Citrumelo Rootstock Research Articles

Softwood Grafting of Clementine Mandarin as Affected by Thidiazuron Treatment and Rootstock Type

Background: Citrus are propagated on a commercial scale by budding on different rootstocks species; nevertheless, one of the main problems nurserymen encounter is the slow growth of these rootstocks in the early phases of their growth, which drives up production costs. Therefore, the process of softwood grafting is one of the promising methods for quick propagating species of citrus. Methods: The present study on softwood grafting in clementine mandarin was conducted under lath house conditions during 2022-2023, to illustrate the impact of dipping the scion shoots in Thidiazuron at a concentration of 0, 10 and 20 mg L-1 and types of rootstocks (Sour orange, Rough lemon, Rangpur lime and Swingle citrumelo) on the percentage of grafting success and some vegetative characteristics of resultant saplings. A factorial experiment was executed out utilizing a randomized complete block design (RCBD) with three replications. Result: The two factors of study, particularly the treatment with 20 mg L-1 of Thidiazuron and Rough lemon rootstock, showed significant superiority in all traits, such as earliness in the initiation and completion of sprouting, graft success per cent, graft survival per cent, plant height, scion and rootstock diameter, as well as the number of branches and leaves plant-1. On the other hand, the un-treated with growth regulator and Swingle citrumelo rootstock recorded the lowest values for these traits.

Rootstocks affect the vulnerability to embolism and pit membrane thickness in Citrus scions.

Embolism resistance of xylem tissue varies among species and is an important trait related to drought resistance, with anatomical attributes like pit membrane thickness playing an important role in avoiding embolism spread. Grafted Citrus trees are commonly grown in orchards, with the rootstock being able to affect the drought resistance of the whole plant. Here, we evaluated how rootstocks affect the vulnerability to embolism resistance of the scion using several rootstock/scion combinations. Scions of 'Tahiti' acid lime, 'Hamlin', 'Pera' and 'Valencia' oranges grafted on a 'Rangpur' lime rootstock exhibit similar vulnerability to embolism. In field-grown trees, measurements of leaf water potential did not suggest significant embolism formation during the dry season, while stomata of Citrus trees presented an isohydric response to declining water availability. When 'Valencia' orange scions were grafted on 'Rangpur' lime, 'IAC 1710' citrandarin, 'Sunki Tropical' mandarin or 'Swingle' citrumelo rootstocks, variation in intervessel pit membrane thickness of the scion was found. The 'Rangpur' lime rootstock, which is known for its drought resistance, induced thicker pit membranes in the scion, resulting in higher embolism resistance than the other rootstocks. Similarly, the rootstock 'IAC 1710' citrandarin generated increased embolism resistance of the scion, which is highly relevant for citriculture.

Germplasm Diversification in Citrus Orchards in a Mesothermal Climate in Brazil

The limited scion-rootstock combinations used in sweet orange orchards in Brazil reduce the commercial potential of citrus and lead to greater susceptibility to pests and diseases. Aiming to provide germplasm diversification, the research evaluated the Folha Murcha (FM) and Charmute de Brotas (CB) orange trees grafted onto ‘Rangpur’ lime (RL) and ‘Swingle’ citrumelo (SC) rootstocks. The study was conducted in the southern center of the state of São Paulo, in the subtropical region of southeastern Brazil. The grafted trees were planted in September 2016. The field experiment was conducted over two seasons. The combinations were evaluated based on the duration in days and degree-days of the phenological intervals, growth and yield performance, alternate bearing, fruit ripening, and quality. The trees of the two scion cultivars grafted onto RL had the shortest crop cycle, the earliest fruit ripening, and the largest canopy volume. SC produced fewer alternate bearings with greater production efficiency. The CB/SC combination produced fruits with a better color index, higher total soluble solids content, and a higher technological index. This assessment can be useful in planning orchards for dual-purpose markets, such as table fruit and the juice industry.

