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

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  • 1 University of Florida, IFAS, Horticultural Sciences Department, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850

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.

Abstract

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.

Sweet oranges [Citrus sinensis (L.) Osb.] are grown in many places throughout the world and are popular as fresh fruit, especially navel and ‘Valencia’ oranges (Saunt, 2000). Sweet oranges are also grown for processing primarily in Florida and Brazil. The principal oranges in Florida are ‘Hamlin’ and various ‘Valencia’ clones and ‘Pera’ in Brazil, but other orange cultivars are grown in both places so that once harvesting begins, fruit are available for an uninterrupted harvest season.

Oranges grown in Florida and Brazil must meet certain minimum maturity standards. Processed orange juices produced in Florida or imported for sale in the United States are rated on the basis of U.S. Standards for Grades for color, flavor, and defects using a 100-point scoring system (Fellers, 1990). In that system, 40 points each are assigned to color and flavor. For a product to be graded A, it must score a minimum of 36 in each category. Therefore, the juice of different orange cultivars is often blended to meet those criteria because the juice of one or more of the individual cultivars may be inadequate to be used on its own.

The Florida citrus processing season begins with a relatively poor-quality orange, ‘Hamlin’, which is normally harvested in December and January. It is a very productive cultivar, but its juice has relatively poor color and flavor and typically must be combined with ‘Valencia’ and the juice of other cultivars to meet Grade A standards (Wutscher and Bistline, 1988). ‘Valencia’ fruit mature late in the season and are usually harvested in March to June. It is the standard cultivar for quality and easily meets the color and flavor criteria and the same is generally true for ‘Pera’ orange. These three cultivars have been the mainstay of their respective citrus juice industries for many years. However, because growers are compensated for yield and/or juice quality (usually soluble solids content), efforts to find improved selections have continued (Grosser et al., 2009). Moreover, single-strength pasteurized juice (also known as not from concentrate) has emerged as a popular product that has changed the processing industry by placing more emphasis on blending, juice flavor, and color. Therefore, we conducted a worldwide search with the objective of identifying, introducing, and evaluating new sweet orange selections with higher yield and better juice quality than the standard commercial cultivars.

Materials and Methods

Plant material and propagation.

Various citrus collections throughout the world and locally were either visited to examine and identify sweet orange selections with promising attributes for the Florida processing industry or someone was contacted with specific requests for seeds. Many of the selections were named cultivars of some level of commercial interest and are described in various publications (Hodgson, 1967; Saunt, 2000). In some instances, fruit samples were collected and the juice extracted and analyzed. Seed counts were noted along with a visible determination of juice color. From that effort, ≈100 sweet orange selections were identified (Table 1). Some selections were introduced as seeds; others were available in Florida and also initiated from seed.

Table 1.

Expt. 1: Sweet orange scion selections.

Table 1.

Approximately 100 seeds of each selection were germinated and seedlings grown for 1 year. Sweet oranges generally produce uniform seedling populations. The seeds have multiple embryos of which a high proportion is of nucellar origin (Frost and Soost, 1968). After 1 year, the occasional visible off-type was removed from each population of seedlings. Buds were taken from among ≈50 to 75 seedlings of each selection and used for propagation onto Swingle citrumelo rootstock seedlings being grown at the field site. The rootstock seedlings were either grown in a nursery setting and ≈1 year later, the budded plants were moved to their nearby trial locations or the seedlings were started in their trial locations and budded in place. In some instances, extra trees from the first planted trials (EM.1 and V.1; Table 1) were moved after 1 year by tree spade to positions in EM.2 and budded with a new scion selection. In field trials started with trees produced in a commercial nursery, tree age is set at 0 years at planting. For our trees to be considered equivalent, we set tree age 0 as 1 year after budding. Certain common commercial selections were also propagated as standard nursery trees using mature bud sources for comparison purposes.

Field trials.

Our grower–cooperator provided an 8-ha site (lat. 28°15″11.79′; long. 81°14″19.30′) in St. Cloud, FL, with standard grove microsprinkler irrigation, nutrient (Obreza and Morgan, 2008) (160 kg N/ha/year) and pest management and arranged for harvesting each season. The soil series at the site is mapped as Myakka fine sand, a Spodosol in the family of Aeric Haplaquods. The soil has a surface layer of very dark gray sand ≈18 cm thick underlain by 50 cm of light gray sand. Below those horizons is the spodic horizon, which consists of sand cemented by organic material and is ≈12 to 25 cm thick. Site drainage was provided by a perimeter ditch.

Six trials were established with all trees spaced 4.3 × 6.7 m within and between rows, respectively. The first two trials consisted of selections maturing from early- to midseason (EM.1) to late (V.1), October to June, respectively (Table 1). The same range of selections was included in the third and fourth trials (EM.2; V.2; Table 2). ‘Pera’ selections were evaluated in the fifth trial (P.1) and one ‘Pera’ selection on several rootstocks was tested in the sixth trial (P.2). ‘Pera’ orange is a cultivar known to be incompatible with trifoliate orange and its hybrids (Table 2) (Salibe, 1963). A crease forms at the bud union; thus, in P.1, each two-tree plot had one tree propagated directly on Swingle citrumelo rootstock and the second tree was propagated on Swingle citrumelo also but with an interstock of another sweet orange selection, usually ‘Hamlin’. ‘Pera Olympia’ (P.2) was the only scion on four rootstocks: rough lemon (C. jambhiri Lush.), Palestine sweet line (C. limettioides Tan.), Cleopatra mandarin (C. reshni Hort. ex Tan.), and Volkamer lemon (C. volkameriana Tan. & Pasq.) (Table 2). All trials consisted of four or five replicates of one- or two-tree plots in a randomized complete-block design. The P.2 trees were planted on the ends of rows in single-tree plots.

Table 2.

Expt. 2 and ‘Pera’ sweet orange scion selections and experiment assignment.

Table 2.

Data collection and analysis.

Tree height was measured periodically. Annual samples of 50 to 60 fruit for standard juice quality assessments were collected usually from three plots near the time of harvest. Juice was extracted and analyzed with commercial equipment at the Citrus Research and Education Center, Lake Alfred, FL. Juice color was measured in subsamples of extracted juice (Fellers et al., 1990). The color measurement is known as the color number. The number is allowed to be rounded off to generate the color score used in business. Fruit yield was measured annually during commercial harvest by placing fruit in tubs holding 400 kg and recording volume. Seeds were counted in samples of five fruit/tree in two separate seasons. Annual field notes were taken in the early and midyears of the trials to record observations of off-type trees, consistency among trees, fruit drop, peel and juice color, flavor, and any otherwise noteworthy traits. Data analyses were conducted according to the experiment design using analysis of variance with mean separation by the least significant difference test (PROC GLM; SAS Institute, Cary, NC). Some variables were compared by simple linear correlation analysis.

