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William S. Castle

A rootstock trial with a ‘Marsh’ grapefruit (Citrus paradisi Macf.) scion was established in the flatwoods of the Florida east coast Indian River region in 1990. The trees were planted in an Alfisol of the Pineda series. The trial consisted of trees on 16 rootstocks, primarily citranges [C. sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.], citrumelos (C. paradisi × P. trifoliata), mandarins (C. reticulata Blanco) and various hybrids, in three or six replicates of three-tree plots in a randomized complete block design. Tree growth and survival, yield, and juice quality were measured annually or periodically for 10 years. In three seasons, whole-tree crops were sized in the field. Using the fruit size distribution data, crop value or income/tree was estimated. Tree height after 10 years ranged from 1.7 to 4.1 m and survival was greater than 90% with a few exceptions. Mean cumulative yield was 1202 kg/tree and ranged from 649 (Hamlin + Flying Dragon trifoliate orange) to 1615 kg/tree for a hybrid of trifoliate orange × Milam. At tree age 9 years, mean soluble solids production was 3594 kg·ha−1 with a 240% difference between the lowest and highest value. There were differences in the distribution of four commercial-sized categories based on analysis of individual sizes and using PROC FREQ to examine and compare whole-tree distributions. When the yield and fruit size data were combined for 3 years and converted to income/tree using commercial Florida Freight On Board prices in November and March, the trees on a trifoliate orange × Milam hybrid (1584) had the highest estimated income ($354 U.S., March data) followed by Calamandarin ($321) and Norton citrange ($292). The lowest income/tree was $112 (Hamlin + Flying Dragon trifoliate orange). When all data were considered, the best matches to current grower interest in smaller sized trees, and high yield and fruit quality, were the hybrid 1584 and C-35 citrange rootstocks.

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William S. Castle and James C. Baldwin

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.

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Graham H. Barry, William S. Castle, and Frederick S. Davies

Juice quality of `Valencia' sweet orange [Citrus sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks was determined for fruit harvested by canopy quadrant and separated into size categories to ascertain the direct role of rootstock selection on juice soluble solids concentration (SSC) and soluble solids (SS) production per tree of citrus fruit. SS production per fruit and per tree for each size category was calculated. Juice quality was dependent on rootstock selection and fruit size, but independent of canopy quadrant. Fruit from trees on Carrizo citrange had >20% higher SSCs than fruit from trees on rough lemon, even for fruit of the same size. Large fruit accumulated more SS per fruit than smaller fruit, despite lower juice content and SSC. Within rootstocks, SS content per fruit decreased with decreasing fruit size, even though SSC increased. Rootstock effect on juice quality was a direct rather than an indirect one mediated through differences in fruit size. The conventional interpretation of juice quality data that differences in SSC among treatments, e.g., rootstocks or irrigation levels, or fruit size, are due to “dilution” of SS as a result of differences in fruit size and, hence, juice volume, is only partly supported by these data. Rather, accumulation of SS was greater for fruit from trees on Carrizo citrange than rough lemon by 25% to 30%.

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Graham H. Barry, William S. Castle, and Frederick S. Davies

Citrus rootstocks have well-known effects on tree size, crop load, fruit size, and various fruit quality factors. Fruit from trees budded on invigorating rootstocks are generally larger with lower soluble solids concentration (SSC) and titratable acidity compared to fruit from trees budded on less invigorating rootstocks. Although it is unclear how rootstocks exert their influence on juice quality of Citrus L. species, plant water relations are thought to play a central role. In addition, the larger fruit size associated with invigorating rootstocks and the inverse relationship between SSC and fruit size implies that fruit borne on trees on invigorating rootstocks have lower SSC due to dilution effects in larger fruit. To determine how rootstock type affects sugar accumulation in fruit of Citrus species, controlled water-deficit stress was applied to mature `Valencia' sweet orange [C. sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks. Withholding water from the root zone of citrus trees during stage II of fruit development decreased midday stem water potential and increased the concentrations of primary osmotica, fructose and glucose. Sucrose concentration was not affected, suggesting that sucrose hydrolysis took place. Increased concentrations of sugars and SSC in fruit from moderately water-stressed trees occurred independently of fruit size and juice content. Thus, passive dehydration of juice sacs, and concentration of soluble solids, was not the primary cause of differences in sugar accumulation. Controlled water-deficit stress caused active osmotic adjustment in fruit of `Valencia' sweet orange. However, when water-deficit stress was applied later in fruit development (e.g., stage III) there was no increase in sugars or SSC. The evidence presented supports the hypothesis that differential sugar accumulation of citrus fruit from trees on rootstocks of contrasting vigor and, hence, plant water relations, is caused by differences in tree water status and the enhancement of sucrose hydrolysis into component hexose sugars resulting in osmotic adjustment. Therefore, inherent rootstock differences affecting plant water relations are proposed as a primary cause of differences in sugar accumulation and SSC among citrus rootstocks.

