In fruit tree cultivation, an ideal rootstock should possess the ability to be easily propagated asexually, have strong environmental adaptability, exhibit strong compatibility with grafting, and effectively improve the growth of scions and enhance fruit quality. These are also the main breeding objectives for rootstocks of stone fruit trees.
3.1. Efficient Clonal Propagation
Clonal rootstocks, due to their identical genotypes, exhibit minimal genetic variation among individuals, which allows for the maximum maintenance of uniform scion traits, thereby stabilizing yield and quality. In production, there are often potential rootstock varieties that possess multiple desirable traits but are unable to be propagated asexually on a large scale due to difficulties in adventitious root formation. Therefore, evaluating the adventitious root-forming ability of rootstocks is an important aspect of rootstock breeding. When breeding stone fruit rootstocks with ease of asexual reproduction as the primary selection trait, screening from a large pool of materials with rich genetic variation is a viable approach. Korkmaz et al. [
5] compared the adventitious root formation ability of 79 plum rootstock cuttings, selecting 39 superior materials based on a threshold of root length of 33.50 mm, root number of 3.00, and rooting rate of 30.00%. Evaluating the rooting ability of rootstocks with other desirable traits is a method for further selection. A comparison regarding the adventitious root formation ability and quality of 30 rootstocks potentially tolerant to peach tree short life syndrome (PTSL) showed that ‘GL-ERA-09-33’ (
P. persica) had the best rooting efficiency, making it an excellent rootstock for tolerance to PTSL [
6]. Furthermore, an evaluation conducted by Rosa et al. [
7] on the adventitious root formation ability of semi - hardwood cuttings from four
Prunus rootstocks—‘Okinawa’ (
P. persica), ‘Tsukuba-1’ (
P. persica), ‘Julior’ (
P. insititia ×
P. domestica), and ‘Mirabolano 29C’ (
P. cerasifera); it was found that ‘Mirabolano 29-C’ had a rooting rate exceeding 80% with the highest number and quality of adventitious roots. Among five
Prunus rootstocks (‘Adafuel’ (
P. dulcis ×
P. persica), ‘Adarcias’ (
P. dulcis ×
P. persica), ‘Cadaman’ (
P. persica ×
P. davidiana), ‘Garnem’ (
P. dulcis ×
P. persica), and ‘GF 677’ (
P. dulcis ×
P. persica)), it was observed that the hardwood cuttings of ‘Garnem’ showed the best adventitious root formation ability, while ‘GF677’ and ‘Cadaman’ were less likely to root [
8]. The difficulty in rooting ‘Cadaman’ is associated with a lower ratio of carbohydrates to nitrogen [
9], while the difficulty in adventitious root formation for ‘GF677’ may be due to its higher activities of putrescine, polyamine oxidase, and catalase, as well as lower activity of diamine oxidase [
10].
3.2. Abiotic Stress Tolerance
Stone fruit trees exhibit a rich diversity of species and demonstrate remarkable adaptability to a wide range of environments. However, they often face various environmental stresses, including drought, waterlogging, soil salinization, and nutrient deficiencies. To mitigate these challenges, breeders have undertaken extensive screening efforts to select rootstocks with enhanced environmental adaptability. This review synthesizes current research progress in these key areas.