Results and Discussion

General tree performance.

Virtually all trees grew to a height of ≈4 to 5 m within 6 to 7 years and were largely upright in growth habit with vigorous thorny branches typical of juvenile trees. Sweet orange trees propagated with buds from mature sources normally begin commercial cropping in Florida 3 years after planting. In our trials, cropping was delayed only 1 year in some selections, but most selections produced enough fruit to record yield in the third year and were producing commercial quantities of fruit in the fourth season after planting (Tables 3, 5, 7, 9, and 12). Cropping initially occurred in the upper portions of the canopy at the ends of vigorous, upright shoots. That cropping pattern confirms earlier observations that the least juvenile (first fruiting) part of a canopy is at the perimeter and upper extremities (Furr et al., 1947). Heavy cropping at the ends of branches among some selections led to broken limbs as the trees aged. The damage was substantial in some cases and probably affected the yield data. Fruit also had juvenile characteristics, including an initial tendency to large sizes, puffiness, and thicker peel in some selections. After the trees had cropped for a few seasons, most fruit began to look more like fruit typical of trees grown from mature buds. Juice content and quality of the first crops were generally commercially acceptable.

Tree survival was essentially 100% with a few exceptions explained in the discussion of each trial. However, it was apparent after examining the trees over several seasons that there were some off-type trees based on their appearance. The typical tree judged to be an off-type was noticeably shorter with odd fruit, leaves, and shoots by comparison with the other trees of the same selection. The percentage of off-types varied from zero to ≈10% among selections. The off-type trees were apparent within ≈3 to 5 years after planting and were removed to make way for new selections. The remaining trees within a selection were uniform in appearance and performance. In some instances, it was apparent within a few years that some selections were excessively vigorous, unproductive, or produced undesirable fruit and were removed from the trial.

EM.1: Early- and midseason selections.

By the third season after “planting,” selections of a similar age were bearing a crop ranging from 44 (‘Pineapple’) to greater than 110 kg/tree (‘Fiwicke’, ‘Homosassa’, ‘Midseason’, and ‘Perao’) (Table 3). The ‘Jincheng’ trees had the lowest yield because they were planted 1 year later; however, their yield in the fourth year was comparable to or exceeded that of most of the other selections. During the eight seasons of measurement, mean yield/tree increased from 67 to 257 kg/tree with five selections yielding greater than 300 kg/tree in the eighth season and one selection, ‘Pineapple’, producing 525 kg/tree. Alternate bearing was evident after the third bearing year, but the degree appeared to be no more severe than that experienced among commercial Florida citrus groves (Fla. Agric. Stat. Serv., 2010). There were a few exceptions, primarily midseason-maturing seedy types like ‘Pineapple’, a cultivar well known for severe alternate bearing (Davies and Jackson, 2009). Cumulative yield ranged from 1751 kg for the ‘Midseason’ (‘Sunstar’) trees to 1027 kg for the ‘Masry’ trees, a 70% difference.

Table 3.

Mean yield (kg/tree; n = 5) of EM.1 early- and midseason sweet orange selections on Swingle citrumelo rootstock, St. Cloud, FL.z

Table 3.

The mean seed count over two seasons was 10/fruit (Table 4). The seediest selections were ‘Sanford’ (21 seeds) and ‘Kona’ and ‘Pineapple #8’ (19 seeds). ‘Ackay’, ‘Moro’, ‘Pera 15’, and ‘Tarocco’ were virtually seedless with less than three seeds/fruit (data not shown). Those four selections were eventually removed as a result of excessive vigor, lack of productivity, or undesirable fruit. The ‘Pera 15’ trees were removed because of tree decline resulting from the bud union incompatibility well known to occur between selections of this sweet orange and trifoliate orange and its hybrids (Carlos and Donadio, 1996; Salibe, 1963). The ‘Pera 7’ trees also declined later. The ‘Perao’ trees did not exhibit any decline although they are a supposed selection of ‘Pera’.

Table 4.

Mean seed number/fruit (n = 5) determined when the trees were 4 and 8 years old and juice quality (n = 3) of fruit harvested in late November to early December from EM.1 early- and midseason sweet orange selections budded in place on Swingle citrumelo rootstock in June 1986 at St. Cloud, FL.

Table 4.

Juice quality was assessed when the trees were 5 and 8 years old. The mean values were not markedly different between those years; however, they are not strictly comparable because of different fruit sampling dates. Mean juice quantity across both years was typical for Florida-grown fruit and ranged from ≈55% to 62% by weight (Table 4). The soluble solids concentration ranged from less than 10% to a few values greater than 12% and acid concentration ranged from ≈0.5% to a few values greater than 1.0%. The soluble solids:acid ratios reflect fruit maturity with the larger numbers indicating the most mature selections on a particular date. Mean ratio was 17.1 and 12.0 when the trees were 5 and 8 years old, respectively. Those values would ordinarily be considered typical of young versus older trees, but in this instance, the later sampling date (December versus November, respectively) for the younger trees was responsible for the difference.

Juice color is an especially important attribute of early-season oranges. Their color is often below the minimum score of 36 required to make Grade A juice (Fellers, 1990). As a result, their juices must be blended with the juices of other selections with better color such as ‘Valencia’. Juice color is not usually a concern with most midseason-maturing oranges. Their juice will achieve a color number greater than 36 by the time of harvest and likewise for ‘Valencia’ orange juices, which are usually 37 or greater. In this trial, only one of the six early-maturing oranges, ‘Partin Delicious’, had a juice color number greater than 36. The standard early-maturing orange, ‘Hamlin’, had color numbers below 36. Among the midseason selections, only ‘Perao’ and ‘Pera 7’ had a color number above 36 in both seasons (Table 4). However, the color numbers for the other midseason oranges may be misleading because those selections would normally be harvested later in the season and the numbers would then be above 36.

