Cuttings of Malus Rootstock Resources: Comparison
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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.

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.

  • apple
  • resources
  • stock
  • cutting

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 cuttin[1]g. 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].

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][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][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

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][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][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][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][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][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][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][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].

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