Field Performance of ‘Valencia’ Sweet Orange Trees Grafted onto Pummelo Interstocks and Swingle Citrumelo Rootstocks under Huanglongbing (HLB) Endemic Conditions

Interstocks have been used in fruit tree cultivation to regulate tree size and improve fruit production and quality. In this study, several Huanglongbing (HLB)-tolerant open-pollinated pummelo interstock candidates were evaluated as interstocks between the Swingle rootstock and the ‘Valencia’ scion, with Swingle serving as the control interstock. After 5 years in the field, most trees did not exhibit visual HLB symptoms, although the trees were infected with HLB, and the CaLas Ct values in the ‘Valencia’ leaves of the different interstock treatments ranged between 25.88 and 27.82. Although the foliar chlorophyll content among the interstock treatments was not highly significant (p-value = 0.0313), the foliar starch content was significantly different (p-value = 0.0018). ‘Valencia’ grafted onto 5-1-99-3 and HBJL-4 interstocks (both open pollinated seedlings of the Hirado Buntan pummelo) exhibited the highest total phenolic compound (TPC) levels (46.44 and 46.36 mg gallic acid g−1 FW). Transcripts of CsPR1 and CsPR2, two pathogenesis-related (PR) genes, were upregulated in ‘Valencia’ grafted onto open pollinated seedling selections of the red shaddock pummelo, Liang Ping Yau pummelo, and Hirado Buntan pummelo compared with ‘Valencia’ grafted onto Swingle. All interstocks influenced the tree growth rate and improved canopy volume in the field compared to the control trees without any interstocks (p-value = 0.0085). The 5-4-99-7 (red shaddock pummelo) and 8-1-99-1B (Liang Ping Yau pummelo) interstock trees had the highest canopy volume among all the treatments. We propose, based on our current results, that HLB-tolerant citrus accessions, when judiciously used as interstocks, may enhance plant defense and provide increased HLB tolerance to susceptible scions.

Leaf physiological responses of the citrus commercial varieties grafted onto rootstocks

The use of rootstocks provides choices to growers to increase fruit quality and yield, obtain high-density planting, avoid juvenility, and control tree size. The choice of rootstock for a citrus orchard was an important consideration. The aim of the study was to evaluate the effects of citrus rootstocks on leaf mineral composition and leaf physiology of the citrus commercial cultivars (‘Pontianak’ tangerine, ‘Pamindo’ pamelo and ‘Montaji’ lemon). For this purpose, the citrus commercial cultivars were budded onto Japansche citroen (JC) and Citrumelo rootstocks and then leaf mineral composition (total nitrogen, phosphor and kalium concentration) and leaf physiology (chlorophyll index, respiration rate, stomatal conductance and photosynthetic rate) of each scion were measured. Results showed that rootstock type had different influence on leaf mineral composition and leaf physiology in leaf of each scion. For total nitrogen, total kalium and stomatal conductance variables, rootstocks had no effect. For total phosphor variable, JC rootstock induced higher value than Citrumelo in all scion. For chlorophyll Index variable, Citrumelo induced higher value on ‘Pamindo’ and lower value on ‘Montaji’ than JC but had no effect on ‘Pontianak’ tangerine. For respiration rate and photosynthetic rate variable, Citrumelo induced higher value than JC in ‘Pamindo’ but had no effect on ‘Pontianak’ tangerine and ‘Montaji’. Therefore, ‘Pamindo’ was better to be budded on Citrumelo, ‘Montaji’ and ‘Pontianak’ tangerine were better to be budded on JC.