For Florida growers of oranges for processing, the quantity of soluble solids produced/ha (a combination of juice volume, soluble solids concentration, and tree yield) is the basis of a crop's monetary value. Today, ≈3000 kg·ha−1 is considered to be necessary for a profitable grove operation. That number was exceeded by most of the selections regardless of season of maturity. The ‘Hamlin 1-4-1’ trees propagated with mature buds had one of the lowest soluble solids production levels, ≈3700 kg·ha−1 (Table 4). Trees of ‘Hamlin’ propagated using juvenile buds produced 4652 and 5371 kg·ha−1 when they were 5 and 8 years old, respectively, or ≈30% more soluble solids than the trees propagated with mature buds. That difference was not statistically significant, but it could be explained by the difference in canopy volume because the latter trees grew more vigorously and were clearly taller by the time the trial was 8 years old. Among the midseason oranges, ‘Pineapple’ is the commercial standard, but we did not include trees propagated with mature buds for comparison. There were a few selections such as ‘Homosassa’ (‘Midsweet’, presently a moderately important commercial cultivar) that produced larger amounts of soluble solids as 8–year-old trees than ‘Pineapple’, but the differences were generally less than 10% and not statistically significant.

V.1: Late-season selections.

There were significant differences in fruit yield among selections in individual years with the highest cumulative yields being those of ‘10-12-7’ and ‘9-8-17’ (Table 5). Both of those selections are among the most popular and productive selections used commercially in Florida. Yield at first cropping was 20 kg/tree and increased to nearly 200 kg/tree (Table 5). Seed counts were not made of these selections because they are all ‘Valencia’ oranges, which usually contain only a few seeds. Juice content was ≈60% by weight with no differences among selections (Table 6). Mean soluble solids concentrations were mostly between 12% and 13% and acid concentrations varied among the selections from less than 0.80 to greater than 1.00%. There were no differences in fruit maturity within either harvest season as indicated by the solids:acid ratios. Juice color numbers were mostly 37 to 40 and typical for Florida-grown ‘Valencia’ oranges. At tree age 8 years, soluble solids production ranged from 5204 kg·ha−1 to 3051 kg·ha−1. The production of all selections except ‘Olinda’ was significantly higher than that of the ‘1-14-31’ trees, which were propagated using mature buds and included for comparison. The latter trees were smaller and, thus, less productive.

Table 5.

Mean yield (kg/tree; n = 5) of V.1 ‘Valencia’ sweet orange selections budded in place on Swingle citrumelo rootstock in June 1986 at St. Cloud, FL.

Table 5.
Table 6.

Mean juice quality (n = 3) of fruit harvested in early April (age 5 years) or early March (age 8 years) from V.1 late-season sweet orange selections budded in place on Swingle citrumelo rootstock in June 1986 at St. Cloud, FL.

Table 6.

EM.2: Early- and midseason common and blood orange selections.

To interpret the yield and juice quality data, note that tree ages are different among selections. Measurements were taken from the 1992–1993 through the 1997–1998 seasons when the majority of the trees were between the ages of 3 and 8 years, respectively. The yields of those selections budded 1 year later were measured over the same seasons, so, e.g., the yield of ‘Sanguinello Riccio’ shown in Table 7 for tree age 3 years is actually for tree age 2 years, a difference considered in the discussion.

Table 7.

Mean yield (kg/tree; n = 4) of EM.2 early- and midseason sweet orange selections budded in place on Swingle citrumelo rootstock in May 1988.z

Table 7.

No selections were eliminated from this trial except ‘Roble’, which suffered decline from bud union creasing (Garnsey et al., 2001). The ‘Roble’ trees remained reasonably healthy during the evaluation period and declined afterward. The mean yield of the 41 selections increased from 78 kg/tree at age 3 years to 186 kg/tree after six cropping seasons, but there were few significant differences among selections in individual years (Table 7). Cumulative yield ranged from 1003 kg/tree (‘Aziza’) to less than 700 kg/trees among several blood orange selections. Most of the less productive selections were 1 year younger, but their yields still would be relatively low even if ≈100 kg was added to their cumulative totals to compensate for their younger age.

The cropping of several selections at tree age 3 years was greater than 100 kg/tree. Especially notable were the ‘Aziza’, ‘Sanguine Grosse Ronde’, ‘Roble’, and ‘Marrs’ trees, which all produced greater than 125 kg/tree, yield values well above normal for commercial trees in Florida. Those selections cropped substantially more than the comparison selection ‘Hamlin 1-4-1’, (700 kg/tree), which was propagated using mature buds. The productivity of the highest yielding selections can generally be explained by one or both of two factors: precocity and a resulting propensity to heavy early bearing, e.g., ‘Aziza’ and ‘Marrs’, or above average tree vigor/canopy volume (data not given), e.g., ‘Sanguine Grosse Ronde’ and ‘Roble’, or a combination of those factors, e.g., ‘Olivelands’ and ‘Rotuma Island’. The trees of selections like ‘Aziza’ and ‘Marrs’ exhibited juvenile traits, but less so than many of the other selections. Shoots of ‘Aziza’ and ‘Marrs’ were less thorny and vigorous and the trees seemed to require fewer shoot flushes before their shoots took on a less juvenile nature. As a result, flowering began sooner after budding leading to heavier, early cropping that also tended to reduce tree canopy growth.

Mean fruit seed count was nine but varied from virtually seedless (‘Salustiana’, ‘Cadenera’, ‘Marrs’, and ‘Moro #18’) to 20 or more seeds/fruit, e.g., ‘Bidwell's Bar’ and ‘Indian River Sweet’. Mean fruit juice content was 61.2% by weight, soluble solids concentration was 9.8%, and acid concentration was 0.79%. Most juice content values were greater than 58%, typical for Florida fruit. Soluble solids concentrations are not strictly comparable because the trial consisted of early- and midseason-maturing selections all harvested at one time, which was not optimal for all selections. Among those considered to be early-maturing, ‘Roble’ had the highest soluble solids concentration (11.7%) followed by a group with values ≈10%, but they were not significantly different from ‘Hamlin 1-4-1’. Exceptions to the preceding results were the mid- to late-season selections ‘Wetumpka’ and ‘Bidwell's Bar’ that had soluble solids values of 10.0 and 10.1, respectively, on 14 Nov. 1995. Season of maturity classification is largely based on soluble solids:acid ratio. Notable values were those of ‘Vainiglia’, ‘Lima’, and ‘Mangaratiba’. The former two selections are acidless, which explains their high ratios. ‘Mangaratiba’ proved to be a selection that matured very early primarily because of low acid concentrations. The range in juice color numbers was small and there were few differences among selections. Most selections had values below the desired color number of 36. The highest values were those of ‘Ruby Nucellar’.

