Almond root stocks have been shown to alter root, shoot, trunk, and fruit development, probably by affecting the allocation of carbon assimilates between these tissues. Khadivi-Khub and Anjam evaluated the Iranian cultivar ‘Rabiee’ grown on P. scopariaand ‘Estahban’ root stock under normal and rainfed conditions. They reported significant differences in tree height, trunk diameter, annual growth, and internode length, observing reduced scion growth when grafted on P. scoparia root stock. P. scoparia, suggesting potential as a dwarfing root stock. Parvaneh et al. evaluated three Iranian cultivars on bitter almond, sweet almond, and peach root stocks and found that cultivars grafted on peach had greater vegetative growth, while scions grown on both bitter and sweet almonds had reduced growth, resulting in smaller trees. The magnitude of the effect varied with cultivar. In a regional root stock trial at California State University, Fresno, significant differences among root stocks were found in canopy growth, tree height, and tree circumference. Almonds grafted on peach root stock had larger scion diameters than on almond root stocks. Preliminary results from a vigor study showed that trunk diameter of the scion cultivar depends on the scion-root stock interaction. The root stock effect differed depending on the cultivar grafted and scion vigor itself. Lordan et al. studied the performance of two Spanish almond cultivars, ‘Marinada’ and ‘Vairo’, grafted onto different root stock genotypes and reporting strong root stock effects on vigor, bloom, and ripening dates, yield, and kernel weight. The effect of root stock on tree architecture is less clear. Rootstock effects on shoot length and shoot diameter have been reported,container growing raspberries but the magnitude of the effect varied as a function of specific scion-root stock combinations.
Similarly, the scion can influence root structure, primarily by altering auxin and cytokinin responses. This suggests the regulatory feedback between the root stock and scion ultimately modulates final tree architecture. The underlying molecular mechanisms of these interactions remains unknown. Studies of the effect of root stock on pecan scion vigor have demonstrated that common pecan root stocks vary by geographic region and have a diverse effect on scion growth. Before introducing clonal root stocks, open-pollinated seed stocks widely used for the vegetative propagation of commercial pecan cultivars had different growth responses. Grauke and Pratt evaluated bud growth of three pecan cultivars on seven open-pollinated seed stocks including ‘Curtis’, ‘Burkett’, ‘Elliott’, ‘Moore’, ‘Riverside’, ‘Apache’, and ‘Sioux’. They reported that scion growth was significantly influenced by root stock, with bud growth of ‘Candy’ on ‘Elliot’, and ‘Curtis’ root stocks were more than ‘Sioux’, ‘Riverside’, ‘Apache’, and ‘Burkett’ root stocks. Liu et al. studied the grafting-responsive MicroRNAs which are involved in growth regulation of grafted pecan and identified some miRNAs that regulate grafted pecan by regulating inorganic phosphate acquisition, auxin transport, and cell activity. The root stock effect on vigor of other nut trees has been less studied. In hazelnut, new root stocks have produced superior vigor compared to own-rooted varieties. This is an important improvement when trees are trained to a trunk, and not grown as bushes with many stems. Graft success depends on the root stock-scion physiological compatibility and the proper alignment of tissues in the graft union. Graft incompatibility is a complex physiological process defined by the adjustment of the metabolisms of the cultivar– root stock combinations, growth conditions, the presence or absence of viruses, environmental conditions, the nutritional status of trees, and as other stresses. Graft incompatibility can be detected by a variety of symptoms including poor graft success, yellow-colored leaves, slow vegetative growth, drying of the scion, a generally diseased appearance, symptoms of water stress, overgrowth in the graft area, thicker bark tissues of scion, and excessive sprouting on the root stock .