Profile of Bioactive Compounds in Orange Juice Related to the Combination of Different Scion/Rootstocks, Packaging and Storage

Citrus scion/rootstock combinations alter the concentration of bioactive compounds in orange juice. The shelf life of freshly squeezed juice can be maximized through packaging and storage. The profiles of ascorbic (AA), dehydroascorbic acid (DHAA), and phenolic compounds were analyzed in juices of four sweet orange scions, Sanguínea de Mombuca (SM), Rubi (R), Lue Gin Gong (LGG), and Valência Delta Seedless (VDS), grafted onto ‘Rangpur’ lime (RL) and ‘Swingle’ citrumelo (SC) rootstocks. The juices obtained from the combination of the ‘Rubi’ orange in both rootstocks stood out by their higher concentration of ascorbic acid (AA) and dehydroascorbic acid (DHAA). Overall, all SC-grafted scions showed higher AA and DHAA and some phenolic compound concentrations. In all combinations, phenolic compounds showed the highest concentrations in the juices at the time of fruit extraction and decreased during storage. Dark packaging provided higher bioactive compounds in juices stored for longer periods. These findings can contribute to the diversification of scion/rootstock cultivars in order to increase the variety of orchards by choosing the best combinations for pasteurized orange juice with higher nutritional value.

Performance and Fruit Ripening of Red Orange Tree Grafted onto Two Rootstocks

The cultivar Sanguínea de Mombuca (Citrus sinensis [L.] Osbeck) bears lycopene-rich fruits with attractive red pulp and high nutritional value; thus, this cultivar has the potential to diversify orchards for the production of fruits for natural consumption and juice. Therefore, studies on the productive performance in combination with rootstocks are required. This study evaluated the phenological cycles, yield, ripening curve, and fruit quality of this cultivar grafted onto ‘Rangpur’ lime and ‘Swingle’ citrumelo. The study was conducted in the Midwest of the state of São Paulo, in the subtropical region of southeastern Brazil, and the field experiment was performed over two consecutive seasons. Rootstocks had little effect on the duration of the phenological cycles. The fruit ripening curve predicted the time of harvest as approximately 240 days after anthesis, regardless of rootstock. Fruits harvested from trees grafted onto ‘Swingle’ citrumelo showed higher levels of soluble solids and sugars and higher technological indices. The larger number of fruits and greater fresh weight confirmed that better yield performance occurred with the trees grafted onto the ‘Rangpur’ lime as opposed to the ‘Swingle’ citrumelo rootstock.

Better tolerance to Huanglongbing is conferred by tetraploid Swingle citrumelo rootstock and is influenced by the ploidy of the scion.

Huanglongbing (HLB) is a disease that is responsible for the death of millions of trees worldwide. The bacterial causal agent belongs to Candidatus Liberibacter spp., which is transmitted by psyllids. The bacterium lead most of the time to a reaction of the tree associated with callose synthesis at the phloem sieve plate. Thus, the obstruction of pores providing connections between adjacent sieve elements will limit the symplastic transport of the sugars and starches synthesized through photosynthesis. In the present article, we investigated the impact of the use of tetraploid Swingle citrumelo (Citrus paradisi Macfrad × Poncirus trifoliata [L.] Raf) rootstock on HLB tolerance, compared to its respective diploid. HLB-infected diploid and tetraploid rootstocks were investigated when grafted with Mexican and Persian limes. Secondary roots were anatomically studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to observe callose deposition at the phloem sieve plate and to evaluate the impact of the bacterium's presence at the cellular level. Voltammetry of immobilized microparticles (VIMP) in roots was applied to determine the oxidative stress status of root samples. In the field, Mexican and Persian lime leaves of trees grafted onto tetraploid rootstock presented less symptoms of HLB. Anatomical analysis showed much stronger secondary root degradation in diploid rootstock, compared to tetraploid rootstock. Analysis of the root sieve plate in control root samples showed that pores were approximately 1.8-fold larger in tetraploid Swingle citrumelo than in its respective diploid. SEM analyses of root samples did not reveal any callose deposition into pores of diploid and tetraploid genotypes. VIMP showed limited oxidative stress in tetraploid samples, compared to diploid ones. These results were even strongly enhanced when rootstocks were grafted with Persian limes, compared to Mexican limes, which was corroborated by stronger polyphenol contents. TEM analysis showed that the bacteria was present in both ploidy root samples with no major impacts detected on cell walls or cell structures. These results reveal that tetraploid Swingle citrumelo rootstock confers better tolerance to HLB than diploid. Additionally, an even stronger tolerance is achieved when the triploid Persian lime scion is associated.

