Swingle 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.

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.

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.

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.

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.

Compatibility and horticultural performance of Pera sweet orange clones grafted to Swingle citrumelo rootstock

ABSTRACT Despite its interesting agricultural traits, Swingle citrumelo (Citrus paradisi Macfad. ‘Duncan’ × Poncirus trifoliata L. Raf.) rootstock exhibits severe incompatibility when grafted with Pera sweet orange [Citrus sinensis (L.) Osb.] scions, and this fact limits its use for propagation purposes. Nonetheless, the existence of certain Pera sweet orange clones that are compatible with Swingle citrumelo represents a substantial gain for the entire citrus industry. This study investigated the compatibility between seven Pera sweet orange clones and Swingle citrumelo rootstock, regarding the presence of a necrotic line at the graft union junction, the difference in diameter between rootstock and scion, the influence of clonal cleaning by micrografting, and the horticultural performance of the selected combinations. No trees died because of grafting incompatibility, even after twenty years (trial I), except for clone CV1, or after six years and nine months (trial II) from planting. The rootstock/scion trunk diameter ratio was not related to the presence of a necrotic line in the graft-union region. Additionally, these variables did not influence fruit yield and quality, and they were not affected by clonal cleaning or pre-immunisation of the old Pera sweet orange clones.

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.

Nutrient Status and Root Density of Huanglongbing-Affected Trees: Consequences of Irrigation Water Bicarbonate and Soil pH Mitigation with Acidification

The Candidatus Liberibacter asiaticus bacterium, associated with Huanglongbing (HLB) disease of citrus trees, moves downward in the phloem and infects the roots soon after transmission by the Asian Citrus Psyllid (Diaphorina citri) vector into shoots. Before canopy symptoms appear, 30–50% of the roots are damaged. Without aggressive management to reduce abiotic and biotic stress, root loss increases to 70–80%. An extensive survey of HLB-affected groves in central and south-central Florida indicated that a greater decline in fibrous root health as well as a greater expression of HLB symptoms is observed where irrigation water is high in bicarbonates (>100 mg L−1) and soil pH is >6.5. Over three seasons of survey, acidification of irrigation water in the central and south-central citrus growing regions of Florida reduced the decline in root density associated with HLB. Irrigation water treatment with sulfuric acid and soil amendment with elemental sulfur for 36 months to establish a soil pH range from 4.0 to 7.0 increased root growth, soil nutrient availability, and the uptake of Ca, Mg, Mn, and Zn in response to a gradual reduction in soil pH in young and mature Valencia orange groves on Swingle citrumelo rootstock. The reduction in soil pH increased yield and soluble solids in fruit and so would improve citrus production.

Regulated deficit irrigation benefits the production of container-grown citrus nursery trees

Withholding irrigation at − 15 to − 25 kPa water potential significantly saved water with minimal changes in plant growth, water relations, and percentage of shippable citrus trees on drought-tolerant and -sensitive rootstocks. We evaluated the impacts of regulated deficit irrigation (RDI) on plant growth, water relations, and biochemical variables of Valencia sweet orange nursery trees grafted on drought-tolerant Rangpur lime (RL) and drought-sensitive Swingle citrumelo (SC) rootstocks in pots. Irrigation was withheld to potting mix water potential of − 15 (mild RDI) and − 25 kPa (moderate RDI), with daily irrigation as control, in three graft growth periods (20–60, 61–120, and 20–120 days after grafting, DAG). Plant growth was reduced by moderate RDI, albeit RL induced more vigor than SC. SC induced higher CO2 assimilation rates and water use efficiency (WUE), regardless of the irrigation regime, and only trees grafted on RL decreased WUE under moderate RDI. Most leaf nutrient concentrations were increased with RDI intensity, while K levels were decreased by moderate RDI. Starch reserves were drastically decreased, while reducing sugars were increased by RDI in all plant organs, most notably in the leaves; roots represented the main source of carbohydrates for both rootstocks under water deficit. Starch concentration promptly recovered in all organs after restoring irrigation. Leaf proline concentration was 20 times higher at RDI, but decreased by 50% just 1 day after rehydration. Mild-to-moderate RDI resulted in water savings of 54–80%, but a 0–40% reduction in the percentage of shippable trees at 120 DAG; the biological responses were mainly related to the RDI duration. The production of container-grown citrus nursery trees in greenhouses could greatly benefit from the use of RDI, without major negative effects on tree quality.