Temperature Stress: Temperature is a crucial factor affecting the growth of stone fruits. Differences in temperature due to geographical latitude directly limit the ecological distribution of these fruits. Within suitable habitats, excessively high temperatures in summer or excessively low temperatures in winter can cause growth stagnation or even death of these plants. Therefore, in the context of global climate change, the breeding of heat-tolerant and cold-tolerant rootstocks is of significant importance for maintaining the normal growth of stone fruits and expanding their cultivation areas. The cold tolerance of 48 cherry hybrid rootstocks was tested by Blážková et al. [
11], using ‘Colt’ (
P. avium ×
P. pseudocerasus) as a control, and they identified ‘CPHVODÁRNA,’ ‘CPH 43,’ ‘CPH 17,’ ‘CPH 22,’ and ‘CPH 49’ as new cold - tolerant rootstocks. Another study comparing the growth and physiological state of scions grafted onto cherry rootstocks ‘Gisela 5’ (
P.cerasus ×
P.canescens) and ‘Mazzard’ (
P. avium) under cold adaptation conditions showed that ‘Mazzard’ was better able to maintain fruit sugar metabolism and had higher soluble sugar content, making it a more cold - tolerant cherry rootstock [
12]. In addition, the response of seven
Prunus rootstocks (‘Garnem,’ ‘GF 677’, ‘Marianna 2624’ (
P. cerasifera ×
P. musonianna), ‘Myrobolan 29C’ (
P. cerasifera), ‘Rootpac 20’ (
P. besseyi ×
P. cerasifera), ‘Rootpac 40’ ((
P. amygdalus ×
P. persica) × (
P. amygdalus ×
P. persica)), and ‘Rootpac R’) to drought, heat shock, and their combination revealed that ‘Rootpac 40’ and ‘Marianna 2624’ consistently showed superior tolerance under stress, exhibiting a stem length increase of up to 30% and a 25% increase in stomatal conductance [
13].
Water stress: Water stress encompasses both drought and waterlogging, with the former preventing roots from absorbing adequate water and inhibiting various biological processes, while the latter causes oxygen deprivation in the roots, leading to the loss of normal biological functions. In cherries, a comparison of the root structure and anatomical characteristics of different genotypic rootstocks revealed that ‘SV2-7’ (
P. fruticosa) and ‘OV14’ (
P. cerasus) are drought tolerant rootstocks [
14]. Additionally, ‘Daqingye’ (
P. pseudocerasus) and ‘Gisela 6’ (
P. cerasus ×
P. canescens) were found to have higher levels of ROS scavenging ability and were more tolerant to waterlogging [
15]. In peaches, studies have shown that the wild almond (
P. ramonensis) exhibits higher drought resistance than the common peach rootstocks used in orchards, and is considered a high-quality material for breeding highly resistant rootstocks [
16]. The rootstock ‘MP-29’ (
P. umbellata ×
P. persica) is more flood-tolerant than the commonly used rootstock ‘Flordaguard’ (
P. persica), making it suitable for promotion in subtropical regions prone to flooding [
17]. Comparative studies of peach and plum rootstocks have found that among ‘Mr. S. 2/5’ (
P. cerasifera), ‘Monegro’ (
P. persica ×
P. dulcis), and ‘Nemared’ (
P. persica ×
P. davidiana), the plum rootstock ‘Mr. S. 2/5’ has superior root porosity and greater tolerance to flooding [
18]. Klumb et al. [
19] also found that the plum rootstock ‘Marianna 2624’ has stronger waterlogging tolerance than the peach rootstock ‘NR0170401’ (
P. persica) by assessing gas exchange parameters and changes in gene expression involved in glycolysis and ethylene metabolism under flooded conditions.
Salinization: Soil salinization is another factor that affects the growth of root systems in stone fruit rootstocks. Over the past decade or so, there has been some success in the breeding of salt-tolerant rootstocks. A comprehensive salt tolerance analysis of seven
Prunus rootstocks was conducted, identifying ‘Mariana 2624’, ‘Garnem’, and ‘Colt’ as having strong salt tolerance, with ‘Mariana 2624’ being recognized as the most salt - tolerant among the plum rootstocks [
20]. Among peach and almond rootstocks, ‘Empyrean 1’ (
P. persica ×
P. davidiana), ‘Cornerstone’ (
P. persica ×
P. dulcis), and ‘Bright’s hybrid 5’ (
P. dulcis ×
P. persica) also exhibit strong salt tolerance [
21]. Furthermore, the high salt tolerance of the almond rootstock ‘Empyrean 1’ was further confirmed, with suggestions that its mechanism may involve upregulating ion transport proteins and enhancing the deposition of suberin and lignin in the root cortex to cope with salt stress [
22].