Drought tolerance memory transmission by citrus buds

Plants face recurrent drought events, and previous stresses can influence their responses to subsequent stress episodes. Studies on drought stress memory are recent in citriculture, although they show promise as a tool for crop improvement. Here, we investigated whether stress memory mechanisms can be detected in citrus plants grafted with buds from plants subjected to recurrent water deficit. Three rootstock varieties, namely ‘Rangpur Santa Cruz’ lime, ‘Sunki Maravilha’ mandarin and ‘Sunki Tropical’ mandarin, in combination with ‘Valencia’ orange, were either maintained under full irrigation or subjected to one, two, or three water deficit cycles. Buds from ‘Valencia’ orange were grafted onto ‘Swingle’ citrumelo rootstocks and were evaluated. This combination displayed improved physiological and biochemical performance under water limitation, especially ‘Valencia’ buds grafted onto ‘Sunki Maravilha’, with better photosynthetic performance under water deficit. These findings indicate that genotype-dependent epigenetic memory is a key factor in restoring citrus plants’ capacity to rely on previous stress experiences to restore better photosynthetic and physiological responses when undergoing new water deficit events. Therefore, epigenetic marks can be stored and transmitted to new citrus plants and are a promising alternative to enable increased water deficit tolerance when plants are then challenged by drought-prone environments.

SUBSTRATE MIXING FORMULATIONS FOR CITRUS NURSERY MANAGEMENT

The length of citrus seedling development is determined by several factors, including the physical and chemical qualities of the substrate, which affect rootstock growth and, as a result, the quality of field seedlings. The purpose of this study was to see how the ‘Swingle’ citrumelo rootstock developed with different substrate formulations. The experiment was carried out in a seedling nursery from seeding to grafting, and six treatments were carried out, as follows: 60% peat moss, 30% fine grade horticultural vermiculite, 10% rice hulls (1); 60% peat moss, 30% fine grade horticultural vermiculite, 10% rice hulls (2); 50% peat moss, 30% fine grade horticultural vermiculite, 20% rice hulls (3); 50% peat moss, 30% fine grade horticultural vermiculite, 20% rice hulls (4); 50% peat moss, 20% fine grade horticultural vermiculite, 30% rice hulls (5); 50% peat moss, 20% fine grade horticultural vermiculite, 30% rice hulls (6). In addition, the experiment was divided into two stages: seeding (stage 1) and seedling nursery (stage 2). In a completely randomized design, the experiment has six treatments, four replications, and 51 plants per plot. When container transplanting was performed, the emergence percentage of seedlings was determined; thus, eight plants per plot were examined. For the second stage, was used a completely randomized design with 6 substrate formulations, 4 replications, and 20 seedlings per plot. Stem height, stem diameter, leaf number, area meter, root dry matter, leaf and stem dry matter, and quality index were measured on seedlings every 28 days. All substrate formulations improved seedling development until grafting, except for the 30% rice hulls, which hampered seedling development in ‘Swingle' citrumelo.