Rehabilitation of Huanglongbing-affected Citrus Trees Using Severe Pruning and Enhanced Foliar Nutritional Treatments

Citrus trees affected by huanglongbing (HLB) become diminished, weak, and develop dieback resulting in reduced production. Decline in fruit yield ultimately prevents economically acceptable commercial citrus production. The objectives of this study were to evaluate the effects of severe pruning in combination with an enhanced foliar nutritional treatment on growth, yield, and juice quality of HLB-affected orange trees. The bacterial titer within the trees was monitored before and after treatments, and a cost–benefit analysis provided an economic evaluation of the treatments. Fifteen-year-old ‘Valencia’ orange (Citrus sinensis Macf.) trees on Swingle citrumelo rootstocks [C. paradisi × Poncirus trifoliata (L.) Raf.] with 100% incidence of HLB, confirmed by real-time polymerase chain reaction (PCR), were severely pruned back to the main scaffold branches. Between 2010 and 2015, foliar nutrients were sprayed on both pruned and nonpruned trees to target new flush growth. Three enhanced nutritional foliar treatments were evaluated and compared with a control foliar nutritional treatment that was considered to be a standard practice before endemic HLB. The enhanced nutritional treatments included a mixture of micro- and macronutrients commonly known as the “Boyd cocktail,” a micronutrient package labeled Fortress © (Florida Phosphorus LLC, Key Largo, FL) sprayed with potassium nitrate (KNO3), and the Fortress © micronutrient package sprayed with urea. The experiment was a split-plot with seven replications, with pruning as the main plots, and a foliar nutritional treatment as subplots. Tree pruning was performed in Feb. 2010 before the spring flush. Pruned trees grew longer shoots than the controls the year after pruning. Canopy volume and leaf area were greater with nonpruned trees, but the chlorophyll content per cm2 leaf area was higher in the pruned trees compared with nonpruned trees in 3 years of the 5-year experiment. Pruned and nonpruned trees bloomed and set fruit the first year of the experiment in the spring of 2010–11. The fruit crop for the 2010–11 and 2014–15 seasons, and the overall total fruit crop for the 2010–15 season on pruned trees were significantly lower than those on nonpruned trees. However, no significant yield differences were found between pruned and nonpruned trees in the 2011–12, 2012–13, and 2013–14 growing seasons. Fruit yields from pruned trees never surpassed the yields from nonpruned trees, and this was possibly due to the severe-pruning treatment. Thus, severe pruning, as used in this trial, was not cost effective through the first 5 years after pruning. The rapid regrowth response of the pruned trees, however, may indicate that a reduced pruning approach could be effective at rejuvenating the HLB-affected trees, and an alternative to tree removal and replanting. Enhanced foliar nutrition treatments provided some yield benefits, especially in the early years of the trial. However, the enhanced foliar nutrition treatments did not prove to be cost effective.

Non-phytotoxic concentration of cucurbitacin-containing phytonematicides and the overall sensitivities to Swingle citrumelo seedling rootstock

Nemarioc-AL and Nemafric-BL phytonematicides contain cucurbitacins A and B active ingredients, respectively. The major drawback of the two products had been their phytotoxicity to various crops, which was resolved using the Curve-fitting Allelochemical Response Data (CARD) computer-based model, which also provided overall sensitivities (Σk) of the plant to the test phytonematicides. Two of the seven biological indices were adapted to provide the non-phytotoxic concentration, technically referred to as the Mean Concentration Stimulation Point (MCSP)-which is product-specific and crop-specific. The objective of this study was to determine MCSP of Nemarioc-AL and Nemafric-BL phytonematicides on Swingle citrumelo seedling rootstock and the related Σk of the rootstock to the phytonematicides. Six-months-old seedlings were subjected on weekly basis to 0, 2, 4, 8, 16, 32 and 64% for each phytonematicide. At 84 days after the treatments, MCSP values for Nemarioc-AL and Nemafric-BL phytonematicides on seedlings were 2.97 and 5.6%, with Σk values of 3 and 2 units, respectively. In conclusion, both phytonematicides had MCSP and Σk values which suggested that the two products would be suitable for use on Swingle citrumelo seedling rootstock as a supplement for managing nematode population densities in citrus production.