Nutritional stress: The essential nutrients required by stone fruits include macronutrients such as N, P, K, and Ca, as well as micronutrients like Fe, Mn, and Zn, with both excess and deficiency of these nutrients leading to poor plant growth. Under nutrient-poor soil conditions, rootstocks with efficient nutrient uptake can help reduce the incidence of physiological diseases in the scion and decrease the amount of fertilizer needed. Reports show significant differences in the kinetic parameters of NO3
− and NH4
+ absorption among different peach rootstocks, with varieties such as ‘Aldrighi’ (
P. persica), ‘Tsukuba1’ (
P. persica), and ‘Clone 15’ (
P. persica) possessing a high-affinity transport system for NH4
+, which facilitates nutrient translocation to the scion [
23]. Similarly, the peach rootstock ‘Shannong-1’ exhibits higher nitrogen use efficiency compared to ‘Maotao’, a difference that is associated with the differential expression of nitrogen metabolism regulatory genes [
24]. Additionally, the peach rootstock ‘Garnem’ may exhibit higher calcium use efficiency than the peach rootstock ‘GF677’ due to its larger cortical cells and secondary enlarged xylem vessels [
25]
. However, other reports suggest that ‘GF677’ has stronger tolerance to iron chlorosis, which may be attributed to the high expression of genes related to iron ion transport proteins, defense systems, and photosynthesis [
26]. Additionally, using tolerant rootstocks to iron chlorosis is the best choice to prevent chlorosis. An evaluation of 17
Prunus rootstocks, conducted by measuring root ferric chelate reductase enzyme activity, leaf SPAD values, and field performance, identified ‘Adesoto’ (
P. insititia), ‘Felinem’ (
P. dulcis ×
P. persica), ‘GF 677’, ‘Krymsk 86’ (
P. cerasifera ×
P. persica), and ‘PAC 9921-07’ ((
P. besseyi ×
P. salicina) ×
P. armeniaca) as having stronger tolerance to iron chlorosis [
27]
. In soils with excess elements, tolerant rootstocks are also a useful tool for improving scion growth. Studies have found that in soils with high zinc content, the cortical cells of the root systems of peach and plum rootstocks ‘Rigitano’ (P. mume) and ‘Tsukuba-1’ (P. persica) are prone to rupture, while ‘Flordaguard’ shows no significant changes in root tip morphology and anatomy, making it recommended as a rootstock with high zinc tolerance [
28].
3.3. Biotic Stress Tolerance
In the production of stone fruit trees, pests and diseases not only directly cause poor growth but also lead to a decline in fruit quality and yield loss. The main pests and diseases affecting stone fruit trees include sharka, crown gall, root rot, and nematodes.
Viral diseases:Plum pox virus (PPV) is one of the most dangerous viruses affecting stone fruit trees, causing host leaves to curl and lose their green color, leading to fruit deformities, altering fruit composition, and even resulting in premature fruit drop [
29,
30]. PPV typically spreads through infected propagation materials and aphid vectors, and has now been reported in at least 33 countries [
30]. To develop rootstocks resistant to PPV, Polák and Oukropec [
31] evaluated the resistance of rootstocks ‘Barier’ (
P. davidiana ×
P. persica), ‘Fire’ (
P. amygdalus ×
P. persica), ‘Cadaman’, ‘GF677’, ‘MRS’ (
P. cerasifera ×
P. spinosa), ‘NBS 540-73’ (
P. cerasifera ×
P. holoserica ×
P. domestica), and ‘Pumiselect’ (
P. pumila), finding that ‘GF677’ exhibited high resistance to PPV, while ‘Cadaman’ and ‘Fire’ showed moderate resistance. Subsequently, Polák and Komínek [
32] further discovered that ‘GF677’ and ‘Myrobalan BN4Kr’ (
P. cerasifera) rootstocks are more suitable for propagating plums, apricots, and peaches in areas affected or threatened by PPV. A recent report also indicates that ‘GF677’ is resistant to a unique PPV strain, PPV-T, found in Turkey, while common rootstocks such as ‘Nemaguard’ (
P. persica ×
P. davidiana), ‘Myrobalan 28B’ (
P. cerasifera), and ‘Myrobalan 29C’ (
P. cerasifera) are sensitive to PPV-T [
33].
