: the morphological and anatomical factors involved in the rooting of cuttings of Malus plants; internal and external factors affecting apple rootstocks; and Malus rootstock resources and the characteristics of dominant areas of apple production areas in China. Based on the above content, we present future prospects for selecting and breeding apple rootstocks that are conducive to easy cutting and rooting in the main areas of apple production in China.

2. Morphological and Anatomical Study on the Rooting of Cuttings of Malus Plants

It is generally believed that the types of rooting that cuttings undergo include the phloem rooting type (an easy rooting type), the callus-induced type (a difficult rooting type), and the mixed-rooting type (involving both phloem and callus rooting types) [7,8,9,10]. Malus halliana Koehne, Gatt. Pomac (Malus halliana) undergoes the callus-induced rooting type [11]. The growth of adventitious roots on cuttings first requires the generation of the root primordium. The adventitious root primordium can then be divided into the latent root primordium and the inductive root primordium according to formation time [12].
Malus halliana and Malus prunifolia (Willd.) Borkh. (Malus prunifolia) do not have a latent root primordium; their adventitious roots are instead formed by an inductive root primordium originating from the division and differentiation of cells at the junction of the primary ray and vascular cambium [11,13]. The root primordial of apple rootstock ‘SH40’ may be formed in the phloem, cortex, and pulp rays [14]. The adventitious roots of ‘M9’ originate from the stem’s vascular cambium cells [15]. At present, all apple rootstocks are categorized as inductive root primordium types, and adventitial roots can be formed from the vascular cambium, phloem parenchyma cells, and callus. However, there are many kinds of apple rootstocks, considering the abundance of apple resources in China [3]. Therefore, the types and anatomical structures of adventitious roots still need to be studied in more depth.

3. Internal Factors Affecting the Rooting of Apple Rootstock Cuttings

Regarding the issue of apple propagation using cuttings, extensive exploratory research has been conducted. Apple propagation using cuttings is highly dependent on both the intrinsic and extrinsic factors influencing the cuttings and the environment. Understanding the internal factors that influence the efficiency of rooting is critical for increasing the efficiency of apple propagation using cuttings.

3.1. Genetic Factors

One of the most important factors influencing the rooting of cuttings is genetics. Due to genetic differences, the rooting ability of different apple rootstock cutting varies significantly [16]. Significant differences were found in the softwood cutting effects of four rootstocks: ‘Liaozhen 2’, ‘Zhaai 76’, ‘SH40’, and ’77–34’. ‘Liaozhen 2’ had the best rooting ability and a 50% rooting rate, while ‘SH40’ had the worst rooting rate of only 20.5%, with ‘Zhaai 76’ and ‘77–34’ having rooting rate values between these two [17]. Trial results of softwood and hardwood cuttings showed that the capacity to root was in the following order: ‘MM106’ > ’M26’ > ’M9T337’. MM106, ‘MM111’, and ‘M9’ were easy to root by cutting, but ‘M3’, ‘M4’, and ‘M11’ were hard to root by cutting [18]. The rooting rate of leafy cuttings (from high to low) was as follows: Malus xiaojinensis Cheng et Jiang(Malus xiaojinensis), ‘B9’, ‘P22’, ‘MM106’; however ‘LG80’, ‘GM256’, ‘M7’, and ‘M26’ barely took root [16]. Another hardwood cutting experiment showed that ‘JM7’ was easy to root and that ‘M9’ was hard to root [19].
The expression of genes is different for plants with different cutting rooting rates. A plant’s WOX genes are especially important for the formation of adventitious roots [20,21,22]. MdWOX4a, MdWOX4b, MdWOX 5b, MdWOX11/12a, and MdWOX11/12b may play important roles in the adventitious root development of apples. Adventitious rooting ability was shown to be enhanced in MdWOX4b transgenic tobacco lines [23]. The ARRO-1 gene, isolated from the apple rootstock ‘Jork 9’, is an important gene that regulates hormone homeostasis and affects the formation of adventitious roots in apple plants. The apple rootstock ‘M26’ was transformed with an RNAi-ARRO-1 construct. The transgenic clones, as confirmed by PCR and a Southern blotting analysis, showed significantly reduced adventitious root formation both with microcuttings and stem discs, indicating the involvement of ARRO-1 in adventitious root formation [24]. The study of miRNAs and their target genes is also very important for the growth of adventitious roots in apple plants. It was shown that mdm-miR160 played a negative regulatory role in the formation of adventitious roots of apple rootstocks; the regulation of mdm-miR160a’s expression (and that of its target genes, MdARF16 and MdARF17) also significantly affected the formation of adventitious roots in apple rootstocks [25]. In apple rootstocks that were easy to root, low content of CTK inhibited the expression of MdTCP17 and promoted the expression of MdWOX11. The interaction between MdTCP17 and MdWOX11 was reduced, and MdWOX11 bound to the promoter of MdLBD29, thereby encouraging the formation of adventitious root primordia in apple [26].