Candidatus Liberibacter asiaticus causes damage to citrus fibrous roots before visual decline of Huanglongbing-infected citrus trees

Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus (Las), was first detected in Florida in late 2005 and is now widely distributed throughout the commercial citrus-growing regions. In recent seasons, concurrent with freeze and drought episodes, symptomatic HLBinfected trees were much more affected by the extremes of temperature and moisture than trees without HLB. Symptoms exhibited by the stressed trees were excessive leaf loss and premature fruit drop even when HLB-infected trees were managed with good nutritional and irrigation practices recommended to support health of HLB-affected trees. This stress intolerance may be due to a loss of fibrous roots. To assess root status of HLB-infected trees on Swingle citrumelo rootstock (Citrus paradisi × Poncirus trifoliata), blocks of 2,307 three-yr-old Hamlin orange trees and 2,693 four-yr-old Valencia orange trees were surveyed visually and with a real time polymerase chain reaction (PCR) assay to determine Las infection status. The incidence of Las-infected trees (pre-symptomatic: Las+, visually negative, and symptomatic: Las+, visually positive) trees was 89% for the Hamlin block and 88% for the Valencia block. Las+ trees had 30 and 37% lower fibrous root mass density for pre-symptomatic and symptomatic trees, respectively, compared to Las- trees. In a second survey, 10- to 25-yr-old Valencia trees on ‘Swingle’ citrumelo or ‘Carrizo’ citrange (C. sinensis (L.) × P. trifoliata) rootstock were sampled within 3-6 months after identification of visual HLB status as symptomatic (Las+, visually positive) or non-symptomatic (Las-, visually negative) in orchards located in the central ridge, south-central and southwest flatwoods. Pairs of HLB symptomatic and non-symptomatic trees were evaluated for PCR status, fibrous root mass density and Phytophthora nicotianae progagules in the rhizosphere soil. Las+ trees had 27-40% lower fibrous root mass density and in one location higher P. nicotianae per root but Phytophthora populations per cm 3 soil were high on both Las+ and Las- trees. Fibrous root loss from HLB damage interacted with P. nicotianae depending on orchard location and time of year.

Nitrogen‐fertilizer forms affect the nitrogen‐use efficiency in fertigated citrus groves

AbstractThe fertigated area of the Brazilian citrus industry has grown rapidly during recent years, and an efficient management of nitrogen (N) application at these sites is required for sustainable citrus production. Therefore, a field trial with Valencia orange trees [Citrus sinensis (L.) Osbeck] on Swingle citrumelo rootstock (Citrus paradise Macfad. x Poncirus trifoliata L. Raf.) was conducted for 8 years to evaluate the effects of N rates (80, 160, 240 and 320 kg ha–1 y–1) applied by fertigation, either as ammonium nitrate (AN) or calcium nitrate (CN), on soil solution dynamics, fruit yield, nutritional status, and N‐use efficiency (NUE) of trees. The maximum fruit yield was reached with 240 kg N ha–1 for AN, whereas a linear response and greater fruit yield was observed for N supplied as CN. The NUE was reduced for both N forms with increasing N rates. However, the NUE for CN was 14 to 38% greater than the NUE for AN. The lower fruit yield and NUE for AN compared to CN‐treated trees was associated with the increased acidification of the soil solution with increased AN rates (pH ≤ 4.0). This limited nitrification resulted in a high ammonium (NH$ _4^+ $) concentration in the soil solution and a reduction in the net absorption of cations by the trees, particularly calcium (Ca). Due to the improved ion balance as well as the higher pH of the soil solution (pH ≥ 6.3) and diminished NH$ _4^+ $ availability, gains in both fruit yield and NUE in fertigated citrus groves in tropical soils can be obtained with the use of CN as a source of N.