Bacterial diseases: Crown gall, caused by
Agrobacterium tumefaciens, is one of the most susceptible bacterial diseases for stone fruit saplings such as apricot, peach, and almond. An evaluation was conducted to screen for resistant rootstocks to mitigate the damage of this disease to stone fruit production, assessing the resistance of
Prunus rootstocks ‘GF677’, ‘Antafuel’ (
P. amygdalus ×
P. persica), ‘St.Julien GF 655/2’ (
P. insititia ×
P. domestica), and ‘Gisela 5’, and the results showed that ‘St.Julien GF 655/2’ is insensitive to
A. tumefaciens and exhibits the strongest resistance [
34]. Bacterial canker, caused by
Pseudomonas syringae pv syringae (Pss), is a serious disease of stone fruit orchards causing severe yield reductions and the death of entire trees.
Fungal diseases: Root rot mediated by
Armillaria mellea or
A. tabescens is one of the leading causes of death in stone fruit trees. According to existing reports, the plum rootstock ‘Sharpe’ (
P. angustifolia ×
Prunus spp.) and ‘MP-29’ exhibit resistance to
A. tabescens [
35,
36]. ‘Myrobalan’ (
P. cerasifera) and its interspecific hybrids not only show some resistance to
A. tabescens but also to
A. mellea [
37].
Pest damage: Root-knot nematodes (RKN), including
Meloidogyne hapla,
M. arenaria,
M. incognita, and
M. javanica, are among the most damaging pests to stone fruit trees. Since 1929, breeders have developed some peach rootstocks resistant to RKN, including ‘Nemaguard’, ‘Hansen’ (
P. persica ×
P. amygdalus), ‘Flordaguard’, ‘Okinawa’, and ‘Nemared’ [
38]. Eliwa et al. [
39] evaluated that most genotypes of the local Egyptian peach variety ‘Mit-Ghamer’ (
P. persica) exhibit resistance or moderate resistance to
M. incognita and
M. javanica. Unlike peach rootstocks, which are specifically resistant to certain types of RKN, the plum rootstock ‘Myrobalan’ has been found to be completely resistant to over 30 species of RKN, significantly mitigating the damage they cause to plum production [
40]. Subsequent research further revealed that the broad-spectrum resistance of ‘Myrobalan’ is primarily attributed to three resistance genes:
Mal,
Ma2, and
Ma3 [
41], which may activate the phenylpropanoid and flavonoid metabolic pathways to produce defensive secondary metabolites [
42].
3.4. Graft Compatibility
The grafting compatibility between the scion and the rootstock is a crucial factor in determining the performance and longevity of an orchard. Generally, varieties and species with close genetic relationships exhibit good scion-rootstock compatibility, which is specifically manifested by the ability to establish effective vascular connections between the scion and rootstock. Scion-rootstock incompatibility is typically divided into two scenarios: the first is "displacement" graft incompatibility, which manifests in the first year after grafting as growth cessation, leaf drop, and leaf discoloration; the second is "localized" incompatibility, which appears later in development as a disruption in the continuity of vascular and cambial patterns. This also indicates that scion-rootstock incompatibility is delayed and unpredictable, often leading to significant economic losses. Therefore, it is essential to clearly understand scion-rootstock compatibility.
Apricot: When selecting rootstocks for apricot scions, there is a strong universality in choice, as apricots have moderate compatibility with plum rootstocks (
P. cerasifera,
P. salicina,
P. mariana,
P. insititia,
P. domestica). A comparison of the grafting compatibility of 13 apricot varieties with four plum rootstocks: ‘Marianna 2624’, ‘Miragreen’ (
P. cerasifera ×
P. davidiana), ‘Mirared’ (
P. cerasifera × (
P. persica ×
P. davidiana)), and ‘Montclar’ (
P. persica), found that over 90% of apricot varieties showed good compatibility with ‘Miragreen’ and ‘Mirared’, making them suitable as universal rootstocks for grafting apricot [
43]. The Aula Dei Experimental Station observed through anatomical studies that apricot varieties (‘Búlida’, ‘Canino’, ‘Moniquí’, and ‘Paviot’) all exhibited good grafting compatibility with hexaploid plum rootstock (
P. insititia) [
44]. Moreover, apricot scions can also successfully graft onto peach, almond, or their hybrid seedlings [
45]. Similarly, sand cherry (
P. pumila,
P. besseyi) and its interspecific hybrids are also suitable as rootstocks for apricots, but apricots should not be grafted onto nanking cherry (
P. tomentosa) [
45].