Response of Citrus spp. germplasm from Puerto Rico grafted on two rootstocks to early infection of huanglongbing

Huanglongbing (HLB), associated with the proteobacterium ‘Candidatus Liberibacter asiaticus’(CLa), is a destructive disease in citrus. Resistance/tolerance has been identified in some citrus species, wild relatives, and transgenic citrus genotypes. The objectives of this study were to: (1) evaluate the response of the citrus germplasm collection from Puerto Rico (namely ‘Rico’) to HLB, (2) determine the most appropriate rootstock-genotype combination, and (3) quantify the presence of the bacterium in the plant tissue by quantitative polymerase chain reaction (qPCR). Seventeen genotypes were grafted onto ‘HRS 812’ and ‘Swingle’ citrumelo rootstocks. The plants were inoculated with HLB infected buds and evaluated at 12 months after inoculation in a greenhouse in Adjuntas, Puerto Rico. The genotypes ‘Lares’ and ‘Padilla’ had significantly higher incidence (>65.0%) and severity (3.2–4.3) compared to ‘Mexican Lime’ and ‘Tahiti Lime’ that had lower scores (<18.0%, 1.1–1.3) using both rootstocks. In contrast, the genotypes ‘Fronton’, ‘PR-Sel-1’, and ‘Rico 7–376’ grafted on HRS 812; and ‘Pietri 334’ grafted on Swingle citrumelo had lower HLB incidence (<10%) and severity (<1.5). Significantly higher mean cycle threshold (Ct) values were only observed for the genotypes Fronton (37.0), Tahiti Lime (36.5), and Pietri 334 (36.2) compared to Padilla (34.7 and 32.7 when HRS 812 and Swingle citrumelo was used as rootstock, respectively). Citrus genotypes having lower HLB scores were identified, however, their reaction varied according to the rootstock used.

Rootstock Affects Floral Induction in Citrus Engaging the Expression of the FLOWERING LOCUS T (CiFT)

In Citrus, flower induction represents the transition from vegetative to reproductive growth. The regulation of flower induction is mainly triggered by exposure to low temperatures and water-deficit stress, which activates the signaling cascade leading to an increased expression of the citrus orthologs of the FLOWERING LOCUS T (CiFT). In this study, the relationship between rootstock and flower induction under Mediterranean field conditions was investigated by monitoring the expression levels of the floral promoter CiFT2 in leaves of the pigmented sweet orange “Tarocco Scirè” grafted onto “C35” citrange and “Swingle” citrumelo rootstocks. The latter two are known to confer, respectively, high and low yield efficiency to the scion. In both rootstock/scion combinations, CiFT2 showed a seasonal expression with a peak during the inductive period in January triggered by cold temperature. The “Tarocco Scirè”/”C35” citrange combination showed the highest expression levels for CiFT2; this increased expression was correlated with yield and a higher number of flowers in the following spring, suggesting a significant effect of rootstocks on flower induction mediated by the overexpression of the CiFT2 gene.

Compatibility of Rimau Gerga Lebong (RGL) mandarin grafted onto three genotypes of citrus rootstock

Introduction of Citrumelo and Kanci, an alternative citrus rootstock, into citrus cultivation system in Indonesia as complement of Japansche citroen (JC) is very appreciated. Compatibility of Citrumelo and Fortunella to commercial citrus in Indonesia have not been evaluated. The aim of the study is to assess the compatibility of Rimau Gerga Lebong (RGL) mandarin citrus grafted onto JC, Citrumelo, and Kanci based on anatomical, morphological and physiological parameters. The results showed that 100% of RGL mandarin shoot bud grew on Citrumelo and JC rootstocks, but only 88.75% on Kanci rootstock. Anatomically, RGL mandarin at 60 days after budding (DAB) had perfectly adhered on Citrumelo rootstock and not yet fully adhered on JC and Kanci rootstock. Morphologically, RGL-Citrumelo and RGL-JC combinations at three years old plant resulted in the same plant growth parameters, better than RGL-Kanci combination. Physiologically, RGL-Citrumelo and RGL-JC combinations had the same chlorophyll index, better than RGL-Kanci combination. RGL-Citrumelo and RGL-JC combinations showed graft compatible and RGL-Kanci combination showed late incompatibility.