Phosphorus availability and rootstock affect copper-induced damage to the root ultra-structure of Citrus

The control of several citrus diseases requires continuous applications of fungicides containing copper (Cu) which favor to the accumulation of this metal in the soil. Therefore, the evaluation of how nutrient availability and rootstock interact with Cu toxicity in the citrus trees is required to maintain sustainability of fruit production in Cu-contaminated soils. Valencia orange trees on Sunki mandarin (SM) or Swingle citrumelo (SC) rootstock were grown in nutrient solutions combining adequate Cu (1.0μmolL−1), excess Cu (50.0μmolL−1), deficient phosphorus (P) (0.01mmolL−1) and sufficient P (0.5mmolL−1). The excess Cu reduced root and shoot growth, chlorophyll and relative water content in the leaves of the trees compared to those under adequate Cu supply. Furthermore, excess Cu caused severe damage to the root ultra-structure, characterized by the degeneration of the middle lamella and the presence of a thin and sinuous cell wall, as well as, starch accumulation in the plastids, disruption of the mitochondrial membranes and cellular plasmolysis. The damage caused by excess Cu in the cell wall and middle lamella on the root cells of SC was less severe than SM. Sufficient P supply improved the structure of the cell wall and middle lamella of trees subjected to excess Cu in comparison to P-deficient ones. Thus, the occurrence of more preserved cell wall and middle lamella supports the idea that sufficient P availability in the rooting medium and the use of SC rootstock might contribute to increase the ability of young citrus trees to cope with Cu toxicity.

Daily Temperature Amplitude Affects the Vegetative Growth and Carbon Metabolism of Orange Trees in a Rootstock-Dependent Manner

Both instantaneous and average growth temperatures affect plant metabolism, and the physiological importance of daily variations in temperature is frequently underestimated. To improve our understanding of the environmental regulation of citrus trees, we hypothesized that vegetative growth would be stimulated in orange plants subjected to large daily temperature variations, even without changes in the average daily air temperature or the amount of energy given by degree-days. This hypothesis was tested with orange plants grafted onto Rangpur lime or Swingle citrumelo rootstocks and grown for 20 days under thermal regimes (day/night) of 25/25°C or 32.5/17.5°C. Such regimes imposed growth conditions with daily temperature variations of 0 and 15°C. Plant growth, photosynthesis, respiration, and carbohydrate availability in leaves, stems, and roots were measured under both thermal conditions. The daily temperature variation affected the carbon metabolism of young citrus trees; plants grown under daily variation of 15°C used more of the carbon stored in mature leaves and roots and the energy generated by respiration for the biosynthesis of vegetative structures, such as leaves and branches. Thus, there was a significant increase in the leaf area of plants subjected to high daily temperature variation. Current photosynthesis was similar in the two thermal regimes; however, the photosynthetic rates increased under the 15°C variation when measurements were normalized to 25°C. In addition to the stimulatory effect of the source–sink relationship on photosynthesis, we suggest a probable involvement of hormonal regulation of plant growth through gibberellin metabolism. The rootstock affected the response of the canopy to daily temperature amplitude, with the Rangpur lime improving plant growth through higher carbohydrate availability in roots. This is the first report that highlights the importance of daily temperature variations for citrus growth and physiology under nonlimiting conditions.

Young-tree Performance of Juvenile Sweet Orange Scions on Swingle Citrumelo Rootstock