For the year presented, the juice data combined with yield resulted in greater than twofold difference in soluble solids/ha among the selections (Table 8). Seventeen of 42 selections produced greater than 4000 kg·ha−1 and two selections, ‘Salustiana’ and ‘Vainiglia’, produced greater than 5000 kg·ha−1.

Table 8.

Mean seed count and juice quality (n = 4) of EM.2 early- and midseason sweet orange selections budded in place on Swingle citrumelo rootstock in May 1988.z

Table 8.

During the trial or soon after data collection was completed, the ‘Cadenera’, ‘Joao Nunes’, ‘Macetera’, ‘Ruby’, Sanguinello Moscato’, ‘Tarocco’, Tomango’, and ‘Trovita’ trees were removed at the suggestion of the cooperator because of excessive vigor and poor yields. Other selections such as ‘Aziza’, ‘Budd Blood’, ‘Earlygold’, and ‘Itaborai’ were further evaluated elsewhere in Florida (Castle and Baldwin, 2008).

V.2: Late-season selections.

Mean yield was 31 kg/tree when they began cropping after 2 years in the field and increased to ≈160 kg/tree (Table 9). ‘Vernia’ and the selection for comparison, ‘1-14-31’, were precocious in that they produced well in the first year of cropping but fell behind in subsequent years leading to average cumulative yields after six seasons. Several selections produced cumulative yields greater than 700 kg/tree of which ‘Williams Budded’ was the largest. Included in that group were ‘Appleby’, ‘Smith’, and ‘Jenner’, selections that had been previously tested in Australia and found to be among the best producers. Fruit samples were collected in April when the trees were 4 and 8 years old. Mean juice content was 62.2% and 56.3%, soluble solids concentration was 11.2% and 12.2%, and acid concentration was 0.79% and 0.65%, respectively (Table 10). Most selections did not differ significantly in these variables and all selections produced commercially acceptable quality juice. The higher soluble solids concentration and soluble solids:acid ratio of the ‘Vernia’ trees suggested that it matures at the beginning (February) of the Florida late season. ‘Natal’ is one of the latest maturing orange selections in Brazil and it appeared to perform similarly in our trial (Donadio et al., 1995). There were virtually no differences among the selections in juice color. All color numbers were mostly between 38 and 39, typical for Florida late-season oranges. Soluble solids production at age 8 years ranged from 4442 kg·ha−1 (‘Williams Budded’) to 2055 kg·ha−1 (‘Frost’), but there were no differences among the top 11 of 13 selections. Soluble solids production/ha differences were largely explained by yield (r = 0.72; P < 0.01).

Table 9.

Mean yield (kg/tree; n = 4) of V.2 late-season sweet orange selections budded in place on Swingle citrumelo rootstock, May 1988, St. Cloud, FL.

Table 9.
Table 10.

Mean juice quality (n = 3) of V.2 late-season sweet orange selections budded in place, May 1988, on Swingle citrumelo rootstock, St. Cloud, FL.z

Table 10.

P.1: ‘Pera’ orange selections.

All the P.1 ‘Pera’ selections without an interstock, except ‘Pera Acidless’, eventually exhibited a bud union crease, declined, and were removed (Table 11). However, by the end of the trial period, an interstock did not ensure longevity because among all the selections, survival varied from 25% to 75%.

Table 11.

Tree survival (n = 4) among P.1 ‘Pera’ sweet orange selections budded in place on Swingle citrumelo rootstock, May 1988, St. Cloud, FL.

Table 11.

As observed in Brazil (Carlos and Donadio, 1996), there were no effects among the trees with and without an interstock on yield and juice quality over the course of 3 years when the trees in each two-tree plot were measured separately; thus, the data were combined within each plot in all years. Also, the ‘Pera’ trees had a more mature tree (rounder rather than upright) appearance soon after they began cropping with less vigorous shoots, reduced thorniness, and the fruit had smoother, thinner peel than the oranges of the other selections. The trees began to crop well when they were 3 years old, which coincides with the apparent quick loss of juvenility compared with the other sweet oranges in EM.1 and EM.2. Mean yield was 86 kg/tree in the first cropping year and increased to 240 kg/tree when the trees were 8 years old (Table 12). There were significant differences among selections only when the trees were 6 and 8 years old. More apparent was the alternate bearing, particularly in the last three measured crops when yield varied among 162, 96, and 240 kg/tree, respectively. However, alternate bearing is considered to be related to juvenility and may explain the performance of the ‘Pera’ selections and those in the other trials (Cameron and Frost, 1968). Mean cumulative yield was nearly 800 kg/tree over six crops with ‘Bianchi’ selections producing two of the largest cumulative values and some of the largest and smallest annual values.

Table 12.

Mean yield (kg/tree; n = 4) P.1 ‘Pera’ sweet orange selections budded in place on Swingle citrumelo rootstock, May 1988, at St. Cloud, FL.z

Table 12.

The fruit were moderately seedy with approximately five to 10/fruit. Samples for juice quality measurements were collected in January, which is midseason when ‘Pera’ selections are typically harvested in Brazil. Fruit juice content was ≈58% (Table 13), which is lower than the values of the early- and late-season selections. Mean soluble solids concentration was ≈11.7 with some significant differences in each year. At age 4 years, the fruit from the ‘Vacinada’ (12.3) and ‘Olympia’ (12.2) selections had the highest and those from ‘Acidless’ (11.0) had the lowest soluble solids, a similar situation when the trees were 8 years old. The mean soluble solids:acid ratios were skewed to high values because of the low acid–high ratio values of the ‘Acidless’ fruit. Juice color numbers were ≈37 except for the low values of the ‘Acidless’ fruit. The numbers were one or two units below many of those for the ‘Valencia’ selections as is commonly observed for ‘Pera’ as indicated by juice importers who bring juice from Brazil to Florida. The significantly higher yields of the ‘Bianchi 26’ and ‘Premunizada’ selections when those trees were 8 years old, combined with average juice quality, resulted in the highest soluble solids/ha.

Table 13.

Mean juice quality (n = 3) of P.1 ‘Pera’ sweet orange selections budded in place on Swingle citrumelo rootstock, May 1988, St. Cloud, FL.zy

Table 13.

P.2: ‘Pera’ orange selections.

