In good agreement with other studies, Solibacteres, mainly Candidatus Solibacter, previously suggested to be adapted to nutrient-limited environments, was associated with the conventional farming system. Interestingly, taxa capable of degrading xenobiotic compounds were also enriched. The nitrification process was enhanced, as ammonia oxidizing bacteria and archaea wereparticularly induced after applying the conventional fertilization, and that response was consistent over the 3 months of the experiment, similar to longer-term studies. Thaumarchaeota archaea’s enrichment has been previously observed after a long-term application of organic fertilizers and in several long-term fertilization experiments with inorganic N treatment. In addition, several denitrifying bacteria responded to the chemical fertilization, such as Gemmatimonas , Pseudomonas , Achromobacter , Nocardia, and Rhodococcus . In this study, functional profiles were more resistant to intervention than community composition. This agrees with the conclusions of Pan et al., who proposed that the coexistence of organisms with overlapping ecological functions confers functional stability. Fierer et al. found that, under high concentrations of inorganic nitrogen, the relative abundance of the DNA/RNA replication, electron transport, and protein metabolism functions increase. Likewise, Carbonetto et al. evidenced that the relative abundances of intracellular trafficking, secretion and vesicular transport, energy production and conversion, and amino acid transport and metabolism were greater in soils under conventional farming system than in uncultivated soils, consistent with a copiotrophic strategy. Ding et al. reported changes in functional groups associated with nitrogen cycling when conducting a metagenomic analysis, observing the greatest effect for functional groups associated with aerobic ammonia oxidation, nitrite reduction, anaerobic ammonia oxidation, and nitrate reduction. Chen et al., besides, found no significant differences in functional genes, predicted from 16S RNA using PICRUSt, involved in denitrification , nitrification ,blueberries in containers growing and N-fixation when analyzing soils managed over 18 years that included organic and conventional farming.
Similarly, in the present study, when evaluating PICRUSt predicted functions, the Kruskal–Wallis test did not detect any differentially abundant functions between the conventional and organic soil samples, which included those N transformation functions. However, according to LDA, organic soils had greater predicted abundances of glutathione metabolism, which plays an important role in the defense of microorganisms and plants against environmental stresses. In addition, it is also involved in the regulation of sulfur nutrition and plays a key role in the nitrogen-fixing symbiotic interaction. However, the functional results reported here are based on predictions obtained from the 16S rRNA gene, which resulted in NSTI values that were moderately high, as expected for phylogenetically diverse samples such as soil, suggesting that those predictions must be interpreted with caution. In addition, functional differences might have been hidden, as de novo OTUs were eliminated for the analysis to conduct PICRUSt predictions. Nut trees are among of the most important horticultural tree crops. Both production and consumption are increasing dramatically due to strong economic returns and the nutritional value of their products. The world’s tree nut production has increased by 48% over the last 10 years . The world-wide export value of tree nut crops amounted to approximately 34.5 billion dollars in 2019, an increase of ~107% compared to the prior 10-year period. Technical knowledge regarding nut tree production has also rapidly increased as a result of the demand for higher production and quality, multiple destinations of nuts fruit in current consumption and food industry, but also of the growing importance accorded to the nuts in a balanced and healthy diet and in the prevention of various diseases. Among the areas of interest and progress has been the use of root stocks to adapt to climate and edaphic factors including soil borne diseases and abiotic stresses, control scion vigor, increase yield, and improve fruit quality. the selection of the scion cultivar is the grower’s top consideration for long-term productivity and profitability, root stock selection is becoming more important. Now, the root stock scion and interaction per se is considered when planting an orchard. The advantages of selected root stocks have been recognized and utilized in the nut trees’ production, but they do not have a long history of use in many species.