Agronomic characterization of citrandarin fruits and seeds

In citrus cultivation, rootstocks are of fundamental importance and affect several characteristics of the variety used as canopy. Despite the great diversity within Citrus and related genera, the production of rootstocks in Brazil is restricted to a small number of varieties, making the citrus culture vulnerable to the appearance of phytosanitary problems. The aim of this study was to agronomically characterize fruits and seeds of seven citrandarins [Citrus sunki (Hayata) hort. ex Tanaka x Poncirus trifoliata cv. Rubidoux (L.) Raf.], obtained by controlled crossing. The orchard was installed in randomized blocks, with three replicates, in the municipality of Cordeirópolis, SP, where 20 fruits were collected in each replicate, obtained from free pollination of seven citrandarins, Swingle citrumelo and Rangpur lime. The following variables were evaluated: fruit mass, height, diameter, total number of seeds and percentage of viable seeds per fruit; number of embryos per seed, mass of one thousand seeds, number of seeds in 1.0 kg, final emergence rate, number of seedlings per seed, polyembryony rate, emergence speed index and seedling height at 60 days after sowing. For fruit size, the highest values were obtained for Swingle citrumelo. For number of embryos per seed, seedlings obtained through seed and polyembryony, citrandarin TSxPT 245 showed the highest values. Although citrandarin fruits had smaller size than fruits from commercial Rangpur lime and Swingle citrumelo rootstocks, characteristics related to seeds such as viability, polyembryony and emergence rate, were similar or superior, and can be considered potential new rootstocks for the production of citrus plants.

Impact of HLB on the physiological quality of citrus rootstock seeds and non-vertical ‘Candidatus Liberibacter asiaticus’ transmission

Huanglongbing (HLB) is the main disease of citriculture worldwide, being mainly caused by the bacterium known as ‘Candidatus Liberibacter asiaticus’ (CLas), limited to the phloem and naturally transmitted by vector insects known as psyllids. For the production of citrus seedlings, it is necessary to use rootstocks that are propagated by seeds from fruits produced by mother plants grown in field, however being submitted to natural HLB transmission. Although there is no evidence of CLas transmission from seeds to seedlings, little is known about the effects of this bacterium on the physiological quality of seeds obtained from CLas-infected plants. Therefore, the aim of this work was to evaluate the physiological quality of seeds from ‘Rangpur’ lime and ‘Swingle’ citrumelo rootstocks, from CLas-infected plants, with symptomatic and asymptomatic branches. The parameters evaluated, in addition to fruit development in mother plants, were germination rate, tetrazolium test, electrical conductivity, seedling emergence in greenhouse, and vegetative development and molecular analysis via qPCR for the detection of bacteria (leaf petiole, fruit peduncles, roots, seed coats, and seed endosperm). The presence of CLas in plants of both rootstocks has negative impact on fruit development, as well as on the number of normal seeds. In addition, there is reduced germination and seed emergence. Although CLas was detected in different tissues (leaf petiole, fruit peduncles and seed coat, and seed embryo), no vertical bacteria transmission was detected. Results have shown that seeds from CLas-infected branches have impaired physiological quality, but without bacterial transmission from seeds to seedlings.

Optimization of sampling and monitoring of vegetative flushing in citrus orchards.

Citrus trees produce flushes throughout the year, but there are no criteria established for a precise shoot monitoring in orchards under tropical climate. Methods for quantification of flush dynamics would be useful for horticultural and pest management studies because different insect vectors feed and reproduce on flushes. We estimated the minimum number and distribution of trees for sampling and determined the flushing pattern over time in ‘Valencia Late’ orange trees grafted onto ‘Swingle’ citrumelo rootstock. Shoots within a square frame (0.25 m2) on two sides of the canopy were counted and classified by their phenological stage. The minimum number of samples was estimated using the mean number of shoots and area under the flush shoot dynamics (AUFSD). The temporal and spatial distribution analysis was performed by Taylor’s power law and by multiple correspondence analysis (MCA). Additionally, a shoot maturity index (SMI) based on visual qualitative assessment of flushes is proposed. Considering the mean number of shoots, it was necessary to sample two sides of 16 trees to reach a relative sampling error (Er) of 25%, whereas by the AUFSD, only five trees were necessary to reach an Er of 10%. Flushes were predominantly randomly distributed over time and space. Testing eight transects, sampled trees should be distributed throughout the block, avoiding sampling concentration in a certain area. MCA showed that the west side and the upper sampling positions of trees were more likely to be associated with younger shoots. AUFSD and the evaluation of both sides of the canopy yielded a smaller number of trees to be assessed. The SMI was a reliable metric to estimate the shoot phenology of orange trees, and correlated well (R2 > 70%) with the mean number of shoots within the square frame. Therefore, SMI has the potential to make shoot monitoring in the field more practical.

