Rootstock Cultivars Research Articles

Evaluation of rootstock cultivars for almond in California

Evaluation of rootstock cultivars for almond in California

Vulnerability of pear (Pyrus) genetic resources in the U.S.

AbstractPears (Pyrus) represent an important part of consumer diets, and have the fourth highest production of non-citrus fruits, measured by fresh weight, in the U.S. They are maintained clonally and grown as composite plants, consisting of a scion (fruit bearing) cultivar grafted onto a rootstock cultivar. Up to 98% of existing production relies on only a few scion and rootstock cultivars, leaving the standing crop vulnerable to threats. Pears are faced with a wide range of biotic and abiotic threats and production vulnerabilities, some of which can be limited by integrating resistance and horticultural traits from wild and cultivated materials from around the world. The National Clonal Germplasm Repository (NCGR Corvallis), part of the USDA-ARS National Plant Germplasm System, maintains a large Pyrus collection from across the globe, consisting of 2793 Pyrus accessions from 37 species. The collection represents an important resource for preservation, research, and breeding efforts for pears. The crop vulnerability status of pears in the U.S. is currently moderate to high, with increasing threats and challenges. Breeding and preservation efforts, along with genetic, crop protection and production research are, however, actively targeting these needs.

Effects of Salt Stress on Salt-Repellent and Salt-Secreting Characteristics of Two Apple Rootstocks.

The effects of NaCl-induced salinity on biomass allocation, anatomical characteristics of leaves, ion accumulation, salt repellency, and salt secretion ability were investigated in two apple rootstock cultivars (Malus halliana '9-1-6' and Malus baccata), which revealed the physiological adaptive mechanisms of M. halliana '9-1-6' in response to salt stress factors. This experiment was conducted in a greenhouse using a nutrient solution pot. Salt stress was simulated by treating the plants with a 100 mM NaCl solution, while 1/2 Hoagland nutrient solution was used as a control (CK) instead of the NaCl solution. The results showed that the two rootstocks responded to salt environments by increasing the proportion of root biomass allocation. According to the stress susceptibility index, '9-1-6' exhibits a lower salt sensitivity index and a higher salt tolerance index. The thickness of the leaf, upper and lower epidermis, palisade tissue, and mesophyll tissue compactness (CTR) of the two rootstocks were significantly decreased, while the thickness of sponge tissue and mesophyll tissue looseness (SR) were significantly increased, and the range of '9-1-6' was smaller than that of M. baccata. With an extension of stress time, the accumulation of Na+ increased significantly, and the accumulation of K+ decreased gradually. The stem and leaves of '9-1-6' showed a lower accumulation of Na+ and a higher accumulation of K+, and the roots displayed a higher ability to reject Na+, as well as young and old leaves showed a stronger ability to secrete Na+. In conclusion, within a certain salt concentration range, the '9-1-6' root part can maintain lower salt sensitivity and a higher root-to-shoot ratio by increasing the proportion of root biomass allocation; the aerial part responds to salt stress through thicker leaves and a complete double-layer fence structure; the roots and stem bases can effectively reduce the transportation of Na+ to the aerial parts, as well as effectively secrete Na+ from the aerial parts through young and old leaves, thereby maintaining a higher K+/Na+ ratio in the aerial parts, showing a strong salt tolerance.

Integrative physiological, metabolomic, and transcriptomic analysis reveals the drought responses of two apple rootstock cultivars

