In addition, hazelnut trees on C. colurna root stocks are frequently more variable in size and yield than self-rooted trees of C. avellana. In a trial using ‘Barcelona’ as a scion cultivar, the graft unions were overgrown and nut yields declined with age, at ~20–25 years. Due to these disadvantages, the Faculty of Agriculture, at Novi Sad in Serbia, has focused on identifying non-suckering selections of C. avellana. Currently, seeds of selected C. colurna are used as hazelnut root stock as it has been demonstrated to be long-living, resistant to frost and drought, has wide adaptability to soil conditions, and the trees are more vigorous and productive than self-rooted trees. Hazelnut root stock breeding started in Oregon in 1968. In nursery rows, open pollinated seedlings of C. colurna seedlings whose traits were intermediate between C. colurna and C. avellana were selected and propagated. During twenty years, approximately 150 potential root stocks were selected from 20,000 seedlings investigated.Both root stocks are thought to be interspecific hybrids because their nut and husk characteristics differ from those of the maternal parent. In 2000, a root stock trial was established at the IRTA-Mas Bové Research Center ,blueberries in pots with Spanish cultivar ‘Negret’ grafted onto four different root stocks, ‘Dundee’, ‘Newberg’, and two open pollinated C. colurna seedlings, compared to own-rooted ‘Negret’ as the control.
The results showed that ‘Dundee’ and ‘Newberg’ root stocks improved agronomic performance, solving the problem of suckering, increasing productivity and vigor, and producing increased yield at lower cost. However, the search for additional non-suckering root stocks necessary for commercial hazelnut orchards remains topical and is continuing. Root stocks can be vegetatively propagated by micropropagation, layering, or cuttings. The rooting ability of root stocks and the most effective propagation methods vary by species and genotype. Among walnut root stocks, Persian walnut is more difficult to root than black walnut × Persian hybrids. Japanese × European chestnut hybrids are more easily propagated by cuttings or layering than European chestnuts. Many efforts have been made to propagate walnuts by layering, cuttings, and micropropagation. In vitro propagation of walnuts obtained seems quite difficult. Generally, the effects of genotypes, but also the culture medium in vitro on proliferation, rooting, and survival rates are significant. Along with genotypes, nutritive support in vitro has an important influence on all stages of micropropagation. In addition to the well-known Murashige and Skoog culture medium, the Driver and Kuniyuki medium is also widely used for tissue culture of walnuts. However, depending on nutritive improvements of the medium, large variations of the results can be obtained regarding the success of in vitro culture . Vahdati et al. reported in-vitro rooting of Persian walnut cultivars ‘Sunland’ , ‘Chandler’ , and ‘Vina’ . Vahdati and Khalighi and Vahdati et al. evaluated stool layering of Persian walnut and found the greatest root number and root length was obtained using 5000 and 10,000 ppm IBA + IAA + NAA, respectively. Vahdati et al. obtained up to 81% and 82% rooting of Paradox walnut semi-hardwood and hardwood cuttings respectively, using 8000 ppm of IBA. Dong et al. reported a range of 60.5 to 87.5% rooting in a study of six cultivars.
Currently, the nursery pathogen problems have demonstrated it is better to produce plants in inert medium, and micropropagate the root stocks and graft the material in the nursery. Vahdati et al.found rooting success of low-vigor walnuts was better than more vigorous ones in response to stool layering. Peixe et al. achieved ex-vitro rooting rates exceeding 80% for microcuttings of ‘Vlach’ hybrid walnut. RolABC genes , derived from the bacterium A. rhizogenes, were inserted into somatic embryos of Px1 to increase the rooting potential. In a field trial, the rolABC genes produced shorter internodes and a more fibrous root system. Water and nutrient uptake are one direct effect of root stock on nut tree yield. These are regulated by complex interactions between the scion and root stock. Hormones, macromolecules, and miRNAs act as long-distance signaling molecules that regulate nutrient uptake. Water and nutrient uptake are enhanced by root stock vigor. In addition, the rate of vascular bundle development in a graft union determines the transfer potential of water and nutrient to the scion. Insufficient vascular bundle connection in a graft union leads to decreased water flow and subsequently altered nutrient translocation and hormonal signaling. Vertical and lateral root development plays an important role in water and nutrient uptake. Root stocks with a vigorous root system, i.e., long roots with many lateral branches and root hairs, are able to exploit water and nutrients from different soil depths and textures . Water uptake by roots is both parallel symplastic and apoplastic pathways. Root system hydraulic conductivity defines the root’s ability to conduct water across a water-potential gradient between the root surface and the stem xylem. Root stock effects on the canopy nutrient content are influenced not only by the roots physical characteristics but also depending on the chemical composition of the soil and environmental conditions. Pistachio root stocks differ in efficiency of macro- and micronutrient uptake.