Although nut trees are grown around the world, root stock studies are limited to only a few tree nut species. Initially, most root stocks were open-pollinate seedling, or seed stock. Seed stocks are not as genetically uniform as clonal root stocks, but they have advantages such as deep root system and tolerance to edaphic abiotic stresses. However, seed stocks have high heterozygosity in terms of different traits. Hence, the type of seed and location in which it is grown is important for choosing seed stocks. Seed stocks should be uniform, vigorous, disease resistant, and readily available. Therefore, several studies have been performed to study the growth vigor of seed stocks and improve seed germination in nut trees. In addition to seed stocks, a wide range of clonal root stocks are now being developed. Numerous root stock breeding programs have begun to introduce clonal root stocks to meet important challenges, including excess vigor, low yield, poor nut quality, poor soil, climate change, drought and salt stress, suckering, diseases, and graft incompatibility. Common tree nut root stocks, especially clonal root stocks, and their main characteristics are listed in Table 1. Advances in the development of temperate nut trees root stocks until 2003 were last reviewed by Grauke and Thompson. Given the recent advances in root stock breeding for tree nut crops, this review will focus on the physiological and molecular effects of root stocks on scions under different edaphic and climatic conditions. The main purpose of this review paper is to present studies on various aspects of breeding and physiology of nut trees root stock, as well as, draw a comprehensive vision to accelerate future research in this field using combination of traditional and modern methods. To this end, we first provide overall information on vigor, root stock-scion com-patibility, suckering, and rooting ability which can be useful for tree nut crops researchers and growers. Next, we review water and nutrient uptake on nut trees. In the following, we review phenology and yield related traits which are important in industry and marketing. Then, we comprehensively review abiotic and biotic stresses studies on tree nut crops. Finally, we briefly review root stock-scion transfer of macromolecules and small interfering RNAs in nut trees. Since nut tree crops have a long juvenile period, development of a new variety or root stock may take more than 20 years via classical breeding. Therefore, in the conclusion and perspectives section, we note the future prospects of molecular breeding in nut tree crops using novel technologies for rapid generation advancement.The nut trees growth is strongly controlled by the distribution of organic and inorganic constituents within the tree trunk, canopy, and the root system. The vascular system plays a role in this long-distance signaling.
Hypothetically, root stocks impact scion vigor by controlling water and nutrient transfer and hormones signaling and RNAs which move up through the graft union. Numerous studies have been conducted regarding the effect of root stocks on the growth of nut trees. Pistachio growers and breeders are seeking vigorous root stocks. Kallsen and Parfitt reported ‘Kerman’,planting blueberries in containers the previously primary female pistachio cultivar in California, has a rapid growth habit that produces trunk circumferences larger than that of the root stocks. Matching the scion and root stock growth rates produces stronger graft unions. Highly vigorous root stocks produce more uniform graft unions and reduce bark damage from trunk shaking harvesters by uneven graft unions. They report that UCB1 is a better root stock for ‘Kerman’ as it produces a smoother trunk compared to Pistacia integerrima root stocks. Caruso et al. evaluated one seedling and eight clonal pistachio root stocks and reported that root stock had a significant effect on growth rate of the scion and nut yield. Clones of P. integerrima and P. atlantica are highly to intermediately vigorous root stocks. The pistachio cultivar ‘Bianca’ onto P. integerrima seedling root stock had significantly better growth than on P. terebinthus or P. atlantica clonal root stocks. Scions grown on P. terebinthus root stocks had the least vigor. When ‘Bianca’ scions were budded onto eight in vitro propagated clonal root stocks and observed for 4 years, trunk cross-sectional areas on P. integerrima were three times higher than on P. terebinthus root stocks. Ak and Turker reported the cultivars, ‘Kirmizi’ and ‘Siirt’, grafted onto