BackgroundDrought is considered the main environmental factor restricting apple production and thus the development of the apple industry. Rootstocks play an important role in enhancing the drought tolerance of apple plants. Studies of the physiology have demonstrated that ‘ZC9-3’ is a strong drought-resistant rootstock, whereas ‘Jizhen-2’ is a weak drought-resistant rootstock. However, the metabolites in these two apple rootstock varieties that respond to drought stress have not yet been characterized, and the molecular mechanisms underlying their responses to drought stress remain unclear.ResultsIn this study, the physiological and molecular mechanisms underlying differences in the drought resistance of ‘Jizhen-2’ (drought-sensitive) and ‘ZC9-3’ (drought-resistant) apple rootstocks were explored. Under drought stress, the relative water content of the leaves was maintained at higher levels in ‘ZC9-3’ than in ‘Jizhen-2’, and the photosynthetic, antioxidant, and osmoregulatory capacities of ‘ZC9-3’ were stronger than those of ‘Jizhen-2’. Metabolome analysis revealed a total of 95 and 156 differentially accumulated metabolites in ‘Jizhen-2’ and ‘ZC9-3’ under drought stress, respectively. The up-regulated metabolites in the two cultivars were mainly amino acids and derivatives. Transcriptome analysis revealed that there were more differentially expressed genes and transcription factors in ‘ZC9-3’ than in ‘Jizhen-2’ throughout the drought treatment. Metabolomic and transcriptomic analysis revealed that amino acid biosynthesis pathways play key roles in mediating drought resistance in apple rootstocks. A total of 13 metabolites, including L-α-aminoadipate, L-homoserine, L-threonine, L-isoleucine, L-valine, L-leucine, (2S)-2-isopropylmalate, anthranilate, L-tryptophan, L-phenylalanine, L-tyrosine, L-glutamate, and L-proline, play an important role in the difference in drought resistance between ‘ZC9-3’ and ‘Jizhen-2’. In addition, 13 genes encoding O-acetylserine-(thiol)-lyase, S-adenosylmethionine synthetase, ketol-acid isomeroreductase, dihydroxyacid dehydratase, isopropylmalate isomerase, branched-chain aminotransferase, pyruvate kinase, 3-dehydroquinate dehydratase/shikimate 5-dehydrogenase, N-acetylglutamate-5-P-reductase, and pyrroline-5-carboxylate synthetase positively regulate the response of ‘ZC9-3’ to drought stress.ConclusionsThis study enhances our understanding of the response of apple rootstocks to drought stress at the physiological, metabolic, and transcriptional levels and provides key insights that will aid the cultivation of drought-resistant apple rootstock cultivars. Especially, it identifies key metabolites and genes underlying the drought resistance of apple rootstocks.

Oxidative stress as markers in identification of aluminum-tolerant peach tree rootstock cultivars and clonal selections

ABSTRACT Peach rootstock and scion cultivars are selected in breeding programs considering resistance to pests and diseases, salt tolerance, drought tolerance, and vigor. However, rootstock tolerance to aluminum (Al), which is markedly present in tropical and subtropical soils of the world, is not considered. Thus, it is essential to define potential markers that can contribute to the selection of Al-resistant or Al-tolerant peach rootstocks. The objective of this study was to identify Al-tolerant peach tree rootstock cultivars and clonal selections using physiological and oxidative stress variables. A completely randomized experimental design was used in a 13 (rootstock cultivars and clonal selections) × 2 (with and without Al) factorial arrangement, with three replications. Nursery peach trees of own-rooted ‘BRS Mandinho’ (without rootstock) and nursery trees of ‘BRS Mandinho’ grafted on different rootstock cultivars and clonal selections were grown in a hydroponic system, consisting of two treatments, with and without 100 mg L -1 of Al. Dry biomass, photosynthetic variables, pigment concentration, hydrogen peroxide content, membrane lipid peroxidation, and activity of the antioxidant enzymes were evaluated. The total dry matter production of the own-rooted ‘BRS Mandinho’ trees and the SS-CHI-09-39 and SS-CHI-09-40 selections is affected by the Al presence, representing a decline of 35.4, 37.2, and 24.4 %, respectively, compared to the treatment with Al. The highest total dry matter production in Al presence was observed for the ‘Sharpe’ rootstock. ‘Capdeboscq’, DB-SEN-09-23, FB-ESM-09-43, JB-ESM-09-13, JAH-MAC-09-77, SAS-SAU-09-71, and VEH-GRA-09-55 rootstock selections are tolerant to Al. The use of physiological and biochemical variables shows potential for the selection of clonal rootstocks tolerant or resistant to Al.