The P. integerrima root stock is less efficient in zinc and copper uptake than P. terebinthus or P. atlantica. Trees on P. integerrima root stocks have much higher sodium , chloride and boron uptake than the reciprocal hybrids of the latter root stock species. This tendency to absorb and translocate Na and Cl ions to the leaves can be harmful to scions on P. integerrima in saline environments. The PGII root stock is more efficient in Zn and Cu uptake than P. terebinthus; P. atlantica is intermediate; UCB1 and P. integerrima are the least efficient. Boron uptake by PGII is somewhat less efficient than P. integerrima root stocks and slightly more efficient than UCB1. Boron uptake by UCB1 is similar to P. atlantica and P. terebinthus. PGII is less efficient than P. integerrima root stocks and a bit more efficient than UCB1. The ‘Kerman’ scion onto different pistachio root stocks demonstrated that leaves of trees on P. terebinthus often have the highest nutrient levels. P. terebinthus was more efficient than other root stocks in absorbing Cu, Zn, and other micronutrients that are often deficient in pistachio orchards. PGII and P. atlantica root stocks were superior to UCB1 and P. integerrima in absorbing Cu. A study of ‘Bianca’ scion budded onto various in vitro propagated clonal root stocks,square plant pot revealed that P. terebinthus was the most efficient at K uptake, but less efficient in uptake of Mg. The P. atlantica and P. integerrima clones seemed to be deficient in K uptake and the most efficient in Mg uptake. A four-year study of two pistachio cultivars budded on six P. vera seedling root stocks demonstrated that, K, P, and Fe absorption differed significantly among these root stocks. Scions on ‘Badami’ and ‘Daneshmandi’ seedlings had the maximum and minimum K absorption, respectively. ‘Akbari’ budded on ‘Badami’ was the most efficient in uptake of K and Zn. ‘Kalle-Ghouchi’ and ‘Daneshmandi’ had the maximum and minimum Fe content, respectively. The minimum K and Zn uptake occurred in ‘Akbari’ budded on ‘Daneshmandi’. ‘Barg-Seyah’ budded onto ‘Kalle-Ghouchi’ gave the maximum Fe and Cu uptake. These results indicate that selecting the appropriate root stock and scion for a particular environment is an important decision that can affect orchard growth and yield. Tavallali and Rahemi reported that leaves of pistachio cultivars grafted on ‘Beneh’ root stock had higher K, P, and Zn uptake than trees on ‘Badami’ and ‘Sarakhs’ root stocks. Leaves of pistachios on ‘Badami’ and ‘Sarakhs’ had the highest Ca and Cu content, respectively. Kernels of cultivars grafted on ‘Sarakhs’ root stock had greater K, P, Mg, Cu, Fe, and Zn content than cultivars on other root stocks. Trees grafted on P. atlantica seedling root stocks were less likely to show B, Ca, or Zn defificiency. These studies identified the effect of pistachio root stocks on nutrient uptake and yield but our knowledge on the effect on nutrient uptake of different cultivars grafted on the same root stock is limited. Surucu et al.grafted 14 pistachio cultivars of different origins on a single source of P. khinjuk seedling root stocks and evaluated nutrient uptake and yield. Scion cultivar ‘Haciserifi’ had the greatest N, P, and K accumulation, while ‘Mumtaz’ had the greatest uptake of Ca, Mg, and Cu. ‘Vahidi’ accumulated the most Fe and Zn, and ‘Sel-150 accumulated the most Mn. ‘Sel-20 , ‘Sel-50 , and ‘Siirt’ scions had the highest percentage nut split and ‘Mumtaz’ had the highest yield.