P. vera, P. khinjuk, and P. atlantica demonstrated different budbreak, flowering time and vegetative growth. P. vera flowered earlier and P. atlantica and P. khinjuk had greater stem diameters. Rahemi and Tavallali studied the effect of ‘Badami’ , ‘Sarakhs’ , and ‘Beneh’ seedling root stocks on growth, yield, and nut quality of the Iranian cultivars, ‘Ohadi’, ‘Kalleh- Ghouchi’, and ‘Ahmad-Aghaei’. ‘Sarakhs’ seedlings had the least vigor, while ‘Badami’ root stocks produced the highest yields and best nut quality. Ghazvini et al. evaluated the ecophysiological characteristics of four seedling root stocks, ‘Badami’, ‘Sarakhs’, P. mutica, and P. atlantica. Photosynthesis, stomatal conductance, and transpiration was highest in trees on the ‘Sarakhs’ root stock and lowest on the P. mutica root stock. P. integerrima is the most vigorous root stock now commonly used in pistachio cultivation but is also the least cold tolerant. It is rapidly being replaced by the more coldand salinity-tolerant hybrids, available as both a seedling and a clone, and P. integerrima × P. atlantica, now available as a clone . In contrast to pistachio, there is no a specific walnut breeding program to select high vigorous root stock. Nevertheless, the major walnut clonal root stocks introduced in the last few years are vigorous. Among the clones of ‘Paradox’ which was introduced by the University of California-Davis, ‘VX2110is highly vigorous and nematodes-tolerant root stock. Furthermore, ‘Grizzly’ clonal walnut root stock has been recently introduced as a highly vigorous root stock. The mother tree of ‘Grizzly’ is a Tulare variety grafted on a seedling Paradox root stock. This root stock showsgood performance in poor soil structure with low nutrition and heavy populations of lesion nematodes. In addition, high vigorous trees are very important for the wood industry. Numerous interspecific hybrids were carried out in Juglans genus between J. regia with J. cinerea, J. nigra, and J. major. Compared to the parent, most of them such as ‘NG230 , ‘NG380 , and ‘MJ2090show high vigor, disease resistance, greater winter-hardiness, and high wood quality. Walnuts are highly vigorous trees with an extended juvenility phase. Dwarf walnut trees could potentially decrease labor costs and increase yields per hectare by allowing increased plant density. Although dwarfing has not generally been the most important objective of walnut root stock breeding programs, identifying sources of this trait is of great interest in countries with high genetic diversity such as Iran, China, Turkey, and Central Asian countries. In these countries, traditional orchards of giant walnut trees are difficult to harvest mechanically. Harvest injuries and death of laborers during manual harvesting have precipitated interest in dwarfing root stocks. Reportedly dwarf walnut trees have a short life span. Therefore, in some countries, breeders are attempting to combine slow-growing scions with vigorous root stocks. Juvenile and mature walnut tree vigor is highly heritable. Wang et al. evaluated Persian walnuts in China and selected six dwarf walnut root stocks; ‘Xinwen 6090 , ‘Xinwen 7240 , ‘Xinwen 9080 , ‘Xinwen 9150 , ‘Xin 9160 , and ‘Xinwen 9170 as potential root stocks for breeding. Analysis of growth traits of ‘semi-cultivated’ local genotypes of Juglans regia on their own roots, in the sands area of south-west Romania, showed that climatic and edaphic factors significantly influenced the annual growth ring width of the trees, but also their adaptability to environmental factors. Precocious and dwarf walnut trees have been evaluated in Iran. These genotypes induce dwarfing and precocity in scions in preliminary experiments, apparently due to a slower growth rate. They have fewer nodes, shorter internodes, and smaller shoot length, smaller root system, and lower sap flow and hydraulic conductivity which are the typic traits of dwarf root stocks in other fruit trees. They also have a better rooting ability and higher grafting success. Dwarfing is a desirable trait for other tree nuts. In China, dwarfing chestnut root stocks are being evaluated. In the USA, Anagnostakis et al. attempted to breed dwarfing chestnut root stocks and suggested that hybrids with Castanea seguinii could be a source of dwarfing. Researchers at the University of Missouri identified various chestnut cultivars as potential sources of dwarfing. Studies of graft compatibility, vegetative growth, and productivity of these trees are continuing to determine if dwarf chestnut root stocks are feasible.