Rootstock vigor dictates the canopy light environment that regulates metabolite profile and internal fruit quality development in peach

Five rootstock cultivars of differing vigor: vigorous (‘Atlas™’ and ‘Bright's Hybrid® 5’), standard (‘Krymsk® 86’ and ‘Lovell’) and dwarfing (‘Krymsk® 1’) grafted with ‘Redhaven’ as the scion were studied for their impact on productivity, mid-canopy photosynthetic active radiation transmission (i.e., light availability) and internal fruit quality. Αverage yield (kg per tree) and fruit count increased significantly with increasing vigor (trunk cross sectional area, TCSA). Α detailed peach fruit quality analysis on fruit of equal maturity (based on the index of absorbance difference, IAD) coming from trees with equal crop load (no. of fruit cm−2 of TCSA) characterized the direct impact of rootstock vigor on peach internal quality [dry matter content (DMC) and soluble solids concentration (SSC)]. DMC and SSC increased significantly with decreasing vigor and increasing light availability, potentially due to reduced intra-tree shading and better light distribution within the canopy. Physiologically characterized peach fruit mesocarp was further analyzed by non-targeted metabolite profiling using gas chromatography mass spectrometry (GC-MS). Metabolite distribution was associated with rootstock vigor class, mid-canopy light availability and fruit quality characteristics. Fructose, glucose, sorbose, neochlorogenic and quinic acids, catechin and sorbitol were associated with high light environments and enhanced quality traits, while sucrose, butanoic and malic acids related to low light conditions and inferior fruit quality. These outcomes show that while rootstock genotype and vigor are influencing peach tree productivity and yield, their effect on manipulating the light environment within the canopy also plays a significant role in fruit quality development.

In vitro propagation of stone fruit rootstock cultivar Evrica 99 and its influence on some phytochemical traits of fresh apricot fruit

Background: The cultivation of stone fruits is of primary importance in Armenia. Their fruits contain antioxidants, fiber, potassium, vitamin A, C, E, minerals, etc., which have a beneficial effect on human health and prevent many diseases. The concentration of those components varies depending on ecological factors, cultivar, rootstock, cultural practices, etc. Clonal rootstocks are important for increasing orchard density, tree uniformity, and high yields, and they can also affect fruit quality. In vitro culture is a valuable method for rapid propagation of high-quality, virus free plant material. Objective: The purpose of this study was to develop in vitro production technology for the stone-fruit rootstock cultivar Evrica 99, and to determine if rootstocks affect some fresh fruit traits of ´Yerevani´ and ´Sateni´ apricot cultivars. Methods: The shoot apical meristem and lateral bud served as explants for shoot regeneration. Different sterilizing agents at various periods of exposure were used for the explant surface sterilization. Various concentrations of phytohormones, both individually and in combinations, were employed for in vitro regeneration and rooting of plants. The titratable acidity (TA), dry matter (DM), vitamin C, mineral content, total carotenoids (TC), and sugar contents were evaluated in fresh fruit. Results: The most optimal option for explant surface sterilization was the gradual application of calcium hypochlorite [Ca (ClO)2] (2.0% solution, exposure time 10 min) and ethanol (70% solution, exposure time 20 s), as a result of which we had 75.5% survival rate of explants. The efficient medium for in vitro shoot regeneration was MS supplemented with 6-Benzylaminopurine (BAP) 0.8 mg/l, Kinetin (Kin) 0.2 mg/l, and Gibberellic acid (GA3) 1.0 mg/l. The half-strength Murashige and Skoog (MS) medium containing 0.8 mg/l indole-3-butyric acid (IBA) was optimal for in vitro rooting. Rooted plants were successfully adapted with a survival rate of 85.0%. The defined method can be successfully used for 'Evrica 99' cultivar micropropagation. The results obtained showed that fruit quality strongly depended on both the varieties and the rootstock tested. Conclusion: In the current study, an alternative in vitro propagation technology for rootstock cultivar ´ Evrica 99 was developed by direct organogenesis, enabling mass-scale production of virus-free plants that is suitable for commercial purposes as well. The apricot cultivars ´Yerevani´ and ´Sateni´ grafted on the virus-free rootstock cultivar Evrica 99 showed higher fruit quality traits, which are essential for human health and diet. Keywords: ´Evrica 99, fucnctional foods, in vitro regeneration, micropropagation, plant growth regulators, stone fruit rootstock, tissue culture.