3.2. Cutting Method

Cutting propagation is a method of vegetative propagation. A portion of a plant’s vegetative organ is used as the propagation material. The vegetative organs, usually young or mature branches, are inserted into a substrate to grow roots. Cuttings are classified either as hardwood cuttings or softwood cuttings based on the maturity of the branches. Hardwood cuttings propagate easy-rooting tree species, such as grapes and figs, using fully lignified annual branches. Softwood cuttings propagate from tender or semi-lignified new shoots with leaves. Softwood cuttings have greater potential for vigorous growth and are easier to root than hardwood cuttings of the same species; this can be attributed to their tender nature. Softwood cuttings, however, have stricter temperature and humidity requirements for their propagation environment than hardwood cuttings [4].
Apple cuttings were first examined by Gardner in 1929 [27]. The rooting rate of softwood cuttings was higher than that of hardwood cuttings for most apple rootstocks; however, the rooting rate was higher than 90% for Malus Begonia cyclophylla Hook. F. (Malus Begonia) [28]. The rooting rate was higher for softwood cuttings than for hardwood cuttings in Malus hupehensis (Pamp.) Rehd. (Malus hupehensis) and Malus halliana [29], as also observed in Malus prunifolia [30]. Different rooting rates were observed for different rootstocks, even for softwood cuttings. From highest to lowest, the rooting rates were exhibited by ‘MM106’, ‘M26’, and ‘T337’ when using softwood cuttings [18,19,31].

3.3. Age of the Mother Tree

The juvenile is an important factor affecting the formation of adventitious roots of apple dwarfing rootstock, and the loss of the juvenile is an important reason for rooting difficulties sometimes encountered in apple cutting propagation [16]. The age of the mother tree is crucial in the formation of adventitious roots. As the mother tree grows and its physiological development matures, it becomes one of the major reasons that cuttings are difficult to root. Young Malus xiaojinensis cuttings root at a much higher rate (94.00%) than adult Malus xiaojinensis cuttings (15.01%). Rejuvenation through tissue culture can significantly improve the rooting ability of Malus xiaojinensis cuttings [32]. The age of the mother citrus tree has been shown to have a significant impact on the survival rate of young shoot cuttings. Cuttings taken from 2-month-old, 15-year-old, and 30-year-old mother citrus trees survived at rates of 77.33%, 53.33%, and 37.99%, respectively [33,34]. For Malus prunifolia, the rooting rate of the cutting was more than 95% when it was two years old, but the rooting rate of the cuttings decreased with the increase in tree age [30]. The rooting rate of cuttings of Malus halliana also decreased with the age of the mother tree [13].

