Moriana et al. investigated the effect of water stress on ‘Kerman’ grafted onto three pistachio root stocks . All three root stocks showed dehydration leading to reduction in vegetative growth and number of leaves, while root weight was promoted. UCB1 was least affected by drought stress and P. atlantica also showed good tolerance. Drought stress also affects the foliar epidermal anatomy of pistachio trees. Regulated deficit irrigation of ‘Kerman’ grafted onto P. atlantica, P. integerrima, or P. terebinthus root stocks and grown on shallow soils was studied by Memmi et al..The RDI irrigation regime decreased by 40% water compared to normal irrigation. The P. integerrima root stocks had less tolerance to drought than P. atlantica or P. terebinthus. Carbonell-Barrachina et al. investigated the performance of the same root stocks under RDI. Yield, nut weight, mineral content, and consumer satisfaction were all greater for the trees grown on P. atlantica. In another study, nuts produced under RDI on P. terebinthus and P. atlantica root stocks had higher polyphenol and tri-terpenoid content than those produced on P. integerrima. Noguera-Artiaga et al. also studied the effect of RDI on ‘Kerman’ trees budded onthat leaf water use efficiency, Kro, and leaf turgor are useful canopy traits for selecting drought-tolerant root stocks. Given that of the area of Persian walnut origin includes arid and semi-arid regions,grow blueberries in a pot utilization of genetic diversity can be an effective strategy in the development of droughttolerant root stocks. A walnut root stock breeding program based on exploration of genetic diversity started at the University of Tehran, Iran in collaboration with University of California-Davis in 2008.

Preliminary studies led to identification of some drought-tolerant candidate genotypes and to understanding of some physiological mechanisms involved in drought tolerance. Accordingly, several physiological processes, including cavitation resistance via stomatal regulation, maintenance of net assimilation and photosynthetic rate, increasing antioxidative enzyme activity , accumulation of proline and total soluble sugars, and improved WUE, are responsible for drought tolerance in walnut genotypes. WUE differences were studied also in a wild population of J. regia, examining variation in δ13C as a surrogate for intrinsic water-use efficiency. New and advanced biotechnology techniques have accelerated the understanding of the molecular mechanisms involved in drought tolerance in walnut. Considering that WUE is associated with drought tolerance, a natural Persian walnut population that was diverse in WUE was used to study the relationship between phenotypic and genotypic traits, using association analysis and a large data set of SNPs. This study led to identification of drought stress-responsive genes involved in ABA signaling, antioxidant responses, stomatal regulation, osmotic adjustment, transduction of environmental signals, and leaf development . In addition to exploiting genetic diversity, genetic transformation has been used successfully to induce drought and salt tolerance in walnut. Sheikh Beig Goharrizi et al. reported that Persian walnut genetically transformed with a flavodoxin gene had better growth under both osmotic and salinity stress. In contrast to drought stress, studies of salinity-tolerant root stocks are rare in walnut. Salinity is an important environmental stress that mostly affects growth and physiological aspects of nut trees. An examination of the response of Juglans species to salinity stress showed that J. hindsii and its hybrid are more tolerant than Persian walnut. Waterlogging can result in root asphyxiation and later in Phytophthora damage; particularly with spring rains and poorly drained soils. Unlike Chinese wingnut that is very tolerant, Juglans species are highly sensitive to waterlogging, probably due to a shift in cellular metabolism towards production of acetaldehyde and ethanol under anaerobic conditions.