Plum: The selection of rootstocks for plum scions also exhibits considerable diversity. The Aula Dei Experimental Station tested the grafting compatibility of five European plum (
P. domestica) varieties and six Chinese plum (
P. salicina) varieties with 38 hybrid rootstocks. They found that Chinese plums ‘Angeleno’, ‘Black Amber’, ‘Delbarazur’, ‘Songold’, and European plums ‘President’ and ‘Reine Claude Tardive of Chambourcy’ showed perfect grafting compatibility with all tested rootstocks. In contrast, ‘Reine Claude Verte’ exhibited localized incompatibility with ‘Myrobalan B’ and ‘Myrobalan GF 3-1’ in the second year after budding, and ‘Stanley’ was only compatible with plum rootstocks [
46].
Cherry: The selection of rootstocks for cherry scions is highly specific. For example, ‘Mazzard’ (
P. avium), derived from sweet cherry seedlings, shows good grafting compatibility with all varieties of sweet cherry but exhibits incompatibility with rootstocks from different species, such as
P. mahaleb,
P. cerasus,
P. canescens,
P. pseudocerasus,
P. fruticosa, and their hybrid progeny [
47]. The accumulation of phenolic compounds and increased peroxidase activity may be the reasons for grafting incompatibility between cherry species or within the same species [
48,
49].
3.5. Dwarfing
Dwarfing rootstocks can control the growth vigor of fruit trees, effectively improve land use efficiency, increase yield per unit area, and facilitate mechanized harvesting and orchard management, thereby reducing labor costs in production.
Cherry: Sweet cherries are the first stone fruit trees to have commercial rootstocks with comprehensive vigor control, and the breeding effects of dwarfing rootstocks are the most systematic and significant. The semi-dwarfing rootstocks, including ‘Gisela 5’, ‘P-HL-C’ (
P. avium ×
P. cerasus), and ‘Weiroot 158’ (
P. cerasus), reduce tree height by about 50%; the full-dwarfing rootstocks, such as ‘Gisela 3’ (
P. cerasus ×
P. canescens), ‘Lake’ (
P. avium ×
P. fruticosa), ‘Clare’ (
P. avium × (
P. cerasus ×
P. fruticosa)), ‘Cass’ (
P. avium × (
P. cerasus ×
P. fruticosa)), ‘Crawford’ (
P. cerasus × (
P. cerasus ×
P. canescens)) and ‘Weiroot 53’ (
P. cerasus), reduce tree height by about 60% [
50].
Peach: Up to now, breeders have also developed peach rootstocks with varying degrees of dwarfing. The ‘Sirio’ rootstock induces trees that are about 40% smaller than those of ‘GF77’ and can improve fruit yield and quality[
38]. Compared with the standard rootstock ‘Nemaguard’, ‘Controller 5’ (
P. salicina ×
P. persica) reduces tree vigor by 50-60%, while ‘Controller 9’ (
P. salicina ×
P. persica) reduces it by about 90% [
51]. Among ‘Rootpac 20’, ‘IRTA-1’, ‘Adesoto’, ‘Ishtara’, ‘Rootpac R’, and ‘Rootpac 40’, ‘Rootpac 20’ is the most dwarfing rootstock [
52]. In addition, a peach dwarf mutant caused by a single nucleotide mutation in the gibberellin receptor GID1 has also been discovered, which has the potential to become an excellent dwarfing rootstock [
53].