3.4. Source of Cuttings

The survival rate of cuttings is affected by their source, and studies have shown that cuttings taken from the upper part of the branch have a lower rooting rate than those taken from the base [35,36]. Branches growing beneath the tree’s canopy have fewer rooting inhibitors and more auxins, resulting in a stronger rooting ability than those growing above the canopy, which have more inhibitors and fewer auxins. The root collar’s main stem base and lateral branches are relatively tender, have a strong meristematic ability, and are easy to root [37]. The middle section of the same branch is thicker, has more vitality and rich nutrient reserves, and is relatively easy to root and sprout, meaning cuttings are more likely to survive.
The rooting rates of the middle branches of ‘Liaozhen 2’ and Malus hupehensis (Pamp.) Rehd. Var. mengshanensis G.Z. Qian were the highest, while those of the basal branches of ‘SH40’ were the highest [38,39]. The rooting rates of the middle, base, and tip cuttings of ‘Liaozhen 2’ were 88%, 63%, and 47%, respectively [38]. It was found that the rooting rate of the cuttings was significantly higher at the top than at the base, with the middle being between the two, while the rooting rate of the cuttings was significantly higher at the top than in the middle and at the base of dwarf rootstock hybrid single lines, with the exception of some lines [40]. As a result, there are significant differences between varieties, and the most appropriate cutting source must be screened using cutting experiments.

3.5. Endogenous Hormones

Endogenous hormone research is currently concerned with five categories: auxin, abscisic acid, cytokinin, gibberellin, and ethylene [41]. The formation of the root primordium is a complex process that is regulated by hormones, in which auxin plays a key role [42]. According to existing research, auxin primarily influences root growth and development by regulating the distribution of internal nutrients in cuttings, the quantity and quality of protein (including enzymes) synthesis in cuttings, and enzyme generation and activity [39]. Gibberellin stimulates root growth at the root tip but inhibits it before the root tip. It was found that applying gibberellin four days before rooting inhibited the rooting of cuttings, whereas applying gibberellin again from four to six days after rooting promoted the rooting of cuttings [43]. The rooting of cuttings is related to the present cytokinin concentration; low cytokinin concentrations promote the rooting of cuttings, while high concentrations inhibit it. Abscisic acid is an inhibitory hormone that can be reduced in order to promote rooting [44]. Ethylene can promote the germination of dormant root primordia but also inhibits the formation of induced root primordia [4]. IBA can promote the accumulation of carbohydrates and reducing sugars at the base of cuttings, promote starch hydrolysis, and increase the level of IAA at the base, thus promoting rooting [45].
After cutting, the auxin content, the ratio of IAA to ABA, the ratio of IAA to IPA + ZR, and the initiation time of advection root prima were consistent for Malus prunifolia, which proved that the early production of a large amount of auxin is necessary for cuttings. The ratio of IAA to ABA could be used to indicate the rooting ability of the cuttings of Malus prunifolia, with a large ratio resulting in a high rooting rate [13]. The difficulty of rooting ‘Jonathan’ apples is caused by the inhibiting effect of ABA [46].

3.6. Nutrients

Large amounts of nutrients are required during cutting propagation to provide the necessary energy and material basis for cuttings to take root [47]. The cuttings’ roots consume soluble sugars, which provide material support for rooting. The total soluble sugar content and rooting rate show a significant positive correlation [48,49]. In plants, soluble proteins are mostly found in the form of enzymes, and their main functions are to regulate cell growth and differentiation, coordinate material transport, and provide energy. The cuttings’ rooting rate is proportional to the ratio of carbohydrates (C) to nitrogen compounds (N), and a high C/N ratio results in a high rooting rate [50,51].
During the induction period of the adventitious roots of apple stem apex explants, the proportion of starch granules to the proportion of plastids in cambium cells increases significantly. It is speculated that these starch granules may be converted into sugars through hydrolysis to supply the energy required for the initiation of adventitious roots [52]. The rooting rate of ‘SH40’ cuttings can be improved using yellowing treatment, as this treatment can increase the starch content in the cuttings, thus increasing the soluble sugar content [14].