Health Status of Ready-to-Plant Grapevine Nursery Material in Canada Regarding Young Vine Decline Fungi.

Young vine decline (YVD), caused by several taxonomically different fungi, results in the decline and death of grapevines within a few years after planting. Infection can occur in nursery mother blocks and/or at several stages in the nursery propagation process, but the final plant material may remain asymptomatic. Four nurseries that sell ready-to-plant grapevines in Canada were sampled to evaluate the health status with regard to YVD fungi, including Botryosphaeriaceae spp., Cadophora luteo-olivacea, Dactylonectria macrodidyma, Dactylonectria torresensis, Phaeoacremonium minimum, and Phaeomoniella chlamydospora. Plants representing three cultivars, 'Chardonnay', 'Merlot', and 'Pinot noir', either grafted onto '3309C' rootstock or self-rooted, were provided by the nurseries. Samples from the roots, base of the rootstock or self-rooted cultivar, graft-union, and scion were collected from each plant. DNA was extracted, and the total abundance of each fungus was quantified using Droplet Digital PCR. Results revealed that 99% of plants harbored at least one of the fungi studied, with a mean of three different fungal species that were present per grapevine. Droplet Digital PCR results showed that the abundance of the different fungi significantly varied between different sections of each plant, individual plants for each cultivar, and cultivars from the same nursery. Necrosis measurements were recorded from the base of the rootstock or self-rooted cultivars and did not correlate with fungal abundance recorded in that section for each grapevine, but necrosis was consistent across cultivars within nurseries. Five different rootstocks were compared from one nursery, and results showed no differences between rootstocks and their health status. Among all nurseries, C. luteo-olivacea was the most prevalent fungus (97% of the plants), while D. macrodidyma was the least commonly found (13% of the plants). This study shows that ready-to-plant nursery material sold in Canada is likely to be infected with several YVD fungi and that presence and abundance of fungi vary significantly among individual grapevines and nurseries.

Transcriptional regulation of MdPIN7 by MdARF19 during gravity-induced formation of adventitious root GSA in self-rooted apple stock

Self-rooted apple stock is widely used for apple production. However, the shallowness of the adventitious roots in self-rooted apple stock leads to poor performance in the barren orchards of China. This is because of the considerable difference in the development of a gravitropic set-point angle (GSA) between self-rooted apple stock and seedling rootstock. Therefore, it is crucial to study the molecular mechanism of adventitious root GSA in self-rooted apple stock for breeding self-rooted and deep-rooted apple rootstock cultivars. An apple auxin response factor MdARF19 functioned to establish the adventitious root GSA of self-rooted apple stock in response to gravity and auxin signals. MdARF19 bound directly to the MdPIN7 promoter, activating its transcriptional expression and thus regulating the formation of the adventitious root GSA in 12-2 self-rooted apple stock. However, MdARF19 influenced the expression of auxin efflux carriers (MdPIN3 and MdPIN10) and the establishment of adventitious root GSA of self-rooted apple stock in response to gravity signals by direct activation of MdFLP. Our findings provide new information on the transcriptional regulation of MdPIN7 by auxin response factor MdARF19 in the regulation of the adventitious root GSA of self-rooted apple stock in response to gravity and auxin signals.

Induction of Polyploidy in Citrus Rootstocks through In Vitro Colchicine Treatment of Seed-Derived Explants

Polyploidy, frequently observed in citrus species, aids in achieving better adaptation to environmental stresses. In this context, the current work aims to develop stable tetraploids in citrus rootstock cultivars, viz., Rough lemon, Rangpur lime and Alemow, through in vitro colchicine treatments. Seed-derived explants were obtained by culturing sterile seeds on MS basal media. Seedlings with a size of 5–8 mm (hypocotyl) were exposed to colchicine treatment. After treatment, the surviving seedlings were minigrafted onto six-month-old rootstock for better growth. Colchicine concentrations of 0.1%, 0.2%, 0.3% and control for durations of 16 or 24 h were tested with respect to the induction of polyploidisation. Treatment with 0.1% colchicine for 24 h resulted in high rates of mutation for polyploidisation and showed the highest tetraploid induction percentage (18.3%) in all the rootstock cultivars. High colchicine concentration and long exposure time decreased the survival of the observed seedlings. Flow cytometry and cytological methods were used for confirmation of autotetraploidy in the analysed samples. The surviving seedlings were identified on the basis of morphological and cytological variables, such as leaf area and stomata size, which significantly increased with increasing ploidy level. The proposed method was found to be an effective way to induce the polyploidy in Rangpur lime, Rough lemon and Alemow rootstocks.