Root Osmotic Adjustment and Stomatal Control of Leaf Gas Exchange are Dependent on Citrus Rootstocks Under Water Deficit

Drought tolerance is defined by several morpho-physiological mechanisms that together improve plant development under water-limiting conditions. Previously, we found root hydraulic redistribution is one of those mechanisms for water stress avoidance. Herein, we aimed to verify the physiological mechanisms associated with root hydraulic redistribution and its consequences for leaf gas exchange and plant growth. Valencia sweet orange scions were grafted onto either Rangpur lime or Swingle citrumelo rootstock. Each plant had two root systems of the same rootstock in distinct pots, which allowed partial irrigation. Our results revealed that citrus species redistribute water under drought and this varies when comparing rootstocks, with Rangpur lime showing higher ability to redistribute water than Swingle citrumelo. For the first time, root hydraulic redistribution in Rangpur lime was associated with osmotic adjustment in well-watered roots of plants facing water deficit. Rangpur lime also presented an effective stomatal regulation of water loss and decreases in leaf transpiration likely allowed water transport to roots under water deficit. As conclusion, we found that root hydraulic redistribution, osmotic adjustment and stomatal control of leaf gas exchange are important physiological mechanisms associated with drought tolerance induced by Rangpur lime rootstock.

EFFECTS OF CITRUS ROOTSTOCKS ON SOME PLANT NUTRIENT ELEMENTS ABSORPTION OF GRAFTED CULTIVARS

Citrus rootstocks have prime effects on the size of tree and fruit, quality of fruit and micorhyza dependence, which make difference in their leaf mineral elements concentration in leaf of grafted cultivars on them. A pot factorial experiment with two factors including rootstocks (sour orange, Citrange, Rough Lemon, Citrumelo and Yozo) and grafted cultivars (Thomson Navel, Sanguine, Unshiu Tangerine) was carried out in farm conditions with three replications. The studied traits were concentrations of elements in leaves including nitrogen, phosphorus, potassium, sulfur, magnesium, iron, manganese, zinc and copper. The result of analysis of variance revealed significant mean squares of variety, rootstock and their interaction effects for the elements in leaves. Citrumelo rootstock had mainly effect on nutritional elements in leaves. Unshiu mandarin with Citrumelo rootstock had the most amount of nitrogen and sanguine variety with Citrange rootstock made the highest concentration of phosphorus and potassium in leaves of the grafted cultivar. Significant positive correlation of K with Fe and Zn indicating that all rootstocks with high mean value of K in leaf of grafted cultivar will have high amount of Fe and Zn. Significant positive correlation also detected between N and P of leaf of grafted cultivars in combination with different rootstocks; therefore, most of combinations with high mean value of N had also high mean value of P. Unshiu Tangerine had high variation for P, but Thomson Novel had lowest variation for this mineral element. Due to Significant positive correlation of S with Mg and Mn, most of rootstocks with high amount of S in leaf of grafted cultivar had high mean values of Mg and Mn in their leaf of grafted cultivars.