A worldwide search was conducted for sweet orange [Citrus sinensis (L.) Osb.] selections with higher yield and better juice quality than existing commercial cultivars used in Florida primarily by the processing industry. Seeds of nearly 100 selections were introduced, germinated, and used as a source of buds for propagation. The scion selections were divided among six trials established by propagating juvenile buds from ≈12-month-old scion seedlings onto Swingle citrumelo [C. paradisi Macf. × Poncirus trifoliata (L.) Raf.] rootstock plants already in place in the field. Comparison trees using buds from mature sources were produced in a commercial nursery. The trials consisted of four to five replications of one- or two-tree plots with trees planted 4.3 × 6.7 m within and between rows, respectively. The scions were early-maturing (fall to early winter), midseason (winter to early spring), and late-season (early spring to early summer) common orange, blood orange, and ‘Pera’ orange selections. Data collected routinely included seed counts, standard measurements of juice quality, and yield during an ≈13-year period of evaluation. All trees exhibited typical juvenile traits such as vigor and thorniness; however, flowering and first cropping were not substantially delayed. Many selections began fruiting within 3 years after planting, which is the common commercial experience among trees propagated with mature bud sources. Many selections were low-seeded with counts of less than 10/fruit. Mean cumulative yield (8 years) among the early- and midseason selections in the first-planted trial was 1390 kg/tree and ranged to a high of 1751 kg/tree; for the late-season types, the mean was 947 kg/tree with little variability among eight selections. The yields of the early- to late-season selections in the other trials were similar. The blood orange selections proved to be mostly midseason in maturity. They lacked the deep peel and flesh coloration of blood oranges grown in a Mediterranean-type climate, but some selections did develop an enhanced orange color of the juice and the different flavor typical of blood oranges. ‘Pera’ orange selections exhibited a bud union incompatibility and subsequent decline with Swingle citrumelo rootstock and also when another sweet orange was inserted as an interstock. Their mean cumulative yield over six seasons was 797 kg/tree with an ≈30% difference between the lowest and highest values. Juice soluble solids, acid, and color values were typical of ‘Pera’ fruit grown in Brazil. The overall collection of sweet oranges displayed considerable diversity in their traits despite their supposed origin as a monophyletic group. Several early-season selections were released for commercialization, including ‘Earlygold’ and ‘Itaborai’, because of their better juice color and flavor. ‘Vernia’, a midseason selection, was released because of its high juice quality in late winter–early spring and its cropping precocity.

Variação sazonal do crescimento vegetativo de laranjeiras Hamlin enxertadas em citrumeleiro Swingle no município de Limeira, Estado de São Paulo

This work aimed to evaluate the seasonal variation in vegetative growth of Hamlin orange plants grafted on Swingle citrumelo rootstocks grown in a subtropical climate. Biometric evaluations were conducted monthly on the East and West canopy positions of 12 plants between December 2005 and December 2006. On average, three shoots emerged per stem between December and February, while other four shoots appeared in two periods between July and September: two shoots between July and August, and two other shoots between August and September. This pattern of vegetative flush occurred during an atypical year, with high water deficit (between April and November) and increase in air temperature starting in May. The number of leaves per stem increased following the same seasonal pattern of shoot emergence. The amount of purely vegetative shoots (given by the ratio between number of leaves and number of shoots) was 2.5 times higher on the first flush in relation to the others. The main vegetative flush responsible for stem growth and increase of leaf area occurred during the summer. Stem growth was interrupted by endogenous aspects (summer) and by unfavorable environmental conditions (winter). There were no differences between the evaluated variables when comparing canopy positions.

Carbon dioxide and high temperature effects on growth of young orange trees in a humid, subtropical environment

Rising atmospheric carbon dioxide (CO 2) concentration and global warming could impact growth of citrus trees. Five 2-year-old Ambersweet orange trees on Swingle citrumelo rootstocks were transplanted into soil containers in two temperature-gradient greenhouses on 9 August 1994 at Gainesville, FL, USA. Either 360 or 720 μmol (CO 2) mol −1 (air) was maintained in the greenhouses. Two containers were located in each of four temperature zones maintained at 1.5 °C increments between each zone with a 4.5 °C difference between zones 1 and 4. The main objective was to test the hypothesis that biomass growth ratios of CO 2-enriched to ambient CO 2-exposed young sweet orange trees would be similar to the large growth enhancements (about 2.6-fold) reported from Phoenix, AZ, USA during the first 3 years of growth of sour orange trees. One tree per container was harvested in 1995 and four trees per container were harvested in 1996. Growth parameters were different between years except leaf fresh weight and fine root biomass. Elevated CO 2 increased growth parameters except leaf growth and fine root biomass. Biomass response ratios to CO 2 (720/360) for 1995 and 1996, respectively, were 1.57 and 1.18 for shoot wood, 1.34 and 1.15 for total above-ground, 1.46 and 1.08 for tap roots, 1.67 and 1.54 for secondary roots, 1.29 and 0.95 for fine roots (NS-CO 2), 1.40 and 1.19 for total roots, 1.47 and 1.18 for total wood, and 1.37 and 1.17 for total plants. The decrease in response to CO 2 in the second year was attributed to crowding of shoot and root space. Components of shoot wood, total above ground, taproot, fine root, total root, total wood, and total plant biomass increased slightly (0.01 < P < 0.05) with increasing temperature. No CO 2 × temperature interactions were significant. The hypothesis that elevated CO 2 would cause biomass increases of about 2.6-fold compared to ambient CO 2 treatments (as found in the midlattitude desert environment of Phoenix) was not supported. May through September mean maximum daily vapor pressure deficit (VPD) was 5.54 and 2.25 kPa for Phoenix and Gainesville, respectively, a ratio of ≈2.5. High summertime VPD coupled with limited water flow capacity within citrus trees that lead to pronounced midday depressions in photosynthesis in ambient CO 2 is discussed as the underlying environmental cause of the large CO 2 enrichment effects in a midlattitude desert compared to the Gainesville humid subtropical climate.