Genome-Wide Identification and Functional Analysis of the AP2/ERF Transcription Factor Family in Citrus Rootstock under Waterlogging Stress.

Citrus plants are sensitive to waterlogging, and the roots are the first plant organ affected by hypoxic stress. The AP2/ERF (APETALA2/ethylene-responsive element binding factors) can modulate plant growth and development. However, the information on AP2/ERF genes in citrus rootstock and their involvement in waterlogging conditions is limited. Previously, a rootstock cultivar, Citrus junos cv. Pujiang Xiangcheng was found to be highly tolerant to waterlogging stress. In this study, a total of 119 AP2/ERF members were identified in the C. junos genome. Conserved motif and gene structure analyses indicated the evolutionary conservation of PjAP2/ERFs. Syntenic gene analysis revealed 22 collinearity pairs among the 119 PjAP2/ERFs. The expression profiles under waterlogging stress showed differential expression of PjAP2/ERFs, of which, PjERF13 was highly expressed in both root and leaf. Furthermore, the heterologous expression of PjERF13 significantly enhanced the tolerance of transgenic tobacco to waterlogging stress. The overexpression of PjERF13 decreased the oxidative damage in the transgenic plants by reducing the H2O2 and MDA contents and increasing the antioxidant enzyme activities in the root and leaf. Overall, the current study provided basic information on the AP2/ERF family in the citrus rootstock and uncovered their potential function in positively regulating the waterlogging stress response.

Influence of rootstock on endogenous hormones and color change in Cabernet Sauvignon grapes

Different rootstocks for grapes can significantly affect fruit color and quality, possibly by affecting hormone contents, related genetic pathways, and fruit coloring mechanisms in skin. ‘Cabernet Sauvignon’ was grafted to ‘5BB’, ‘SO4’, ‘140R’, ‘CS’, ‘3309M’ and ‘Vitis riparia’ rootstocks, with self-rooting seedlings as the control (CS/CS), and sampled from the early stage of veraison to the ripening stage. The effects of rootstock on the contents of gibberellin (GA3), auxin (IAA), and abscisic acid (ABA) in grape skin were determined alongside the expression levels of eight anthocyanin synthesis related genes using real-time fluorescence quantitative PCR methods. The rootstock cultivars exhibited accelerated fruit color change, and the CS/140R combination resulted in grapes with more color than the control group in the same period. With the development of fruit, the IAA and GA3 contents in the skin of different rootstock combinations showed trends of increasing initially, then decreasing, while the ABA content decreased initially and then increased. During the verasion (28 July), the various ‘Cabernet Sauvignon’ rootstock combinations exhibited varying degrees of increases in GA3, ABA, and IAA contents; correlation analysis showed that, at the start of veraison, the expression levels of the anthocyanin synthesis-related genes VvCHS, VvDFR, and VvUFGT had strong positive correlations with hormone contents, which indicated they are key genes involved in the endogenous hormone responsive anthocyanin biosynthesis pathway. The results of this study showed that rootstock regulates the fruit coloring process by influencing the metabolism level of peel hormones in the ‘Cabernet Sauvignon’ grape.