In pistachio, P. terebinthus, P. atlantica, P. integerrima, P. vera and their interspecific hybrids are commonly used root stocks. P. terebinthus is more difficult to bud than P. atlantica or P. integerrima due to scion-root stock incompatibility problems. Although root stock-scion incompatibility is not a serious problem in pistachio production, some evidence of incompatibility between P. veraas a scion and UCB1 as a root stock was observed in the late 1980s in the USA. This incompatibility appeared to be related to a single paternal P. integerrima tree used to produce the first UCB1 seedlings at the University of California, Berkeley. There have been fewer reports of root stock scion incompatibility since removal of this paternal tree. When facing root stock-scion incompatibility problems in pistachio it is worth testing different individuals within a single species to find a compatible genotype. The success of walnut grafting mainly depends on several factors such as root stock, scion, grafting methods, and environmental conditions. The specific graft incompatibility between different Juglans species has not been reported. Nevertheless, some literatures refer to black line disease as a delayed graft incompatibility in walnuts. California black walnut and its hybrids are considered as interesting root stocks for Persian walnut specially in California due to high vigor, resistance to soil-borne pests, and tolerance to saline and saturated soil. However, if Persian walnut was grafted on California black walnut and its hybrids and the tree was infected with CLRV virus, the symptoms of black line disease would appear, which is similar to a graft incompatibility. Therefore, in regions where there is a possibility of infection with the CLRV virus, Persian walnut is a more suitable root stock that can be used to avoid black line disease. Andrews and Marquez reported that black line disease has a long-delayed incompatibility where a CLRV virus migrates to a graft union. In almond, graft incompatibility appears to be genetically dependent. For example, ‘Nonpareil’ shows distinct graft-incompatibility on plum root stocks while the closely related ‘Carmel’ cultivar does not. Graft-incompatibilities can produce both slow general tree deterioration over time and distinct localized deterioration such as the stem-pitting decline seen on almond-Myrobalan plum combinations.
These more localized types of graft-incompatibility can often be observed as a weakness and occasional breakage at the graft-scion union. Because this often occurs at a critical time, when the tree is coming into bearing, several studies have pursued earlier physiological and molecular predictors of graft-compatibility as an aid to both breeding and orchard management. These studies generally involve anatomical, physiological, or molecular aspects of compatible graft union formation such as the similarities/differences in scion vs. root stock vascular size and configuration. Related studies have identified several molecular candidates that may contribute to compatible graft formation, however,raspberries in containers the specific cause and effect relationships remain vague. Studies have identified several metabolic pathways, including the phenylpropanoid pathway, cell wall biosynthesis, oxidative stress, and auxin signaling, that appear to be associated with graft-incompatibility, supporting the complex genetic control commonly encountered when breeding for this trait. Japanese and Chinese chestnuts are used in chestnut root stock breeding programs due to their root-rot resistance. The potential use of hybrid chestnut cultivars also has been evaluated; while incompatibility has been observed in the hybrids. Tokar and Kovalovsky grafted Chinese, European, and Chinese × Japanese hybrid chestnut cultivars onto European chestnut root stocks. The least successful grafting combinations were the Chinese × Japanese hybrid on European root stocks. Viéitez and Viéitez, used Chinese and European chestnuts for European, Chinese, and European × Chinese chestnut hybrid scions. The least successful grafting combinations were the Chinese root stocks with European chestnut cultivars.Soylu suggested that scions and root stocks of the same species should have better graft compatibility, but genetic intraspecies graft incompatibility was reported in Chinese and European chestnuts. Although graft compatibility in chestnut may be mostly controlled by genetic factors, graft success can also be affected by environmental factors, stress, and their interactions with genotype. Oraguzie et al. suggested that growing the root stock and the scion plant under the same environmental conditions would produce better graft compatibility. Oraguzie et al. divided graft incompatibility into two groups, early and late. Early graft incompatibility can be seen in the first two years and late incompatibility in 5 to 7 