These ‘Pera Olympia’ trees were grown in a field nursery and all were set on the ends of the rows where other trials were located. They were less crowded and, thus, by the end of the measurement period, the trees were greater than 6 m tall. Cropping began when the trees were 3 years old and increased to 240 kg/tree by age 8 years (Table 14). The trees on Cleopatra mandarin rootstock produced the lowest yields and those on rough lemon had the largest yield, a significant difference among rootstocks that was first apparent when the trees were 5 years old. Because of their larger size and position at the ends of rows, these trees produced 6% (Cleopatra mandarin) to 35% (rough lemon) more fruit than the ‘Pera Olympia’ trees in the P.1 trial on Swingle citrumelo rootstock.

Table 14.

Mean yield (kg/tree; n = 4) of P.2 ‘Pera Olympia’ sweet orange trees on four rootstocks planted in Mar. 1989 at St. Cloud, FL.z

Table 14.

Trees on Cleopatra mandarin rootstock are well known to produce fruit with higher juice quality (Castle, 1987; Castle et al., 1993, 2006; Wutscher, 1979) as evident in P.2 (Table 15). Fruit from the trees on Cleopatra mandarin had higher juice quality than that of fruit from trees on the other three rootstocks in this trial. However, as we have reported previously (Castle, 2010; Castle et al., 2010a, 2010b), yield was the primary factor that determined soluble solids production/ha with the trees on rough lemon having the highest value in Year 8 with a maximum difference of 39% among rootstocks.

Table 15.

Mean juice quality (n = 4) of P.2 ‘Pera Olympia’ sweet orange fruit from trees on four rootstocks planted in Mar. 1989 at St. Cloud, FL.zy

Table 15.

Our results have both a research and commercial component. The first issue faced in this project was the matter of introducing the selections to Florida in a timely manner such that a proper replicated trial could be established quickly, an objective also highly desired by the grower–cooperator. Plant introduction of a clonally propagated tree fruit species is normally by some means other than seed. Seeds could be of questionable usefulness. Many, but not all, citrus species produce apomictic seeds and the degree of polyembryony varies. Also, seedlings have different lengths of juvenility. Therefore, from a research standpoint, it is noteworthy that our sweet orange selections produced visually uniform seedling populations. Their use in the young seedling stage as a source of buds for propagation did not seriously lengthen the time to bearing compared with trees produced using buds from mature trees. Furthermore, our project trees fruited earlier than if our trial consisted of seedlings only. Sweet oranges are reported to have juvenile periods as long as 15 years, but that is not well documented (Cameron and Frost, 1968).

In our trials, we did not compare seedlings with budded trees within selections, but the comparison was made in EM.1 between ‘Hamlin’ trees propagated using either juvenile (young-line) buds or a mature (old-line) source. In that comparison, the young-line trees were more vigorous and productive. We did not have the same result among the V.1 selections.

If clonal senescence occurs as proposed by some authors (Cameron and Frost, 1968), then our results support the contention that sweet orange cultivars be periodically restored to a more juvenile, i.e., vigorous and fruitful, condition by occasional regeneration through some means such as seeds or possibly tissue culture (W.P. Bitters, personal communication). Clonal restoration through seed may also add a measure of cold-hardiness because juvenile trees, especially seedlings, have greater tolerance to cold weather than trees grown from old-line tree bud sources (Yelenosky, 1985).

The performance and phenotypic variability among the selections in the field trials illustrate the diversity among sweet oranges and suggests genetic variability. However, citrus systematics is complicated by apomixis and hybridization. Furthermore, C. sinensis is not considered to be a true species. Evidence from studies of morphological, biochemical, and molecular characteristics indicates that sweet oranges comprise a monophyletic group, i.e., all the descendants originated from a single ancestor perhaps by mutation (Barrett and Rhodes, 1976; Moore, 2001; Potvin et al., 1983).

The combined research and commercial objective was to search for new sweet orange selections that would be suitable for the Florida citrus juice processing industry, but candidates for fresh use were not excluded from the evaluations. The initial round at identifying selections with commercial potential included the assessments of a professional fruit buyer from a nearby citrus grower cooperative and data developed by a major juice processor. Among the early-maturing selections, emphasis was placed on yield and juice quality, especially color and flavor (Wutscher and Bistline, 1988). On that basis, ‘Westin’ was chosen in EM.1 and ‘Earlygold’ (tested as selection 8), ‘Ruby’, and ’Itaborai’ in EM.2. Those selections became commercial selections, especially ‘Earlygold’, mostly because they fit the particular juice profile of the processor involved and offered advantages over the standard selection, ‘Hamlin’. Other selections with commercial potential were ‘Tobias’ and ‘Cadena Punchosa’. The former selection was very productive and exhibited one trait of special interest: it was the first selection to flower. Sporadic flowering began ≈2 years before the other selections and only on short shoots with single apical flowers. ‘Cadena Punchosa’ was only moderately productive but yielded large, nearly perfectly round fruit (data not given) that were found to be attractive as a fresh fruit in 2 subsequent years by the fruit buyer assisting in the evaluations.

The importance of midseason oranges has declined in Florida. However, among the selections we tested, our results confirmed the reported performance and subsequent release of ‘Sunstar’ (tested by us as ‘Midseason’), ‘Midsweet’ (‘Homosassa’), and ‘Gardner’ (‘Sanford’) that led to their release (Hearn, 1988). ‘Pineapple’ was the standard midseason orange in Florida for many years and is the comparison selection in our trials but has been supplanted in recent years by ‘Midsweet’. The local selection, ‘Pineapple #8’, did not perform as well as the standard ‘Pineapple’. There were no other midseason selections in EM.1 that exceeded the performance of those already mentioned; however, ‘Jincheng’, a popular midseason orange in China, was notable (Deng et al., 2008). The trees were 1 year younger, which contributed to their lower yield. They were more compact trees and yielded efficiently making them suitable for planting as a juice orange in combination with a size-controlling rootstock and higher density planting (Boswell et al., 1975; Wheaton et al., 1991, 1995). Also, the fruit would be attractive for the fresh market because they were low-seeded with good juice quality although our samples were harvested in most years before peak maturity.

In the EM.2 group of midseason selections, ‘Salustiana’ ranked high in the results. It is an old established variety and its known traits were repeated in our trial, i.e., modest yields of excellent internal quality and large, seedless fruit. The combination of yield and juice quality placed this selection at the top of the results across all selections in EM.2 for soluble solids production when the trees were 6 years old. ‘Madam Vinous’ trees also placed high in the results, but the fruit were undesirably seedy, which would limit its commercial appeal as a fresh fruit.