Vegetative development and production of ‘Tahiti’ acid lime clone selections grafted on different rootstocks

Abstract Brazil accounts for a large percentage of the world’s citrus production, with ‘Tahiti’ acid lime tree standing out among several cultivated species. However, its cultivation is supported by a very narrow genetic base, being composed of few scion and rootstock options. In this way, the aim of this study was to evaluate the vegetative development and production of twelve ‘Tahiti’ acid lime clones on two rootstocks. The experiment was carried out at Fazenda José Guarete, municipality of São Mateus – ES. A randomized block design was used in the split plot scheme, with plot consisting of two rootstocks and subplot of the twelve ‘Tahiti’ acid lime clones. Traits evaluated were: vegetative development, production and productive efficiency, internal and external quality of fruits. Bello Fruit, Iconha, BRS Passos, Itarana, Persian 58, CNPMF 5059 and Elédio clones were superior in terms of production, vegetative traits and fruit quality. ‘Swingle’ citrumelo rootstock showed higher values for vegetative traits compared to ‘Riverside’ citrandarin. The clone and rootstock combinations that provided better agronomic performance in this study were BRS Passos, Bello Fruit, Elédio and Iconha grafted on ‘Swingle’ citrumelo and CNPMF 5059, Iconha and Elédio grafted on ‘Riverside’ citrandarin.

Assessment of Pruning and Controlled-release Fertilizer to Rejuvenate Huanglongbing-affected Sweet Orange

Previous research has shown that Huanglongbing {HLB [causal agent Candidatus Liberibacter asiaticus (CLas)]}-affected sweet orange (Citrus sinensis) trees have a reduced root-to-shoot ratio, potentially due to the high rate of root death. The diminished root system cannot support the existing aboveground canopy and a cycle of imbalance begins. As a result, the tree enters into a continuous carbohydrate stress cycle and, eventually, the tree declines. Therefore, the goal of this study was to evaluate pruning as a strategy to adjust the root-to-shoot ratio to improve growth and productivity of HLB-affected trees. In Jan. 2015, a 3-year trial was initiated on a 14-year-old grove of ‘Hamlin’ sweet orange on Swingle citrumelo (Citrus paradisi × Poncirus trifoliate) rootstock that was symptomatic of HLB and produced less than 180 lb of fruit per tree. The four pruning treatments were as follows: 1) 0% pruning (no canopy removal), 2) 25% pruning (canopy removed), 3) 50% pruning (canopy removed), and 4) 80% pruning (canopy removed). In a split-plot design, two sources of fertilizer were evaluated in combination with the pruning: 1) conventional fertilizer [CNV (dry granular)] applied at 200 lb/acre nitrogen (N) in five split applications per year, and 2) controlled-release fertilizer (CRF) applied at 150 lb/acre N, split in three applications per year. Within each pruning treatment, half of the trees received CNV and the other half received CRF. The fertilizer treatments were applied in each of the 3 years; however, pruning was performed only once in the beginning of the experiment. The trees that were pruned produced new vegetative growth that looked healthy with no visual HLB symptoms (initially); however, the trees remained positive for CLas throughout the study as determined by quantitative real-time polymerase chain reaction. The 80% pruned trees grew vigorously over the course of 3 years but remained significantly smaller in canopy than control trees (0% pruning) for both CRF and CNV treatments. The 25% and 50% pruned tree canopies grew back and were similar in canopy size as 0% pruning (control) treatment by the end of year 2. At the end of the study, the use of CRF on 25% pruned trees resulted in a significantly higher leaf area index as compared with trees receiving CNV. A significant positive linear correlation was observed between canopy volume and root density; the root density decreased with intensive pruning. A significant positive correlation was also observed between canopy volume and yield, and a negative correlation between canopy volume and fruit drop. There were no significant increases in yield resulting from any pruning or fertilization treatments compared with controls (0% pruning). However, with the use of CRF, the amount of N and frequency of application were reduced. Overall, our results indicate that pruning did not improve the productivity of HLB-affected trees over the course of 3 years. Therefore, severe pruning is not a viable option to rejuvenate the HLB-affected trees.