Ethanol production and accumulation in roots is the start of events leading to cell death. The ethanol produced in roots moves up to the leaves and is released to the external environment . During waterlogging, transfer of ABA to the leaves leads to an increase in leaf ABA content and plays a critical role in reducing growth. Almond seedlings have traditionally been used as root stocks in arid and semi-arid regions due to their performance on calcareous soils under limited rainfed conditions. However, almond root stocks are susceptible to fungal diseases and nematodes, as well as to root asphyxia in wet and poorly drained soils. For this reason, other root stock species have been utilized, particularly peach and plum, as well as their interspecific hybrids. In recent years, knowledge of the physiological behavior of hybrid Prunus root stocks under drought stress has improved. In a long-term drought experiment, the almond × peach hybrid, ‘Garnem’ consumed its water reserves during the first days of drought stress in order to maintain shoot growth rate. As water stress became more severe, water consumption diminished in response to the loss of hydraulic conductivity. In shorter-term drought experiments, ‘Garnem’ was able to maintain high leaf water content rates under low water potential, as well as preserve a high cell membrane stability, indicating osmotic adjustment as part of its drought tolerance mechanism. In addition, abscisic acid was demonstrated to be involved in rapid long-distance hydraulic signaling from root to shoot for inducing stomatal closure in drought stressed ‘Garnem’. Recent research has also provided insights into the genetic response of Prunus species under drought, identifying key drought-responsive genes, including those directly related to water use efficiency . These include ERF023TF; LRR receptor-like serine/threonine-kinase ERECTA; and NF-YB3TF as well as the gene ppa008651m coding for a LEA protein homolog to LEA D29 and PpDhn1, and PpDhn2 and DREB2B. No less important has been the characterization of natural sources of drought tolerance. Bielsa et al. investigated differences in 48 Prunus species by evaluating leaf ash content and carbon isotope discrimination , which are strongly correlated with WUE.

Almond and wild peach species showed the lowest ∆13C ratios, and therefore, greater WUE than hybrid genotypes, although, among the GN serie ’Monegro´ showed the greatest WUE. An important abiotic limitation to almond production is root asphyxia on heavy soils. Prunus root stocks vary in their response, demonstrating different levels of susceptibility. European plum and Myrobalan plum root stocks are considered root-asphyxia tolerant, while almond, peach, and their hybrids, are more susceptible to waterlogging damage. The physiological response to hypoxia has previously been shown to be under genetic control. Both gas exchange parameters and photosynthetic activity were strongly affected in sensitive genotypes relative to more tolerant genotypes. In addition, morpho-anatomical changes were shown to be important factors in conferring tolerance. Recent studies of alterations in metabolism and regulatory processes in Prunus under waterlogging stress have led to the identification of candidate genes involved and to clarifying their roles in waterlogging response. Arismendi et al. found groups of differentially expressed genes coding for key enzymes that were upregulated under hypoxia in tolerant,container grown raspberries but not in sensitive, genotypes. These were associated with post-transcriptional protein modifications, such ashexokinases and fructokinases , as well as genes coding for proteins involved in transcription regulation, including AP2 domain-containing, ARR6 , Sin3-like2, and zinc finger proteins. Other strategies have also been demonstrated in tolerant and sensitive genotypes under hypoxia conditions. Rubio-Cabetas et al. demonstrated that the tolerant Myrobalan ‘P.21750 plum represses secondary metabolism gene expression as a strategy to prevent the waste of resources/energy. At the same time, they reported the upregulation of protein degradation genes, which led to structural adaptations conferring long-term tolerance to hypoxia. The more sensitive almond-peach hybrid ‘Felinem’ was found to upregulate a group of signal transduction and transcription factor genes. In addition, three candidate genes involved in the oxygen sensing mechanism were identified as possible biomarkers for hypoxia-tolerant selection, including the genes ERF74/RAP2.12, ACBP1/2, and HCR1. The temperature, especially low or freezing conditions, is an important abiotic factor that affects the growth of pecan trees at various growth stages, and is affected by root stock. The pecan root stocks most used in Georgia are seedlings of ‘Curtis’ and ‘Elliott’.