The main practical result among the late-season selections in V.1 was that those propagated from juvenile sources were no different from those propagated using the mature buds. No selections offered any particular advantage over the standard tree, which would be favored because of its statistically comparable yield but smaller canopy size and thus higher yield efficiency. The same general results were obtained in V.2; however, in a subsequent larger-sized trial conducted by us with Carrizo citrange rootstock, ‘Appleby’ was one of the top performing selections after 9 years. ‘Natal’ remains of interest because of its very late maturity, which explains its commercial use in Brazil and ‘Vernia’ has become a commercial variety in Florida because of its excellent flavor and juice color (traits determined by the cooperating commercial juice processor).

The two remaining groups, the blood oranges and ‘Pera’ selections, are treated separately here because diverse selections of each group have not been formally compared in Florida. Blood oranges are not considered to be suitable for Florida's subtropical climate because of inadequate cool temperature during ripening to bring forth the anthocyanin pigments that give those selections their well-known red to purple coloration. ‘Moro’ and ‘Tarocco’ were included in EM.1 but removed early because of the poor performance described. Anthocyanin pigment never appeared in the peel in the first few cropping years, but light flecking was present in the flesh. Cropping was light in both selections and seeds were rare. In EM.2, there were 12 blood orange selections plus ‘Vainiglia’, which is an acidless, seedy orange, which had very pronounced red coloration in the flesh from lycopene pigment. None of the true blood orange selections in EM.2 are commercially important in Florida and did not produce fruit with typical coloration. In general, there were similarities among the selections. The fruit were moderately seedy except for the nearly seedless selections, ‘Tarocco’, ‘Sanguinello Ricco’, ‘Sanguinello Moscato’, and ‘Moro’. All selections had soft flesh easily consumed along with the segment membranes. Blood orange cultivars are normally midseason in maturity, but in EM.2, fruit of some selections, e.g., ‘Orlando Blood’, tended to produce fruit with low acidity and were classified as early-maturing. That trait may change as the trees age.

The blood orange selections we tested have no potential in Florida if evaluated according to their production of typically colored fruit. However, we conducted a short seasonal tracking study in two seasons (data not given) to determine the quality of the blood oranges when allowed to remain on the trees into midwinter, their normal season of maturity. Later harvest had little impact on most selections, but the juice color of ‘Budd Blood’ improved 1.0 to 2.0 points on the 100-point system not because of red pigmentation, but because of enhanced orange coloration. Also, judging from causal sampling in the field, flavor changed from the typical experience of orange juice to the more distinctive flavor of a blood orange with a raspberry note.

The ‘Pera’ orange is the principal cultivar in Brazil and is thought to be similar to an old Florida variety, ‘Lamb's Summer’, that never became established commercially in Florida (Hodgson, 1967). In our trials, no compelling reasons emerged to support adding ‘Pera’ to the Florida citrus industry. The general ‘Pera’ results were typical for the cultivar and would be commercially acceptable (Bassanezi et al., 2009; Cavalcante et al., 2006). However, the ‘Pera’ selections had midseason maturity, which is not an important seasonal niche for new selections, and rootstock compatibility would be a serious issue in any industry using trifoliate orange-based rootstocks.

Conclusions

An important question to the project participants was: Given that our project was a large undertaking involving considerable time and resources on the parts of the grower–cooperator and the research team, was it worthwhile? The answer is yes, for the biological and practical reasons presented in the discussion. Comparative evaluations of large numbers of scions are uncommon. The project showed that sweet orange selections could be easily assembled and propagated from seed without a significant loss in time resulting from juvenility. The nearly 100 selections in the project demonstrated sweet orange diversity and, as a resource beyond the original objective, they proved useful for making rootstock crosses, e.g., we selected ‘Tobias’ as a parent for short juvenility and early flowering.

The scope and size of the project attracted considerable citrus industry attention and input leading to the commercialization of several selections. Like in our previous experience with rootstock trials (Castle, 2010), it is difficult to develop a data set that provides all of the information needed in selecting cultivars for particular commercial uses. Thus, there was not a perfect match between our data and the initial choices of selections for commercialization. Furthermore, field trial constraints in a grower–cooperator situation precluded massive seasonal tracking to learn the full potential of each selection. Nonetheless, the value of conducting such large and time-consuming trials is apparent from the interest growers have shown for these new selections.

Literature Cited

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    • Search Google Scholar
    • Export Citation
  • Bassanezi, R.B., Montesino, L.H. & Stuchi, E.S. 2009 Effects of Huanglongbing on fruit quality of sweet orange cultivars in Brazil Eur. J. Plant Pathol. 125 565 572

    • Search Google Scholar
    • Export Citation
  • Boswell, S.B., McCarty, C.D., Hench, K.W. & Lewis, L.N. 1975 Effect of tree density on the first ten years of growth and production of ‘Washington’ navel orange trees J. Amer. Soc. Hort. Sci. 100 370 373

    • Search Google Scholar
    • Export Citation
  • Cameron, J.W. & Frost, H.B. 1968 Genetics, breeding, and nucellar embryony 325 370 Reuther W., Batchelor L.D. & Webber H.J. The citrus industry Vol. 2 Univ. Calif. Berkeley, CA

    • Search Google Scholar
    • Export Citation
  • Carlos, E.F. & Donadio, L.C. 1996 Interstocks between Pera sweet orange and Rangpur lime Proc. 1996 Intl. Soc. Citricult. 1 221 224

  • Castle, W.S. 1987 Citrus rootstocks 361 399 Rom R.C. & Carlson R.F. Rootstocks for fruit crops Wiley Hoboken, NJ

  • Castle, W.S. 2010 A career perspective on citrus rootstocks, their development, and commercialization HortScience 45 1 5

  • Castle, W.S. & Baldwin, J.C. 2008 Tree survival, growth, and juice quality of early-season sweet orange selections on eight rootstocks in Immokalee Proc. Fla. State Hort. Soc. 121 145 147

    • Search Google Scholar
    • Export Citation
  • Castle, W.S., Baldwin, J.C. & Muraro, R.P. 2010a Rootstocks and the performance and economic returns of ‘Hamlin’ sweet orange trees HortScience 45 875 881

    • Search Google Scholar
    • Export Citation
  • Castle, W.S., Baldwin, J.C., Muraro, R.P. & Littell, R. 2010b Performance of ‘Valencia’ sweet orange trees on 12 rootstocks at two locations and an economic interpretation as a basis for rootstock selection HortScience 45 523 533