3.7. Endogenous Enzyme Activity

The rooting of cuttings is influenced by key enzymes such as peroxidase (POD), polyphenol oxidase (PPO), and indoleacetic acid oxidase (IAAO) [7,53]. At one time, it was believed that POD was related to the ability of cuttings to form a callus [54]. POD activity had previously been linked to the development of root primordia in cutting roots [55]. However, we now know that PPO activity influences phenolic substances’ synthesis and accumulation. High concentrations of phenolic substances accumulate in the branches, which can promote the differentiation of callus tissue [56]. IAAO is an enzyme that oxidizes and degrades indoleacetic acid. It is commonly found in the propagation of plant cuttings, and it influences the initiation of adventitious roots by regulating IAA content [53]. It was discovered that during the induction period of adventitious roots in cuttings, the activity of the three oxidases increased; meanwhile, during the formation period of adventitious roots, the activities of PPO and POD gradually decreased, and IAAO activity continued to increase [57]. The activities of PPO and IAAO decreased during the phase of adventitious roots’ elongation, while POD activity increased and then decreased.
The POD activity of apples, pears, and other difficult-to-root species increases in the early stages and decreases in the later stages. Researchers have considered that POD activity is always high after cutting; they have also hypothesized that the main reason for the difficulty in rooting in apples (among other trees) is that the POD activity does not decrease in the early stages of growth [58]. Studies have shown that the rooting rate of cuttings of Malus halliana and Malus prunifolia has a significant negative correlation with the activity of PPO [13].

4. External Factors

Internal and external factors, as well as environmental factors, are equally important for the rooting of apple rootstock cuttings. The rooting rate of apple rootstocks can be improved by controlling external and environmental factors when internal factors cannot be selected.

4.1. Temperature, Humidity, and Lighting Conditions

The temperature and humidity of the cutting environment are important factors in the rooting of leafy cuttings. High temperatures cause excessive evaporation, making cuttings susceptible to infection and decay, whereas low temperatures prevent the formation of callus tissue and adventitious roots [59,60]. Studies have shown that humidity is generally controlled at about 90% during the early stages of apple rootstocks’ leafy cutting propagation, and it then reduces to 65–80% in the middle and later stages [16,61]. The relative humidity of the air is 80–90% in the early stages of rooting, and it is reduced to 50–70% after new roots grow out [62]. For apple rootstocks, the ideal daytime temperature is 20–30 °C, and the ideal nighttime temperature is 15–20 °C [14,19,38,61,62]. The rooting of cuttings is more likely when the soil temperature is slightly higher than the air temperature [63].
Light can inhibit root development while promoting shoot development. When taking hardwood cuttings, appropriate shading can promote rooting while preventing premature bud sprouting, which can cause a water and nutrient imbalance that affects rooting [64]. Softwood cuttings are appropriate for the use of the full-light intermittent misting method, which not only promotes photosynthesis and rooting but also maintains the water balance in tender shoots, thereby improving the cuttings’ chance of survival. Light quality and intensity are also important factors influencing cutting efficiency. It has previously been shown that blue light treatment significantly increases the expression of IAA synthesis-related genes and improves the efficiency of rosemary cuttings’ growth. The growth rate of adventitious roots has been proven significantly higher under light intensity conditions of 50 µmol photons m−2 s−1 (PPFD; photon fluxdensity) than conditions of 30 PPFD [65]. When taking apple softwood cuttings, appropriate shading is usually adopted in the early stages, and it is gradually removed after rooting occurs [59,66].