Both give good germination and quickly develop large stem calipers, but ‘Curtis’ is more resistant to cold. Among eleven root stocks, ‘Apache’, ‘Giles’, and ‘Peruque’ were in the group of the least damaged after a freeze event on 8–9 October 2000 . Among the scion cultivars, ‘Kanza’ was less damaged than ‘Mohawk’, ‘Mount’, or ‘Creek’. ‘Kanza’ exhibited no injury when other cultivars were severely injured during an autumn and winter freeze in Oklahoma, and is considered most cold hardy. ‘Pawnee’ is resistant to both fall and midwinter freeze damage, but is one of the first cultivars to break bud in the spring, making it highly susceptible to spring frost damage. Smith reported that ‘Pawnee’ grafts showed significant damage after freeze events in October 2000, although 1-year-old ‘Pawnee’ grafts were not damaged by a freeze in November 1991. After the same freeze event trees grafted to ‘Kanza’ and ‘Pawnee’ in southwest Missouri experienced the most death, while ‘Posey’ and ‘Dooley’ suffered the least damage. It was concluded that scion cultivar impacts the cold hardiness of the above-ground part of the root stock and that ‘Kanza’ and ‘Pawnee’ scions decreased the cold resistance of the root stock during this early autumn freeze event because they enter into dormancy later in the fall. In addition to autumn freezes, very cold winters can cause serious damage to pecan trees. Symptoms typically are death and browning of the cambium, inner bark, and phloem, as well as splitting and browning of the root stock inner bark and phloem, and delayed bud break. Some root stocks impart sufficient cold resistance for a cultivar to reduce or escape damage on one root stock type, while being severely damaged on another type. Cultivars grafted onto ‘Apache’ seedling root stock showed one third of the damage from a fall freeze on 14 November 1976 than the same cultivars grafted onto ‘Riverside’ seedling root stock . The extent of freeze damage was evaluated by observing the extent of cambium discoloration or browning. ‘Apache’ root stock produced cold-hardy and fast-growing trees. Similar findings were observed by Hinrichs. He observed that ‘Stuart’ scion was killed on some root stocks by cold fall temperatures, while the same scion was not damaged on the ‘Giles’, ‘Major’, and ‘Indiana’ root stocks. ‘Stuart’ exhibited less injury during both fall and winter freeze, and early autumn freezes. ‘Desirable’ and ‘Mohawk’ budded on ‘Apache’ were the most damaged by this freeze, while ‘Wichita’ and ‘Choctaw’ budded to same seeding root stock were the least damaged.The ‘Pawnee’ scion was the most resistant to early fall freeze injury among the nine scion cultivars and un-grafted ‘Elliott’ root stock seedlings. Similarly, freeze resistance by ‘Pawnee’ and ‘Elliott’ was observed by Goff and Tyson. These observations indicate that the scion can also increase the cold resistance of a juvenile root stock, just as the root stock impacts the cultivar susceptibility. The ‘Kanza’ cultivar exhibited much less low winter temperature injury than other cultivars and is one of the last cultivars to break bud dormancy in the spring.The selection of root stocks and cultivars resistant to winter freeze damage is an important aspect to avoiding loss during freeze events. Late-spring frost is another aspect of low temperature limiting tree nut cultivation. Damage caused by a spring freeze to one-year old grafted trees was significantly influenced by root stock and scion, and was directly correlated with stage of bud growth at the time of the freeze event. Pecan root stocks such as ‘Giles’, ‘Peruque’, or ‘Colby’ are considered hardier than ‘Riverside’ or ‘Moore’ to late-spring frost. The southern seedling root stock break bud dormancy earlier in the spring and are more susceptible to spring freeze damage. Scion growth has been observed to vary as a function of root stock, with early leafing root stock also forcing early spring growth in scions. The ‘Stuart’ seedlings tend to begin growth later in the spring, offering some protection from spring freeze. ‘Elliott’ seedlings have early spring growth, making them more susceptible to freeze damage than ‘Moore’. Twelve pecan root stocks including ‘87MX1-2.20 , ‘87MX5-1.70 , ‘Elliott’, ‘Frutoso’, ‘Giles’, ‘Major’, ‘Moore’, ‘Peruque’, ‘Posey’, ‘Riverside’, ‘San Felipe’, and ‘VC1-680 were tested under drought conditions. Among them, ‘Posey’ had the highest resistance and greatest water content under environmental pressure followed by ‘Perque’ with lowest ἑmax value in PV test while ‘Frutoso’ with lowest Va/Vp and shoot tissue water content in transpiration test had the lowest resistance.