    • Search Google Scholar
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    • Export Citation
  • Castle, W.S., Tucker, D.P.H., Krezdorn, A.H. & Youtsey, C.O. 1993 Rootstocks for Florida 2nd Ed Univ. Fla. Coop. Ext. Publ. SP-42

    • Export Citation
  • Cavalcante, I.H.L., Martins, A.B.G. & Stuchi, E.S. 2006 Fruit characteristics of eighteen orange cultivars Rev. de Biol. Ciencias da Terra 6 72 77

  • Davies, F.S. & Jackson, L.K. 2009 Citrus growing in Florida Univ. Press Fla. Gainesville, FL

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  • Deng, X., Peng, C., Chen, Z., Deng, Z. & Li, J. 2008 Citrus varieties in China China Agr. Press

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    • Export Citation
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    • Search Google Scholar
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  • Fellers, P.F., Petrus, D.R., Nyhof, K.K. & Powell, A. 1990 Flavor and color profiles of frozen concentrated orange juice and orange juice from concentrate samples submitted for Florida's seal of approval or Sunshine tree programs, 1986 through 1989 Proc. Fla. State Hort. Soc. 103 265 268

    • Search Google Scholar
    • Export Citation
  • Fla. Agr. Stat. Serv 2010 Florida citrus statistics 2008–2009 Fla. Dept. Agric. Consumer Serv. and USDA Agric. Marketing Serv

    • Export Citation
  • Frost, H.B. & Soost, R.K. 1968 Seed reproduction: Development of gametes and embryos 290 324 Reuther W., Batchelor L.D. & Webber H.J. The citrus industry Univ. Calif. Berkeley, CA

    • Search Google Scholar
    • Export Citation
  • Furr, J.R., Cooper, W.C. & Reese, P.C. 1947 An investigation of flower formation in adult and juvenile citrus trees Amer. J. Bot. 34 1 8

  • Garnsey, S.M., Castle, W.S., Rouse, R.E., Wutscher, H.K. & Kesinger, M.C. 2001 Budunion incompatibilities and associated declines observed in Florida among trees on Swingle citrumelo and other trifoliate-related rootstocks Proc. Fla. State Hort. Soc. 114 121 127

    • Search Google Scholar
    • Export Citation
  • Grosser, J., Castle, B., Gmitter, F. & Ling, P. 2009 What's new in sweet orange improvement? Citrus Industry 90 12 15, 27

  • Hearn, C.J. 1988 The performance of ‘Sunstar’, ‘Midsweet’, and ‘Gardner’ oranges Proc. Fla. State Hort. Soc. 101 33 36

  • Hodgson, R.W. 1967 Horticultural varieties of citrus 431 591 Reuther W., Webber H.J. & Batchelor L.D. The citrus industry Vol. 1 Univ. Calif. Berkeley, CA

    • Search Google Scholar
    • Export Citation
  • Moore, G.A. 2001 Oranges and lemons: Clues to the taxonomy of Citrus from molecular markers Trends Genet. 17 536 540

  • Obreza T.A. & Morgan K.T. 2008 Nutrition of Florida citrus trees 2nd Ed Univ. Fla. Coop. Ext. Publ. SL-253

    • Export Citation
  • Potvin, C., Bergeron, Y. & Simon, J.-P. 1983 A numerical taxonomic study of selected Citrus species (Rutaceae) based on biochemical characters. Syst Bot. 8 127 133

    • Search Google Scholar
    • Export Citation
  • Salibe, A.A. 1963 Studies on budunion crease of citrus trees. Proc. Intern. Org Citrus Virol. 3 187 191

  • Saunt, J. 2000 Citrus varieties of the world Sinclair International Norwich, UK

    • Export Citation
  • Wheaton, T.A., Castle, W.S., Whitney, J.D. & Tucker, D.P.H. 1991 Performance of citrus scion cultivars and rootstocks in a high density planting HortScience 26 837 840

    • Search Google Scholar
    • Export Citation
  • Wheaton, T.A., Whitney, J.D., Castle, W.S., Muraro, R.P., Browning, H.W. & Tucker, D.P.H. 1995 Citrus scion and rootstock, topping height, and tree spacing affect tree size, yield, fruit quality, and economic return J. Amer. Soc. Hort. Sci. 120 861 870

    • Search Google Scholar
    • Export Citation
  • Wutscher, H.K. 1979 Citrus rootstocks Hort. Rev. 1 237 269

  • Wutscher, H.K. & Bistline, F.W. 1988 Rootstock influences juice color of ‘Hamlin’ orange HortScience 23 724 725

  • Yelenosky, G. 1985 Cold hardiness in citrus Hort. Rev. 7 201 238

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Contributor Notes

We are very grateful to Orie Lee and his family for their exceptional long-time commitment of land, tree care, and harvesting assistance that made the field trial possible and to Orie Lee for his intellectual contributions to interpreting the results while teaching us the grower perspective. We also extend our gratitude to Florida citrus growers who supported this work through a self-imposed tax for research funds granted by the Florida Citrus Production Research Advisory Council.

To whom reprint requests should be addressed; e-mail bcastle@ufl.edu.

  • Barrett, H.C. & Rhodes, A.M. 1976 A numerical taxonomy study of affinity relationships in cultivated Citrus and its close relatives Syst. Bot. 1 105 136

    • Search Google Scholar
    • Export Citation
  • Bassanezi, R.B., Montesino, L.H. & Stuchi, E.S. 2009 Effects of Huanglongbing on fruit quality of sweet orange cultivars in Brazil Eur. J. Plant Pathol. 125 565 572

    • Search Google Scholar
    • Export Citation
  • Boswell, S.B., McCarty, C.D., Hench, K.W. & Lewis, L.N. 1975 Effect of tree density on the first ten years of growth and production of ‘Washington’ navel orange trees J. Amer. Soc. Hort. Sci. 100 370 373

    • Search Google Scholar
    • Export Citation
  • Cameron, J.W. & Frost, H.B. 1968 Genetics, breeding, and nucellar embryony 325 370 Reuther W., Batchelor L.D. & Webber H.J. The citrus industry Vol. 2 Univ. Calif. Berkeley, CA

    • Search Google Scholar
    • Export Citation
  • Carlos, E.F. & Donadio, L.C. 1996 Interstocks between Pera sweet orange and Rangpur lime Proc. 1996 Intl. Soc. Citricult. 1 221 224

