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This article for Citrus Industry Magazine examines the relationship of different citrus rootstocks and root system architectures on the tree uprooting resistance.

Citrus is commercially propagated via grafting, which ensures trees have consistent fruit traits combined with favorable traits from the rootstock such as soil adaptability, vigor, and resistance to soil pathogens. Graft incompatibility can occur when the scion and rootstock are not able to form a permanent, healthy union. Understanding and preventing graft incompatibility is of great importance in the breeding of new fruit cultivars and in the choice of scion and rootstock by growers. The rootstock US-1283, a citrandarin generated from a cross of “Ninkat” mandarin (Citrus reticulata) and “Gotha Road” #6 trifoliate orange (Poncirus trifoliata), was released after years of field evaluation because of its superior productivity and good fruit quality on “Hamlin” sweet orange (C. sinensis) under Florida’s growing conditions. Subsequently, it was observed that trees of “Bearss” lemon (C. limon) and “Valencia” sweet orange (C. sinensis) grafted onto US-1283 exhibited unhealthy growth near the graft union. The incompatibility manifested as stem grooving and necrosis underneath the bark on the rootstock side of the graft. Another citrandarin rootstock, US-812 (C. reticulata “Sunki” × P. trifoliata “Benecke”), is fully graft compatible with the same scions. Transcriptome analysis was performed on the vascular tissues above and below the graft union of US-812 and US-1283 graft combinations with “Bearss” and “Valencia” to identify expression networks associated with incompatibility and help understand the processes and potential causes of incompatibility. Transcriptional reprogramming was stronger in the incompatible rootstock than in the grafted scions. Differentially expressed genes (DEGs) in US-1283, but not the scions, were associated with oxidative stress and plant defense, among others, similar to a pathogen-induced immune response localized to the rootstock; however, no pathogen infection was detected. Therefore, it is hypothesized that this response could have been triggered by signaling miscommunications between rootstock and scion either through (1) unknown molecules from the scion that were perceived as danger signals by the rootstock, (2) missing signals from the scion or missing receptors in the rootstock necessary for the formation of a healthy graft union, (3) the overall perception of the scion by the rootstock as non-self, or (4) a combination of the above.

Sainfoin (Onobrychis spp.) is a perennial forage legume that is also attracting attention as a perennial pulse with potential for human consumption. The dual use of sainfoin underpins diverse research and breeding programs focused on improving sainfoin lines for forage and pulses, which is driving the generation of complex datasets describing high dimensional phenotypes in the post-omics era. To ensure that multiple user groups, for example, breeders selecting for forage and those selecting for edible seed, can utilize these rich datasets, it is necessary to develop common ontologies and accessible ontology platforms. One such platform, Crop Ontology, was created in 2008 by the Consortium of International Agricultural Research Centers (CGIAR) to host crop-specific trait ontologies that support standardized plant breeding databases. In the present study, we describe the sainfoin crop ontology (CO). An in-depth literature review was performed to develop a comprehensive list of traits measured and reported in sainfoin. Because the same traits can be measured in different ways, ultimately, a set of 98 variables (variable = plant trait + method of measurement + scale of measurement) used to describe variation in sainfoin were identified. Variables were formatted and standardized based on guidelines provided here for inclusion in the sainfoin CO. The 98 variables contained a total of 82 traits from four trait classes of which 24 were agronomic, 31 were morphological, 19 were seed and forage quality related, and 8 were phenological. In addition to the developed variables, we have provided a roadmap for developing and submission of new traits to the sainfoin CO.

The devastating citrus disease huanglongbing (HLB) associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas) has caused a more than 70% reduction in citrus production since its discovery in Florida in 2005. Most citrus scion cultivars are sensitive to HLB, whereas some cultivars used as rootstocks are tolerant. Using such tolerant rootstocks can help trees to cope better with the disease’s impact. Evaluating rootstock effects on a grafted scion in the field takes many years, but shorter-term evaluation is imperative to aid in rootstock selection for an HLB-endemic production environment. In this study, we investigated grafted healthy and CLas-infected citrus trees under controlled greenhouse conditions. The objectives were to identify traits suitable for assessing grafted tree tolerance in advance of longer-term field studies and aiding in the selection of superior rootstock cultivars. We assessed 10 commercially important rootstocks grafted with ‘Valencia’ sweet orange scion and with known field performance. At 6, 9, 15, and 21 months after graft inoculation (mai), leaf CLas titers were determined and canopy health was evaluated. Plants were destructively sampled at 21 mai to assess plant biomasses and other physiological and horticultural variables. There was little influence of the rootstock cultivar on CLas titers. Surprisingly, few HLB foliar disease symptoms and no differences in soluble and nonsoluble carbohydrate concentrations were measured in infected compared with healthy plants, despite high CLas titers and significant reductions in plant biomasses. Most trees on rootstocks with trifoliate orange parentage were less damaged by HLB than other rootstocks, although results did not always agree with reported field performance. Among the different variables measured, leaf size appeared to be most predictive for grafted tree assessment of HLB sensitivity. The results of this study provide a better understanding of the strengths and weaknesses of assessing rootstock influence on grafted tree performance in a controlled greenhouse environment. Although such studies provide valuable information for cultivar tolerance to HLB, other rootstock traits will ultimately contribute to field survival and productivity in an HLB endemic production environment.