years. Chestnut mosaic virus can also induce graft incompatibility. The first hypothesis was suggested by Santamour et al.. They identified four different cambial peroxidase isozymes patterns in ten chestnut genotypes. They found that C. dentata, C. alnifolia, C. ashei, C. ozarkensis, and C. pumila species have A cambial peroxidase isozymes, C. crenata and C. seguinii have B pattern, C. sativa has A, B, and AB isozymes, C. henryii has A and B and C. mollissima has A, AB, B, and BC isozymes. Grafting plants with different isoenzyme bands could lead to graft incompatibility. Santamour tested his hypothesis with 200 Chinese chestnut seedlings. If the scion and the root stock belonged to the same cambial peroxidase isozymes group, the cambium layer in the graft area united and cambial continuity occurred. If the scion and the root stock cambial peroxidase isozymes groups were different, cambial continuity was interrupted. Thus, he suggested that cambial peroxidase isozymes groups could be used to predict graft incompatibility in Chinese chestnut. However, this hypothesis was not confirmed in subsequent study. The other hypothesis of graft incompatibility in Chinese chestnut is a mismatch of phloem fiber bundles. A very important aspect of this anatomical structure is the presence of a fiber bundle in four or more places in the branch. When the seedlings are 2–3 years old, phloem fiber bundles can be better distinguished. This situation should be considered when grafting, as the cambium of the root stock and scion may not combine uniformly. Given the importance of early detection of graft incompatibility, it is important to find specific markers for prediction in different root stock-scion combinations. Many studies have addressed strategies for compatibility detection such as phenolic marker identification and peroxidase isozyme studies.
Phenolic compounds, whose biosynthesis is triggered by wounding and infections, are produced and accumulated during the callusing phase. This suggests that quantitative and qualitative differences in phenolic patterns between scion and root stock may predict graft union dysfunctions and could be potential markers of graft incompatibility. Research at the University of Torino Chestnut R&D Center, demonstrated different chemical markers: six phenolic acids, five flavonols, two catechins, and two tannins. Chromatographic methods were used to identify and quantify the main bioactive compounds, benzoic acids, binnamic acids, batechins, flavonols, and tannins and obtained specific phytochemical profiles. Benzoic acids , catechins , and tannins were used to establish specific profiles for distinguishing compatible and incompatible chestnut scion-root stock combinations. Another promising technique is the analysis of peroxidase isozyme profiles of root stocks and scions. It appears peroxidases play an important role in grafting, as these enzymes are involved in lignin formation and lignin–carbohydrate bonding. Differences in peroxidase isozymes in root stock and scion graft performance have been reported in Chinese chestnut and peach–plum combinations. Other strategies for evaluating root stock–scion compatibility include describing the phenylalanine ammonia-lyase transcriptomic-level and phenotypic evaluation.Another important trait in root stock selection is suckering. Suckers not only divert water and nutrients from the main trunk, but also increase orchard management costs incurred in removing them. Suckering is an important issue in hazelnut cultivation, requiring four to five herbicide sprays per year in commercial orchards and occasional hand-removal in winter. This situation could be improved by use of non-suckering root stocks. Currently, three types of hazelnut root stocks are in use: C. colurna seedlings, C. avellana seedlings, and two clonal selections from open pollinated C. colurna: ‘Dundee’ and ‘Newberg’. A hazelnut root stock trial in IRTA-Mas Bové, Spain in 1989 led to selection of a clonal C. avellana root stock , which is a seedling of ‘Tonda Bianca’. One of the first European hazelnut root stock trials was conducted in Nebrosi, Sicilia in 1970 to compare self-rooted trees grafted on C. avellana root stock . After 12 years of evaluation, self-rooted trees showed better vegetative and productive behavior than grafted ones. Experience with C. colurna in the U.S.A. has demonstrated that members of this species are more drought tolerant and cold hardy than C. avellana cultivars. The C. colurna was non-suckering, deeply-rooted, and graft-compatible with all C. avellana cultivars and Corylus species, suggesting its potential use as a root stock. Due to differences in bark color and texture, the union between the Turkish and European hazelnut is readily evident. However, the Turkish hazelnut is difficult to propagate and its seedlings often require two additional years before reaching sufficient size for grafting.