Relative Influence of Rootstock and Scion on Asian Citrus Psyllid Infestation and Candidatus Liberibacter asiaticus Colonization

The citrus industry in Florida faces a destructive endemic disease, known as huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus (CLas), a phloem-limited bacterium, and transmitted by the Asian citrus psyllid (ACP). Rootstocks are regarded as critical to keep citrus production commercially viable and help trees cope with the disease. Although most scions are susceptible, some rootstocks are HLB-tolerant and may influence ACP infestation and CLas colonization and therefore the grafted tree tolerance. This study aimed to elucidate the relative influence of rootstock and scion on insect vector infestation and CLas colonization under natural HLB-endemic conditions. Seven commercial rootstock cultivars with different genetic backgrounds were grafted with ‘Valencia’ sweet orange (Citrus sinensis) or were self-grafted (non-‘Valencia’) and planted in an open field where ACP and CLas were abundant. ACP infestation was determined weekly during periods of leaf flushing, and leaves and roots were analyzed every 3 months to determine CLas titers. Trees with ‘Valencia’ scion were more attractive to the psyllids than non-‘Valencia’ scions. This was also associated with a higher number of bacteria and a larger abundance of foliar HLB symptoms. The influence of the rootstock on the psyllid attraction of grafted ‘Valencia’ scion was less evident, and leaf CLas titers were similar regardless of the rootstock. Among the non-‘Valencia’ scions, Carrizo had the lowest and US-942 the highest leaf CLas titers. Root CLas titers also varied among cultivars, and standard sour orange roots harbored more bacteria than some trifoliate orange hybrid rootstocks such as US-942. In some trees, CLas was detected first in the roots 4 months after planting, but root CLas titers remained low throughout the study. In contrast, leaf CLas titers increased over time and were considerably higher than root titers from 7 months until the end of the study, 15 months after planting. Overall, the results of this study demonstrate a greater relative influence of the scion than the rootstock on ACP infestation and CLas colonization during the early stages of infection. This suggests that other cultivar-specific traits, such as the ability to tolerate other stresses and to absorb water and nutrients more efficiently, along with influences on the scion phenology, may play a larger role in the rootstock influence on the grafted tree tolerance during the later stages of HLB progression.

Towards grapevine root architectural models to adapt viticulture to drought.

To sustainably adapt viticultural production to drought, the planting of rootstock genotypes adapted to a changing climate is a promising means. Rootstocks contribute to the regulation of scion vigor and water consumption, modulate scion phenological development and determine resource availability by root system architecture development. There is, however, a lack of knowledge on spatio-temporal root system development of rootstock genotypes and its interactions with environment and management that prevents efficient knowledge transfer into practice. Hence, winegrowers take only limited advantage of the large variability of existing rootstock genotypes. Models of vineyard water balance combined with root architectural models, using both static and dynamic representations of the root system, seem promising tools to match rootstock genotypes to frequently occurring future drought stress scenarios and address scientific knowledge gaps. In this perspective, we discuss how current developments in vineyard water balance modeling may provide the background for a better understanding of the interplay of rootstock genotypes, environment and management. We argue that root architecture traits are key drivers of this interplay, but our knowledge on rootstock architectures in the field remains limited both qualitatively and quantitatively. We propose phenotyping methods to help close current knowledge gaps and discuss approaches to integrate phenotyping data into different models to advance our understanding of rootstock x environment x management interactions and predict rootstock genotype performance in a changing climate. This could also provide a valuable basis for optimizing breeding efforts to develop new grapevine rootstock cultivars with optimal trait configurations for future growing conditions.

Sodicity stress differently influences physiological traits and anti-oxidant enzymes in pear and peach cultivars.