4.2. Cutting Substrate

The substrate provides the cutting with a stable and suitable root environment, and the type or ratio of the substrate has a significant impact on the cutting effect [67]. An effective cutting substrate should feature good ventilation, drainage, and heat and water retention, and it should be free of diseases, pests, weeds, and other potentially harmful substances [68]. River sand and vermiculite are two examples of such a substrate. Different tree species and varieties require different substrates. The rooting rate of dwarfing sweet cherry rootstock Gisela 5 hardwood cuttings can reach up to 90% when using moss as a substrate [69]. It was found that using a substrate of river sand, vermiculite, and peat (in a volume ratio of 1:1:1) for the softwood cuttings of Armeniaca sibirica resulted in an optimal rooting rate [70].
The rooting rate of ‘JM7’, ‘M26’, and ‘M9’ apple rootstock cuttings was higher when using moss and river sand as a substrate than when using river sand alone [19]. The rooting effect of ‘Liaozhen 2’ was optimal in a mixed matrix of perlite–turf–vermiculite (1:1:1), and the same effect in individual matrices was better in perlite, followed by turf and vermiculite [9]. The rooting rate of apple rootstock ‘B9’ cuttings was the highest when vermiculite, river sand, and perlite were used in a ratio of 2:2:1 [62]. The rooting rate of cuttings in a mixed matrix of peat and perlite was the highest for the apple rootstocks ‘Liaozhen 2’ and ‘SH40’ [38].

4.3. Plant Growth Regulator

The type and concentration of plant growth regulators have a significant impact on the initiation and development of root primordia during plant cutting. Today, there are numerous plant growth additives that are available for use in plant cutting propagation [71]. Indole-3-butyric acid (IBA), naphthaleneacetic acid (NAA), ABT rooting powder, and other plant growth regulators are commonly used to promote rooting in plant cuttings [72]. The effectiveness of plant growth regulators is closely related to their type and concentration, treatment time and method, the physiological status of cuttings, and environmental factors.
One study showed that treating Malus baccata (L.) Borkh. (Malus baccata) cuttings with IBA and other plant growth regulators resulted in significantly better rooting than in a water-treated control group [73]. IBA can promote the rooting rate of Malus baccata cuttings [74]. However, other studies have shown that for Malus baccata, the rooting rate is optimal with a 300 mg/L NAA treatment [75]. It is also believed that treatment with a rooting agent increases the survival rate of Malus baccata softwood cuttings [74]. ‘MM106’ and ‘M26’ had the highest rates of rooting in IBA 3500 mg/L, while ‘T337’ had the highest rates of rooting in NAA 2500 mg/L [50]. ‘B9’ had the highest rates of rooting in IBA 1000 mg/L + NAA 100 mg/L, where the rate of rooting was 89.6% [62]. The promoting effect that hormones have on rooting is different in different plants. The effects of IAA were better than those of IBA and NAA in cutting experiments using Malus halliana and Malus prunifolia [13] (Table 1).