Plum and apricot: Breeding of dwarfing rootstocks for plums and apricots has been underway for over 20 years, but progress in selection has been relatively slow, with preliminary evaluations of dwarfing rootstocks still ongoing in many regions. A recent study preliminarily selected 13 effective dwarfing rootstocks with strong rooting ability from 79 wild plum germplasms in the Euphrates River region [
5]. A 16-year grafting experiment conducted at the Aula Dei experimental station found that the plum rootstock ‘Miral 3278 AD’ (
P. cerasifera × P. amygdalus) significantly reduced the growth vigor of two European plum scions (‘Reine Claude of Bavay’ and ‘Reine Claude Tardive of Chambourcy’), while also promoting higher yields, making it a good alternative to the commercial rootstock ‘Myrobalan’ [
54]. However, among the current major dwarfing rootstocks for plums and apricots, including ‘Krymsk 1’ (
P. tomentosa× P. cerasifera), ‘St. Julien GF655/2’ (
P. insititia), ‘Wavit’ (
P. domestica), ‘Pixy’ (
P. insititia), and ‘Pumiselect’ (
P. pumila), most only achieve a semi-dwarfing effect (REF). In contrast, there are reports that two commonly used European plum rootstocks in Italy, ‘Penta’ (
P. domestica × P. cerasifera) and ‘Tetra’ (
P. domestica), can induce apricot trees to achieve a semi-dwarf to dwarf stature [
55].
3.6. Others
Rootstock varieties have functionally differentiated effects on the yield and quality of drupes, with specific rootstocks potentially improving one or several quality indicators of the fruit. In peaches, the rootstock ‘KL-38’ (
P. cerasifera) can increase the total phenol, total flavonoid, and total monomeric anthocyanin content to the highest levels, while the rootstocks ‘NG-1’ ((
P. davidiana ×
P. persica) × (
P. dulcis ×
P. persica)) and ‘NGF-14’ ((
P. davidiana ×
P. persica) ×
P. amygdalus) can enhance the total antioxidant content [
56].The rootstocks ‘Adarcias’ and ‘Cadaman’ can promote the accumulation of sugars, phenols, and flavonoids in peaches [
57]. The peach rootstock ‘Rootpac 40’ can maintain good levels of soluble solids content and titratable acidity while also increasing fruit size [
58].
However, research has found that the effects of rootstocks can be contradictory between fruit yield and quality, and even among different quality indicators. For example, ‘GF677’ significantly improves iron chlorosis tolerance but results in the smallest fruit size, while the rootstocks ‘Barrier1’ (
P. persica ×
P. davidiana) and ‘Citation’ (
P. persica ×
P. salicina) can significantly increase the acidity of fruit but reduce its nutritional value [
59]. The peach-plum hybrid rootstock ‘PS’ (
P. persica ×
P. cerasifera) can increase the sugar, total phenol, and ascorbic acid content in nectarines, enhancing their antioxidant activity, but it is sensitive to iron chlorosis [
58,
60]. Similarly, the sweet cherry rootstock ‘Pikú 3’ (
P. pseudocerasus × (
P. canescens ×
P. incisa)) may result in lower yields for the ‘Newstar’ variety, but it produces larger fruits with higher soluble solids content and firmness, showing potential for producing high-quality sweet cherries [
61]. Milošević et al. [
62] found that rootstocks with stronger growth vigor significantly reduced fruit weight, size, soluble solids content, and titratable acidity, but compared to semi-dwarf and dwarf rootstocks, they increased the content of phenolic compounds.
There is often a significant interaction effect between rootstocks and scions in improving fruit yield and quality. In sweet cherry, it was found that the variety ‘Vera’ (
P. avium) grafted onto ‘Egervár’ (
P. mahaleb), ‘Carmen’ (
P. avium) grafted onto ‘Cemany’ (
P. mahaleb), and ‘Petrus’ (
P. avium) grafted onto ‘Magyar’ (
P. mahaleb) rootstocks achieved the highest yields [
63]. For apricot (
P. armeniaca) varieties ‘E-101’ and ‘E-404’, grafting onto five different rootstocks revealed that the rootstock ‘PAC 00-08’ ((
P. salicina ×
P. cerasifera) ×
P. armeniaca) significantly increased the firmness of ‘E-101’ fruits, while ‘PADAC 01-47’ ((
P. besseyi ×
P. armeniaca) × (
P. cerasifera ×
P. armeniaca)) notably enhanced the firmness of ‘E-404’ fruits [
64]. Therefore, research on the function of rootstocks and the interaction between rootstocks and scions is crucial for developing effective breeding strategies.