  • Castle, W.S. 1987 Citrus rootstocks 361 399 Rom R.C. & Carlson R.F. Rootstocks for fruit crops Wiley Hoboken, NJ

  • Castle, W.S. 2010 A career perspective on citrus rootstocks, their development, and commercialization HortScience 45 1 5

  • Castle, W.S. & Baldwin, J.C. 2008 Tree survival, growth, and juice quality of early-season sweet orange selections on eight rootstocks in Immokalee Proc. Fla. State Hort. Soc. 121 145 147

    • Search Google Scholar
    • Export Citation
  • Castle, W.S., Baldwin, J.C. & Muraro, R.P. 2010a Rootstocks and the performance and economic returns of ‘Hamlin’ sweet orange trees HortScience 45 875 881

    • Search Google Scholar
    • Export Citation
  • Castle, W.S., Baldwin, J.C., Muraro, R.P. & Littell, R. 2010b Performance of ‘Valencia’ sweet orange trees on 12 rootstocks at two locations and an economic interpretation as a basis for rootstock selection HortScience 45 523 533

    • Search Google Scholar
    • Export Citation
  • Castle, W.S., Bowman, K.D., Graham, J.H. Jr & Tucker, D.P.H. 2006 Florida citrus rootstock selection guide Univ. Fla. Coop. Extension Publ. SP-248

    • Export Citation
  • Castle, W.S., Tucker, D.P.H., Krezdorn, A.H. & Youtsey, C.O. 1993 Rootstocks for Florida 2nd Ed Univ. Fla. Coop. Ext. Publ. SP-42

    • Export Citation
  • Cavalcante, I.H.L., Martins, A.B.G. & Stuchi, E.S. 2006 Fruit characteristics of eighteen orange cultivars Rev. de Biol. Ciencias da Terra 6 72 77

  • Davies, F.S. & Jackson, L.K. 2009 Citrus growing in Florida Univ. Press Fla. Gainesville, FL

    • Export Citation
  • Deng, X., Peng, C., Chen, Z., Deng, Z. & Li, J. 2008 Citrus varieties in China China Agr. Press

    • Export Citation
  • Donadio, L.C., Orlando de Figueiredo, J. & Pio, R. 1995 Variedades citricas Brasileiras Funep and Unesp Campinas, Brazil

    • Export Citation
  • Fellers, P.F. 1990 Florida's juice standards for grades and their differences from United States standards for grades and United States Food and Drug Administration standards of identity Proc. Fla. State Hort. Soc. 103 260 265

    • Search Google Scholar
    • Export Citation
  • Fellers, P.F., Petrus, D.R., Nyhof, K.K. & Powell, A. 1990 Flavor and color profiles of frozen concentrated orange juice and orange juice from concentrate samples submitted for Florida's seal of approval or Sunshine tree programs, 1986 through 1989 Proc. Fla. State Hort. Soc. 103 265 268

    • Search Google Scholar
    • Export Citation
  • Fla. Agr. Stat. Serv 2010 Florida citrus statistics 2008–2009 Fla. Dept. Agric. Consumer Serv. and USDA Agric. Marketing Serv

    • Export Citation
  • Frost, H.B. & Soost, R.K. 1968 Seed reproduction: Development of gametes and embryos 290 324 Reuther W., Batchelor L.D. & Webber H.J. The citrus industry Univ. Calif. Berkeley, CA

    • Search Google Scholar
    • Export Citation
  • Furr, J.R., Cooper, W.C. & Reese, P.C. 1947 An investigation of flower formation in adult and juvenile citrus trees Amer. J. Bot. 34 1 8

  • Garnsey, S.M., Castle, W.S., Rouse, R.E., Wutscher, H.K. & Kesinger, M.C. 2001 Budunion incompatibilities and associated declines observed in Florida among trees on Swingle citrumelo and other trifoliate-related rootstocks Proc. Fla. State Hort. Soc. 114 121 127

    • Search Google Scholar
    • Export Citation
  • Grosser, J., Castle, B., Gmitter, F. & Ling, P. 2009 What's new in sweet orange improvement? Citrus Industry 90 12 15, 27

  • Hearn, C.J. 1988 The performance of ‘Sunstar’, ‘Midsweet’, and ‘Gardner’ oranges Proc. Fla. State Hort. Soc. 101 33 36

  • Hodgson, R.W. 1967 Horticultural varieties of citrus 431 591 Reuther W., Webber H.J. & Batchelor L.D. The citrus industry Vol. 1 Univ. Calif. Berkeley, CA

    • Search Google Scholar
    • Export Citation
  • Moore, G.A. 2001 Oranges and lemons: Clues to the taxonomy of Citrus from molecular markers Trends Genet. 17 536 540

  • Obreza T.A. & Morgan K.T. 2008 Nutrition of Florida citrus trees 2nd Ed Univ. Fla. Coop. Ext. Publ. SL-253

    • Export Citation
  • Potvin, C., Bergeron, Y. & Simon, J.-P. 1983 A numerical taxonomic study of selected Citrus species (Rutaceae) based on biochemical characters. Syst Bot. 8 127 133

    • Search Google Scholar
    • Export Citation
  • Salibe, A.A. 1963 Studies on budunion crease of citrus trees. Proc. Intern. Org Citrus Virol. 3 187 191

  • Saunt, J. 2000 Citrus varieties of the world Sinclair International Norwich, UK

    • Export Citation
  • Wheaton, T.A., Castle, W.S., Whitney, J.D. & Tucker, D.P.H. 1991 Performance of citrus scion cultivars and rootstocks in a high density planting HortScience 26 837 840

    • Search Google Scholar
    • Export Citation
  • Wheaton, T.A., Whitney, J.D., Castle, W.S., Muraro, R.P., Browning, H.W. & Tucker, D.P.H. 1995 Citrus scion and rootstock, topping height, and tree spacing affect tree size, yield, fruit quality, and economic return J. Amer. Soc. Hort. Sci. 120 861 870

    • Search Google Scholar
    • Export Citation
  • Wutscher, H.K. 1979 Citrus rootstocks Hort. Rev. 1 237 269

  • Wutscher, H.K. & Bistline, F.W. 1988 Rootstock influences juice color of ‘Hamlin’ orange HortScience 23 724 725

  • Yelenosky, G. 1985 Cold hardiness in citrus Hort. Rev. 7 201 238

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