The growth and physiological responses to sodicity stress of pear and peach are poorly understood. Insights into how sodicity stress alters tree physiology remain vital to developing salt tolerant scion and rootstock cultivars. The effects of sodicity stress (soil pHs ~8.8) on tree growth and physiological traits of field grown trees of pear cultivars Punjab Beauty and Patharnakh, and peach cultivars Partap and Shan-e-Punjab were recorded using standard procedures. Sodicity-induced changes in oxidative stressors, proline, anti-oxidant enzymes and leaf ions were measured to draw inferences. Sodicity-induced reductions in vegetative growth were particularly marked in Patharnakh pear and Partap peach compared with other cultivars. Although sodicity stress triggered a significant increase in leaf malondialdehyde (MDA) and hydrogen peroxide (H2O2), their levels relative to controls were much higher in peach than in pear; reflecting that peach suffered from greater oxidative stress. Interestingly, MDA and H2O2 levels did not seem to be deleterious enough to trigger proline-induced osmotic adjustment in pears. The activities of anti-oxidant enzymes strongly varied with the cultivar; specifically, the sodicity-induced increases in CAT and SOD activities were much higher in Punjab Beauty pear and Shan-e-Punjab peach. Principal Component Analysis revealed an explicit convergence between CAT and SOD activities in Punjab Beauty and Shan-e-Punjab cultivars in response to sodicity-induced oxidative stress. Correlation analysis revealed that leaf Na+ strongly inhibited tree growth in peach than in pear. Leaf K+ and proline were found to be the major osmolytes in sodicity-stressed pear and peach cultivars, respectively. We have for the first time studied the effects of sodicity stress on important tree growth and physiological traits of commercially important pear and peach cultivars. Our findings revealed a marked suppressive effect of sodicity stress on tree growth in peach than in pear. The sodicity-induced upticks in leaf malondialdehyde, hydrogen peroxide and Na+ seemed to induce proline-mediated osmotic adjustment in peach but not in pear. The overall better sodicity tolerance in pear compared to peach was ascribed to increased activities of anti-oxidant enzymes catalase and superoxide dismutase enzymes together with restricted Na+ uptake and better leaf K+ levels. Further investigations are needed to elucidate the effects of sodicity stress on genetic and transcriptional changes, and on fruit yield and quality.

Genetic modulation of Valencia sweet orange field performance by 50 rootstocks under huanglongbing-endemic conditions.

Although the citrus scion cultivar primarily determines the characteristics of the fruit, the rootstock cultivar of the graft combination has a major role in determining the horticultural performance of the tree. The disease huanglongbing (HLB) is particularly devastating to citrus, and the rootstock has been demonstrated to modulate tree tolerance. However, no existing rootstock is entirely suitable in the HLB-endemic environment, and citrus rootstocks are particularly challenging to breed because of a long life cycle and several biological characteristics that interfere with breeding and commercial use. This study with Valencia sweet orange scion documents the multi-season performance of 50 new hybrid rootstocks and commercial standards in one trial that forms the first wave of a new breeding strategy, with the aim of identifying superior rootstocks for commercial use now, and mapping important traits to be used in selection for the next generation of outstanding rootstocks. A large assortment of traits were quantified for all trees in the study, including traits associated with tree size, health, cropping, and fruit quality. Among the quantitative traits compared between rootstock clones, all except one were observed to have significant rootstock influence. Multiple progeny from eight different parental combinations were included in the trial study, and significant differences between parental combinations of the rootstocks were observed for 27 of the 32 traits compared. Pedigree information was integrated with quantitative trait measurements to dissect the genetic components of rootstock-mediated tree performance. Results suggest there is a significant genetic component underlying rootstock-mediated tolerance to HLB and other critical traits, and that integration of pedigree-based genetic information with quantitative phenotypic data from trials should enable marker-based breeding approaches for the rapid selection of next-generation rootstocks with superior combinations of traits that are needed for commercial success. The current generation of new rootstocks included in this trial is a step toward this goal. Based on results from this trial, the new hybrids US-1649, US-1688, US-1709, and US-2338 were considered the four most promising new rootstocks. Release of these rootstocks for commercial use is being considered, pending the evaluation of continuing performance in this trial and the results from other trials.

Drought tolerance classification of grapevine rootstock by machine learning for the São Francisco Valley

Machine Learning (ML) algorithms are increasingly being used in several areas of agricultural studies, such as plant breeding. ML can assist in the recognition of relevant patterns or groups, or even in the prediction of the outcome under new settings, thus accelerating experiments and interpretating their results. The identification and selection of drought-tolerant grapevine rootstock (Vitis spp.) have become more relevant in late years, motivated mostly by global climate change scenarios. However, the grapevine is a perennial species, with polygenic characteristics and a complex traits inheritance by offspring, thus making it very challenging to discover new, drought tolerant cultivars. For this reason, this study's main objective was to compare the performance of six machine learning models on the prediction of drought tolerance levels of grapevine rootstock cultivars. A data set with forty-five distinct cultivars was used to evaluate the methods, and the best performing model (AUC 0.9857) was used to predict the drought tolerance class of three cultivars (IAC 313, IAC 572, and IAC 766) whose drought tolerance level was still unknown. The results predicted a high drought tolerance for IAC 313 and IAC 766 cultivars, and a low tolerance for IAC 572.