Grafting a scion onto a rootstock results in physical and physiological changes in plant growth and development, which can affect tree vigor, productivity, and tolerance to stress and disease. Huanglongbing (HLB) is one of the most destructive citrus diseases and has become endemic in Florida since its introduction in 2005. It is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), which cause severe metabolic disruptions in affected plants. Although most scion cultivars are highly susceptible, some rootstock cultivars are tolerant and allow the grafted tree to cope better with the disease. The objectives of this study were to identify rootstock traits that can be used to assess cultivars under controlled greenhouse conditions in advance of longer-term field trials. We used 10 commercially important rootstocks with different genetic backgrounds and known field performance in graft combination with ‘Valencia’ sweet orange scion. Trees were graft-inoculated with CLas and compared against mock-inoculated trees. Tree health and CLas populations were assessed regularly, and root growth was monitored using a minirhizotron imaging system. Plants were excavated and destructively sampled 21 months after inoculation to assess biomass distributions and other CLas-induced effects. We found significant differences between healthy and infected trees for most variables measured, regardless of the rootstock. In contrast to leaf CLas titers, root titers were significantly influenced by the rootstock, and highest levels were measured for ‘Ridge’ sweet orange and sour orange. Root growth and root biomasses were reduced upon infection but differences among rootstocks did not always agree with reported field performances. Despite severe biomass reductions plants maintained their relative distribution of biomass among different components of the root system, and no dead roots were observed. Root respiration was reduced by CLas infection and was overall higher in tolerant cultivars suggesting its potential as a physiological marker. This study improves our knowledge about the strengths and weaknesses of assessing rootstock traits of grafted trees in a controlled greenhouse setting. Results from the study suggest that in addition to HLB tolerance, other rootstock traits will ultimately have major contributions to field survival and productivity of the grafted trees in an HLB endemic production environment.

Huanglongbing (HLB) is a devastating disease of citrus that is found in most citrus production areas around the world. The bacterium associated with HLB resides in and damages the phloem, restricting the movement of photosynthates throughout the plant and leading to tree decline. Considerable root loss can be observed in affected trees even when few disease symptoms are visible aboveground. Root traits can substantially influence tree performance and use of superior rootstocks is one strategy to manage tree health and reduce production losses in a disease-endemic environment. Citrus rootstocks are typically propagated by seed, but due to the increased demand for some of the best-performing cultivars, propagation by other methods is being used to overcome seed shortages. In this research, differences in root architecture and root growth of six different rootstocks propagated by seed, cuttings, and tissue culture, and their influence on the grafted ‘Valencia’ (Citrus sinensis) scion were investigated. A field trial was established in southwest Florida in 2017. Trees were evaluated for their performance during the first 2 years after planting and a subset of trees was excavated for detailed analysis of root architectures and biomass distribution. Significant differences among propagation methods were found for the rootstock trunk diameter and the lateral (structural) root length, which were largest in seed-propagated rootstocks. Most of the other horticultural and root architectural traits were not significantly influenced by the rootstock-propagation method; however, many of the measured variables were significantly influenced by the rootstock cultivar regardless of the propagation method. The results showed that rootstocks propagated by cuttings and tissue culture were similar to seed-propagated rootstocks in their influence on the grafted tree during the early years of growth in the field.

The article defines the terms graft compatibility and incompatibility between scion and rootstock. It describes incompatibilities that have recently emerged when combining some of the newer citrus scion and rootstock cultivars and illustrates what to look out for when propagating new combinations.

The rootstock plays a large role in modern citrus production because of its influence on tolerance to adverse abiotic and biotic soil-borne stresses, and on the general horticultural characteristics of the grafted scion. In recent years, rootstock has received increased attention as a management strategy to alleviate the devastating effects of the bacterial disease huanglongbing (HLB), also known as “citrus greening.” In commercial citrus nursery production, rootstocks are typically propagated by seed. Because of the increased demand for HLB-tolerant rootstocks, seed supply is often inadequate for the most popular cultivars. Cuttings and tissue culture (TC) propagation are alternative methods to supply adequate quantities of genetically identical rootstocks to be used as liners for grafting. However, there are concerns among nursery owners and citrus growers regarding the possible inferiority of rootstocks that are not propagated by seed. This study investigates the influence of rootstock propagation method on traits of sweet orange trees grafted on four commercially important rootstock cultivars during the nursery stage and during the first year of growth in a commercial citrus orchard. Several of the measured traits during the nursery stage, including rootstock sprouting, grafted tree growth, and root mass distribution were significantly influenced by the rootstock propagation method, but traits were also influenced by the rootstock cultivar. Our results also suggest that for tissue culture-propagated plants, differences in the starting material and the culturing method can affect the grafted tree behavior. Except for canopy spread and scion to rootstock trunk diameter ratio, tree growth during the orchard stage was determined by the combination of propagation method and rootstock, rather than by propagation method alone.