5. Overview of Malus Rootstock Resources and the Characteristics of the Main Areas of Apple Production in China

The Sources and Main Characteristics of Apple Rootstocks in China
Apple rootstocks in China are classified as one of two types: seedling rootstocks or vegetative rootstocks [4]. Seedling rootstocks mainly include Malus robusta (Carr.) Rehd. (Malus robusta), Malus sieversii (Led.) Roem. (Malus sieversii), and Malus baccata [3,78]. Their vegetative rootstocks are primarily ‘M9’, ‘M9T337’, ‘M26’, ‘GM256’, the ‘SH’ series, and the ‘Liaoning’ rootstock series, among others [4,78]. Most seedling rootstocks are used as rootstocks because of their remarkable stress resistance, whereas vegetative rootstocks may also be used as interstocks (Table 2).
The diverse terrain of China and its ecological and climatic conditions have resulted in a rich diversity of species and an abundance of germplasm resources that are suitable for different environments; said resources serve as the material basis for breeding fruit tree varieties with strong resistance.
Apple is the dominant species of deciduous fruit trees in China, accounting for more than 50% of the world’s area and output. Apple production is a vital industry that farmers engage with to increase their income [79]. China has a diverse range of apple-growing regions exhibiting different ecological and climatic conditions. This means that for apples to be cultivated appropriately, growers must select areas with optimal conditions.
There are five major apple production areas in China: the Bohai Bay production area, the Loess Plateau production area, the Yellow River Old Course production area, the Southwest Cold Highland production area, and the Xinjiang Specialty production area [80,81] (Table 3, Figure 1). Different production areas have different climatic conditions and require different types of rootstocks. Each region has apple rootstock resources that are suitable for local adaptation. For example, Malus robusta, Malus baccata, and Malus prunifolia are suitable for the Bohai Bay production area because they exhibit cold resistance and salt tolerance. Malus prunifolia, Malus micromalus Makino in Bot. Mag.Tokyo (Malus micromalus) and Malus kansuensis (Batal.) Schneid. (Malus kansuensis) have strong drought resistance and barrenness tolerance, making them suitable for the Loess Plateau production area. The rootstocks most suitable for the Yellow River Old Course production area are Malus zumi (Mats.) Rehd. (Malus zumi), Malus robusta and Malus hupehensis var. mengshanensis; this is due to their saline–alkali tolerance. The rootstocks most suitable for the Southwest Cold Highland production area include Malus xiaojinensis, Malus rockii, and Malus toringoides (Rehd.) Hughes. (Malus toringoides), as all three exhibit drought resistance, waterlogging tolerance, and barrenness tolerance. The rootstocks most suitable for the Xinjiang Special production area include Malus sieversii, Malus robusta, and Malus hupehensis var. mengshanensis. Malus sieversii has many variants that exhibit excellent stress resistance and significant hybridization value.

6. Prospects

The primary development strategy to be undertaken for Chinese fruit trees is the utilization of resources from the ‘four wastelands’ (wasteland mountains, hills, beaches, and gullies); this is because mountains and slopes do not compete with farmland in food production. This will require a shift in the development of the Chinese fruit tree industry. Modern dwarf and dense planting methods are inappropriate for China. Instead, research carried out to identify fruit tree varieties that are highly adaptable to barren mountains, saline–alkali land, and arid desert areas should be expanded; if this happens, we may gradually form a ‘four wastelands’ fruit tree industry with characteristics particular to China [82]. Therefore, breeding new resistant rootstocks is a project necessary for the development of China’s apple industry, and the rooting ability of these resistant apple resources is crucial for their efficient utilization. Cutting propagation has become the primary method of vegetative propagation due to its short propagation cycle, low cost, ease of operation, and high seedling uniformity.

6.1. Improving the Utilization Efficiency of Easily Rooted Resources

China has made significant progress in developing rootstock varieties that are resistant to stresses and suitable for different regions. Moreover, several rootstock resources with a strong rooting ability have been obtained as a result of screening. However, apples produced domestically are still predominantly exported, and the most commonly used mode of reproduction is layering. Through research conducted in recent years, we have selected resources that are easy to cut and root, such as ‘Zhongzhen 1’, ‘Y-1’, and Malus xiaojinensis. However, these excellent resources are not sufficiently used in production. Therefore, we must strengthen and expand the utilization of these resources and improve the corresponding efficiency of their utilization.

6.2. Exploring New Resources That are Easy to Root

China is one of the largest genetic centers of Malus plants, with abundant Malus germplasm resources and a diverse range of apple-growing regions. This means fruit trees must feature different forms of stress resistance and cultivation methods and must also be allocated to the most suitable production areas. The rooting ability of most apple rootstock resources originating in China is unknown, and there has been insufficient exploration of resources with strong rooting ability. This has limited the scale of the apple rootstock industry’s development. That being said, it implies that there is a significant research opportunity for us to identify the rooting mechanisms of apple cuttings. As a result, further cutting experiments on collected and preserved apple rootstock resources are required. Resources with particularly strong rooting ability can be pinpointed by evaluating the rooting ability of various apple rootstocks. These materials are important for studying cuttings’ rooting mechanisms and for breeding superior apple varieties, making them crucial for the development of the apple rootstock industry.