Knipfer et al. reported that the root hydraulic conductance of ‘RX10 and ‘Vlach’ walnut root stocks was more than 50% greater than ‘VX2110 and possibly, one reason for the tolerance of these two root stocks to drought stress. Under drought stress, ‘Vlach’ and ‘RX10 decreased root hydraulic conductivity to maintain root biomass. Walnut roots selectively absorb ions when they are under stress. A study of the response of own-rooted walnut varieties to salt stress showed that the tolerant varieties accumulate and translocate more K and Ca in shoots than the less tolerant varieties. In other words, the roots of salt-tolerant walnuts not only absorb more K and Ca, but also translocate more to the leaves. A comparison of nutrient uptake between two walnut root stocks, J. hindsii and Paradox, showed that N, P, Ca, Mg, and Mn uptake were significantly higher with Paradox root stock. In almonds, the impact of root stock choice on concentrations of lime, alkali, B, Zn, and K has been well studied. Jiménez et al. reported that high levels of sucrose, organic acids, amino acids, and PEP carboxylase activity in the roots of Prunus root stocks lead to root growth and iron uptake under iron deficient conditions. Trees on almond or almond × peach hybrids show reduced levels of chlorosis from iron deficiency in high-lime soils. Somewhat less tolerant are the Myrobalan root stocks, which will often develop some chlorotic leaves at the shoot tips by late summer. The three-way, and similarly complex, hybrids tend to show more intermediate tolerance to calcareous soils. In general, almond trees on peach perform poorly on calcareous soils, whereas trees on almond root stocks typically perform better. All Prunus root stocks are generally sensitive to alkaline soils or water containing an excess of alkali salt. Trees on almond root stocks appear to be the most tolerant, followed by Myrobalan plum, and peach, with little difference among the latter two. Some peach × almond hybrids have also demonstrated greater tolerance to alkali than peach or Myrobalan. The Prunus scion also appears to have considerable influence on sensitivity to alkaline soils, but the extent of this influence has not been well characterized. Marianna plum and peach show greater tolerance to excess boron than almond, which, in turn, is more tolerant than Myrobalan root stocks. For this reason, almond root stocks are recommended for locations where excess B is a problem. If boron is low, more vigorous root stocks and Marianna plum are generally preferred. Almond and peach root stocks are more likely to experience Zn deficiency than trees on Marianna. Almond trees on almond or Myrobalan root stocks are more susceptible to K deficiency than peach, with tree death possible if not treated. Reid performed a leaf analysis of two pecan scions, ‘Posey’ and ‘Pawnee’, grown on 10 root stocks: ‘Chickasaw’, ‘Colby’, ‘Dooley’, ‘Giles’, ‘Greenriver’, ‘Major’, ‘Mohawk’, ‘Peruque’, ‘Posey’, and ‘Shoshoni’. He concluded that root stock influenced K and Zn concentration. The greatest K accumulation was seen in trees on ‘Posey’ seedlings while scions on ‘Greenriver’ seedlings showed the least. Trees on ‘Chickasaw’ seedling root stocks contained the highest amount of Zn while those on ‘Major’ seedlings had the least. A study of hazelnut root stocks showed that ‘Dundee’ and ‘Newberg’ are more resistant to iron chlorosis and maintain leaves on the tree for a longer period during the season, an important aspect to be considered, as these trees can then absorb soil nutrients up for a longer period. In fruit trees, there is a lag between planting and fruiting, leading to a delay in the profitability of commercial orchards. Root stocks are not only able to induce precocity, but also increase the quality of flowers and ability to set fruits. Previous results on pistachio showed that flowering time of pistachio can be delayed when ‘Badami-e-Zarand’ is used as root stock and ‘Akbari’ as interstock. In contrast, ‘Badami-e-Zarand’ and ‘Fandoghi’ as root stock without interstock had no significant effect on flowering time.