Abstract Cilantro (Coriandrum sativum L.), also called coriander, is an herbaceous, annual plant that is cultivated worldwide for its leaves and seeds. Cilantro has a strong propensity to bolt quickly in hot weather and under long-day (LD) conditions, which affects the flavor and renders the crop unmarketable. High incidence of preharvest bolting in open-field production can cause significant economic loss. The phytohormone gibberellic acid (GA) regulates stem elongation and floral initiation in many LD rosette plants. In pilot experiments, we found that GA induced bolting in greenhouse-grown cilantro and that plant growth regulators (PGRs) with anti-GA activity can delay this process. We then explored the effects of different GA inhibitors on reducing the incidence of bolting in cilantro grown in a commercial open-field environment. Four field trials were conducted on a commercial farm near Clewiston in Florida between Fall 2016 and Spring 2018. Different growth regulators were applied at different times, ranging from 5 to 8 weeks after seeding (WAS), and plants were harvested 2 to 3 weeks thereafter. Applications of GA inhibitors significantly reduced the incidence of bolting in three of the four trials, but the extent depended on the type of inhibitor used. The results from one trial were inconclusive due to changes in weather that prevented bolting in the entire field. Overall, plots treated with prohexadione calcium and paclobutrazol were most effective and reduced bolting by up to 78%. Applying the PGRs at 5 and 6 WAS was more effective than at 7 or 8 WAS.

Citrus is commercially propagated via grafting, which ensures trees have consistent fruit traits combined with favorable traits from the rootstock such as soil adaptability, vigor, and resistance to soil pathogens. Graft incompatibility can occur when the scion and rootstock are not able to form a permanent, healthy union. Understanding and preventing graft incompatibility is of great importance in the breeding of new fruit cultivars and in the choice of scion and rootstock by growers. The rootstock US-1283, a citrandarin generated from a cross of "Ninkat" mandarin (Citrus reticulata) and "Gotha Road" #6 trifoliate orange (Poncirus trifoliata), was released after years of field evaluation because of its superior productivity and good fruit quality on "Hamlin" sweet orange (C. sinensis) under Florida's growing conditions. Subsequently, it was observed that trees of "Bearss" lemon (C. limon) and "Valencia" sweet orange (C. sinensis) grafted onto US-1283 exhibited unhealthy growth near the graft union. The incompatibility manifested as stem grooving and necrosis underneath the bark on the rootstock side of the graft. Another citrandarin rootstock, US-812 (C. reticulata "Sunki" x P. trifoliata "Benecke"), is fully graft compatible with the same scions. Transcriptome analysis was performed on the vascular tissues above and below the graft union of US- 812 and US-1283 graft combinations with "Bearss" and "Valencia" to identify expression networks associated with incompatibility and help understand the processes and potential causes of incompatibility. Transcriptional reprogramming was stronger in the incompatible rootstock than in the grafted scions. Differentially expressed genes (DEGs) in US-1283, but not the scions, were associated with oxidative stress and plant defense, among others, similar to a pathogen-induced immune response localized to the rootstock; however, no pathogen infection was detected. Therefore, it is hypothesized that this response could have been triggered by signaling miscommunications between rootstock and scion either through (1) unknown molecules from the scion that were perceived as danger signals by the rootstock, (2) missing signals from the scion or missing receptors in the rootstock necessary for the formation of a healthy graft union, (3) the overall perception of the scion by the rootstock as non-self, or (4) a combination of the above. [ABSTRACT FROM AUTHOR]

Cilantro (Coriandrum sativum L.), also called coriander, is an herbaceous, annual plant that is cultivated worldwide for its leaves and seeds. Cilantro has a strong propensity to bolt quickly in hot weather and under long-day (LD) conditions, which affects the flavor and renders the crop unmarketable. High incidence of preharvest bolting in open-field production can cause significant economic loss. The phytohormone gibberellic acid (GA) regulates stem elongation and floral initiation in many LD rosette plants. In pilot experiments, we found that GA induced bolting in greenhouse-grown cilantro and that plant growth regulators (PGRs) with anti-GA activity can delay this process. We then explored the effects of different GAinhibitors on reducing the incidence of bolting in cilantro grown in a commercial open-field environment. Four field trials were conducted on a commercial farm near Clewiston in Florida between Fall 2016 and Spring 2018. Different growth regulators were applied at different times, ranging from 5 to 8 weeks after seeding (WAS), and plants were harvested 2 to 3 weeks thereafter. Applications of GAinhibitors significantly reduced the incidence of bolting in three of the four trials, but the extent depended on the type of inhibitor used. The results from one trial were inconclusive due to changes in weather that prevented bolting in the entire field. Overall, plots treated with prohexadione calcium and paclobutrazol were most effective and reduced bolting by up to 78%. Applying the PGRs at 5 and 6WAS was more effective than at 7 or 8 WAS. [ABSTRACT FROM AUTHOR]