6.3. The Factors Affecting the Rooting of Apple Rootstock Cuttings: An In-Depth Study

The rooting of cuttings is affected by internal and external factors, and rooting efficiency varies between different rootstock resources. Moreover, the rooting of the same rootstock cuttings varies between scientific studies. For example, cuttings of ‘M9’ were found to take easy to root easily in Han’s study [4], but it was difficult for them to take root according to Wang’s study [19]. There are few reports on the effect of light on the rooting of apple cuttings. Both the Malus baccata and ‘SH’ series rootstocks were not easy to root, but a better rooting rate was observed after changes were made to the influencing factors [14,83,84]. Therefore, it is necessary to strengthen and expand research on the factors affecting the rooting of apple rootstock cuttings. We must analyze rooting mechanisms and adjust influencing factors to develop excellent rootstock resources that take root more easily.

6.4. The Screening and Breeding of Cuttings of Rootstock Resources That are Suitable for Rooting in the Main Areas of Production

China is one of the largest genetic centers of the apple plant, featuring numerous germplasm resources that are suitable for different production areas. After long-term cultivation and development, apple rootstocks (which are adapted to local climate and soil conditions) have been screened in each producing area and are used regularly. For example, Malus baccata is widely used in Northeast China because it exhibits strong cold tolerance. Owing to its drought–resistant and saline–alkali resistant properties, Malus robusta is widely used in North China and Northwest China. However, these rootstocks are still predominantly produced by seed reproduction, resulting in the inconsistent growth of the grafted apple trees. It is urgent that we screen these rootstocks for cutting roots or screen the cutting roots of hybrid offspring after crossing-breeding with dwarfing rootstocks so as to obtain easily reproducible apple rootstocks with strong adaptability to local climate and soil conditions.

6.5. Strengthening Cooperation among Apple-Producing Countries

At present, most vegetative propagation rootstocks are selected and bred abroad. Although there are abundant germplasm resources of Malus in China, there is a shortage of vegetative propagation rootstocks with strong adaptability; specifically, there is a lack of excellent rootstocks that are easy to cut and root. In the United States, there is rootstock ‘G.935’, which is easy to cut and resistant to replanting disease. In Japan, there is rootstock ‘JM7’, which takes root easily [4,85]. Therefore, combining the advantages of breeds from different countries to select and breed apple rootstocks with strong resistance and cuttings that take root easily will be of great practical significance for apple producers worldwide.

Author Contributions

Conceptualization, D.W., G.W. and K.W.; investigation, S.S., L.W. and W.T.; resources, L.L., X.L., Z.L. (Zhao Liu) and Z.L. (Zichen Li); writing—original draft preparation, D.W. and G.W.; writing—review and editing, D.W.; supervision, Y.G. and K.W. All authors have read and agreed to the published version of the manuscript..