Field performance of peach rootstock cultivars in South Carolina

Field performance of peach rootstock cultivars in South Carolina

Comparative Transcriptomic Analysis of Inarching Invigorating Rootstock onto Incompatible Grafts in Citrus

Grafting is a technique that is widely used in citrus production. Graft incompatibility often occurs in the orchard. Inarching can effectively improve the vigor of incompatible grafts, but its mechanism remains poorly understood. Our previous studies investigated the scion-rootstock interaction of citrus and highlighted the role of hormonal balance and genes in abscisic acid biosynthesis regulation. To further elucidate the mechanism of inarched grafts rejuvenation, Hm/Pt combination (Citrus maxima (Burm.) Merrill cv. 'Hongmian miyou' grafted onto Poncirus trifoliata) were inarched with 'Pujiang Xiangcheng' (a novel citrus rootstock cultivar recently selected from wild Citrus junos populations), and comprehensive analysis was performed to compare the inarched grafts and controls. Compared with incompatible grafts, the results revealed that inarching could recover the leaf metabolism balance, including reducing starch content, increasing chlorophyll content and restoring the cell structure. Additionally, our results corroborated that hormonal balance and hormone-related genes played a central role in inarching compatibility. Furthermore, the roles of Hsf4, ERF1, NCED3 and PYL were highlighted, and a model for explaining inarched grafts recovery invigoration was proposed. This study shed light on the mechanism of inarching regulation tree vigor and offered deep insights into the scion-rootstock interaction in citrus.

Grape Tomato Growth, Yield, and Fruit Mineral Content as Affected by Rootstocks in a High Tunnel Organic Production System

Recently, so-called “vegetative” and “generative” rootstocks have been identified by seed companies as rootstock types that have different impacts on tomato scions. In this experiment of grafted grape tomato production in an organically managed high tunnel system, we characterized the effects of vegetative and generative rootstock cultivars on tomato yield components and fruit mineral contents. Grape tomato scions ‘BHN 1022’ (determinate) and ‘Sweet Hearts’ (indeterminate) were grafted onto ‘DR0141TX’ (vegetative), ‘Estamino’ (generative), and ‘Multifort’ (noncharacterized) rootstocks with self- and nongrafted scions as controls. Experiments were conducted twice with different transplanting dates (Expt. 1: 31 Jan. vs. Expt. 2: 9 Mar.) in 2018. No rootstock by scion interaction effects on whole-season fruit yield components were observed, indicating similar responses of determinate and indeterminate grape tomato scions to all rootstocks tested. For Expt. 1, the three rootstocks increased marketable fruit number, marketable yield, and total yield by 23.3%, 37.9%, and 34.4% on average, respectively, compared with the self- and nongrafted controls, primarily due to improved productivity during the peak and late harvest periods. For Expt. 2, the rootstocks did not significantly benefit any whole-season yield components. ‘DR0141TX’ and ‘Multifort’ increased stem diameter in both experiments, whereas ‘Estamino’ only increased stem diameter in Expt. 2 relative to the nongrafted controls. Consistent increase in aboveground dry biomass of rootstock treatments at crop termination in Expt. 1 corresponded to the greater yield of rootstock-grafted plants in that experiment. All rootstocks in both experiments consistently increased fruit P, K, Ca, Zn, and Fe contents on a dry weight basis at peak harvest regardless of the tomato scion used. Despite a relatively low level of root-knot nematode infestation, plants grafted with ‘DR0141TX’ or ‘Estamino’ tended to have lower root galling index ratings than scion controls and ‘Multifort’-grafted plants, which was more evident in Expt. 1. Given the different environmental conditions during the tomato production period between the two experiments conducted in high tunnels, our findings highlight the important influence of production environment on grafted tomato performance. This study on grafted grape tomatoes in high tunnel organic production systems also demonstrated that so-called “vegetative” and “generative” rootstocks had similar impacts on tomato scion yield components and fruit mineral contents.