Yield and fruit quality of two late-maturing Valencia orange tree varieties as affected by harvest date

Introduction. Since citrus fruits are non-climacteric, commercial harvest for a given variety can occur over a prolonged period in the same orchard. Delayed citrus harvest has pre- viously been reported to influence fruit quality variables during the current season and to reduce the subsequent year's yield. The aim of our study was to investigate the effect of harvest date on tree yield and fruit quality of two late-maturing Valencia orange varieties during two suc- cessive years. Materials and methods. One hundred 9-year-old Campbell and Lue Gim Gong Valencia orange trees budded on Swingle citrumelo rootstock were used. Fruit sampling inclu- ded fruit harvest at five monthly intervals (April-August) during 2003, and only one harvest date (20 June) during 2004. Results and discussion. Based on seasonal variation of various fruit qua- lity variables during the five harvests conducted in 2003, the most stable fruit quality variables were fruit weight, number of seeds per fruit, rind thickness, juice content and soluble solids con- tent. In general, both varieties produced fruit with similar internal quality characteristics. Howe- ver, their mean fruit weight differed in 2004 (Lue Gim Gong had larger fruit than Campbell). Furthermore, the annual and cumulative yields of Lue Gim Gong Valencia orange trees over two years were significantly higher than those of Campbell orange trees. Finally, the yield per tree and fruit quality of both varieties in 2003 and 2004 were not affected significantly by the date of harvest during 2003. Greece / Citrus sinensis / variety trials / harvesting date / fruits / quality / yields / seasonal variation

Orange Tree Fibrous Root Length Distribution in Space and Time

Understanding the growth pattern of fibrous, orange tree [Citrus sinensis (L.) Osbeck] roots enables proper fertilizer placement to improve nutrient uptake efficiency and to reduce nutrient leaching below the root zone. The objective of this study was to develop relationships defining citrus fibrous root length density (FRLD) as a function of soil depth, distance from the tree trunk, and tree size. Root systems of 18 trees with tree canopy volumes (TCV) ranging from 2.4 to 34.3 m3 on two different rootstocks and growing in well-drained sandy soils were sampled in a systematic pattern extending 2 m away from the trunk and 0.9 m deep. Trees grown on Swingle citrumelo [Citrus paradisi Macf. × Poncirus trjfoliata (L.) Raf.] rootstock had significantly greater FRLD in the top 0.15 m than trees on Carrizo citrange (C. sinensis × P. trifoliata). Conversely, Carrizo citrange had greater FRLD from 0.15 to 0.75 m below the soil surface. FRLD was significantly greater for ‘Hamlin’ orange trees grown on Swingle citrumelo rootstock at distances less than 0.75 m from the tree trunk compared with those on Carrizo citrange. Fibrous roots of young citrus trees developed a dense root mat above soil depths of 0.3 m that expanded both radially and with depth with time as trees grow and TCV increased. Functional relationships developed in this study accounted for changes in FRLD with increase in tree size.