Funding

This research was partly funded by the Agricultural Science and Technology Innovation Program (CAAS-ASTIP-2021-RIP-02).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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Horticulturae 10 00217 g001
Figure 1. The five major apple production areas in China [80,81].
Figure 1. The five major apple production areas in China [80,81].
Horticulturae 10 00217 g001
Table 1. Optimal conditions for softwood cutting propagation of apple resources and rootstocks.
Table 1. Optimal conditions for softwood cutting propagation of apple resources and rootstocks.
No.NameSubstrate (v/v)Hormone ConcentrationTreatment TimeRooting Rate
1Malus xiaojinensis [16]River sand, grass charcoal (1/1)IBA 3000 mg/L1min77.33%
2Malus prunifolia var.ringo [18]Grass charcoalIBA 1500 mg/L10s100.00%
3Malus ‘Hongying’ [76]VermiculiteNAA 1000 mg/L5s67.00%
4Malus baccata [73]River sandIBA 2000 mg/L5s90.00%
5Malus hupehensis var. mengshanensis [39] VermiculiteNAA 600 mg/L10s40.00%
6‘Liaozhen 2’ [38]Perlite, peat (1/1)IBA 1000 mg/L30s87.30%
7‘SH40’ [38]Peat, coco, perlite (1/1/1)IBA 1000 mg/L30s55.10%
8‘Y-1’ [77]Peat, perlite, vermiculite (1/2/4)IBA 2000 mg/L5s52.67%
9B9 [62]Vermiculite, river sand, perlite (2/2/1)IBA 1000 mg/L30s85.00%
10JM7 [19]River sandIBA 2000 mg/L30s60%
Table 2. Sources and main characteristics of dominant resources and rootstocks used in China [3,4].
Table 2. Sources and main characteristics of dominant resources and rootstocks used in China [3,4].
No.NameOrigin or Breeding UnitMain Form(s) of Stress Resistance
1Malus sieversiiXinjiang, Chinasaline–alkali tolerance, barren tolerance
cold resistance
2Malus baccatNortheast, Northwest, and North Chinacold resistance, waterlogging tolerance
drought resistance
3Malus robustaHebe, Chinasaline–alkali tolerance
4Malus hupehensisSouthwest, Central, and North Chinawaterlogging tolerance, drought resistance
5Malus rockii Schneid. (Malus rockii)Yunnan, Sichuan, Tibet, Chinawaterlogging tolerance, drought resistance, saline–alkali tolerance
6Malus hupehensis var. mengshanensisShandong, Chinawaterlogging tolerance, apomixis
7Malus xiaojinensisSichuan, Chinasaline–alkali tolerance, barren tolerance
cold resistance, waterlogging tolerance
8‘SH’ seriesInstitute of Fruit Trees, Shanxi Academy of Agricultural Sciences, Chinacold resistance, drought resistance, lodging resistance
9‘Liaoning’ rootstock seriesInstitute of Fruit Trees, Liaoning Academy of Agricultural Sciences, Chinacold resistance
10‘CX’ seriesResearch Institute of Pomology, CAAScold resistance
Table 3. The five major apple production areas and the corresponding suitable rootstock resources.
Table 3. The five major apple production areas and the corresponding suitable rootstock resources.
No.Production AreaSuitable Rootstock Resources
1Bohai Bay production areaMalus robusta, Malus prunifolia, Malus baccata.
2Loess Plateau production areaMalus prunifolia, Malus micromalus, Malus kansuensis
3Yellow River Old Course production areaMalus zumi, Malus robustar, Malus hupehensis var. mengshanensis
4Southwest Cold Highland production areaMalus xiaojinensis, Malus rockii, Malus toringoides
5Xinjiang Specialty production areaMalus sieversii, Malus robusta, Malus hupehensis var. mengshanensis
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Wang, D.; Wang, G.; Sun, S.; Lu, X.; Liu, Z.; Wang, L.; Tian, W.; Li, Z.; Li, L.; Gao, Y.; et al. Research Progress on Cuttings of Malus Rootstock Resources in China. Horticulturae 2024, 10, 217. https://doi.org/10.3390/horticulturae10030217

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Wang D, Wang G, Sun S, Lu X, Liu Z, Wang L, Tian W, Li Z, Li L, Gao Y, et al. Research Progress on Cuttings of Malus Rootstock Resources in China. Horticulturae. 2024; 10(3):217. https://doi.org/10.3390/horticulturae10030217

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Wang, Dajiang, Guangyi Wang, Simiao Sun, Xiang Lu, Zhao Liu, Lin Wang, Wen Tian, Zichen Li, Lianwen Li, Yuan Gao, and et al. 2024. "Research Progress on Cuttings of Malus Rootstock Resources in China" Horticulturae 10, no. 3: 217. https://doi.org/10.3390/horticulturae10030217

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