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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.

There are generally inadequate supplies of seed for the newest rootstocks to satisfy the growing demand for the propagation material to be used in commercial citrus nurseries. Consequently, rootstock propagation, which is traditionally by seed, now often makes use of alternative methods such as cuttings and tissue culture (TC). Propagation through cuttings and TC will generate a root system that is largely composed of adventitious or lateral roots, compared with seed propagation, which will generally promote the formation of a well-defined taproot. In this study, we compared root architecture and growth of seven different rootstock plants, generated from seed, stem cuttings, or TC, during the early weeks of growth in the greenhouse. Based on total dry biomass, root mass fraction of plants generated from cuttings ranged from 11% to 16%, and from 16% to 29% and 21% to 30% for micropropagated plants and seedlings, respectively. Plants propagated through cuttings had the most primary roots (7–10), followed by tissue culture–propagated plants which developed an average of 2–6 primary roots. As expected, plants grown from seeds mostly developed a single and well-defined taproot during the first weeks. The total number of first order lateral roots was highest in the plants propagated as cuttings (108–185) compared with 53–103 and 43–78 for tissue culture–propagated plants and seedlings, respectively. Similarly, specific root length (SRL) was highest in plants derived from cuttings (21–43 m·g⁻¹) and lowest in plants grown from seed (7–20 m·g⁻¹). It is suggested that the larger number and length of roots on rootstock plants propagated through vegetative methods may be better suited for resource acquisition as compared with seed propagated plants.

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.

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 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.