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Graham H. Barry, William S. Castle, and Frederick S. Davies

The objectives of this study were to determine whether juice quality of `Valencia' sweet orange [C. sinensis (L.) Osb.] is affected by the type of inflorescence on which fruit are borne, and to determine the contribution of inflorescence type to within-tree variation in juice quality. During the 1998-99 and 1999-2000 seasons, fruit size and juice quality [soluble solids concentration (SSC) and titratable acidity (TA)] of fruit from `Valencia' sweet orange trees on Carrizo citrange rootstock [Poncirus trifoliata (L.) Raf. × C. sinensis (L.) Osb.] planted in 1987 at Howey-in-the-Hills, Fla., were measured. A 2×2 factorial design (inflorescence type × canopy position) with leafy and leafless inflorescence types, and southwest top and northeast bottom canopy positions was used. The type of inflorescence on which fruit were borne had a minor effect on juice quality, and inflorescence type and juice quality were not directly associated. Rather, juice SSC was associated with the effect of inflorescence type on fruit size, as small fruit tended to have higher SSC than large fruit, regardless of the type of inflorescence on which fruit were borne. The relatively small difference in SSC between fruit borne on leafy and leafless inflorescences (≈3% of mean SSC) was an indirect result of fruit size. Therefore, fruit borne on leafy inflorescences, which tend to be of larger size compared with fruit borne on leafless inflorescences, tended to have marginally lower SSC. Acid content and ratio of SSC: TA were not related to inflorescence type. In addition, the type of inflorescence on which fruit were borne made only a nominal contribution to variability in juice SSC, in contrast to the major contribution of canopy position to within-tree variation in juice SSC. Factors other than inflorescence type are important components of within-tree variation in juice SSC.

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Jeffrey G. Williamson, Karen E. Koch, and William S. Castle

Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] seedlings were budded with `Hamlin' orange [Citrus sinensis (L.) Osb.] and subjected to 3 bud forcing treatments: (1) topping [T] by removing the seedling above the bud union; (2) lopping [L] by cutting half way through the seedling above the bud union and breaking the rootstock over; or, (3) bending [B] the seedling top over and tying it to the base of the plant. As scion buds emerged and grew, plants were sacrificed for dry weight measurements; also, the portion of the rootstock seedling above the bud union was exposed to 14CO2 at 3 stages of scion development. Plants with seedling tops attached (B,L) gained more dry weight and fibrous roots than T seedlings. Scion elongation was greater for B plants than for T plants. Plants usually flushed twice regardless of bud forcing treatment. No treatment differences were noted for time of flushing or scion bud emergence. Labeled photosynthate from attached rootstock leaves was translocated to scions during both flushes suggesting that recently fixed C enhanced scion growth for B and L plants.

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William S. Castle, James C. Baldwin, and Ronald P. Muraro

‘Hamlin’ is a principal sweet orange grown in Florida for processing. It is productive but produces juice with low soluble solids content and poor color. A long-term trial was conducted in central Florida to determine rootstock effects on yield and juice quality and the effect of economic analysis on the interpretation of the horticultural results. The trees were a commonly used commercial selection of ‘Hamlin’ sweet orange [Citrus sinensis (L.) Osb.] propagated on 19 rootstocks planted in a randomized complete block design of three-tree plots with six replicates in a Spodosol soil at a density of 350 trees/ha. Routine horticultural data were collected from the original trial (H1) for 10 years. Trees on some rootstocks that grew and yielded poorly were removed within a few years and replaced with a second trial (H2) with 13 rootstocks from which data were collected for 5 years. The H1 data were financially analyzed to compare the relative usefulness of horticultural and economic data in interpreting results and making rootstock decisions. In H1 after 10 years, tree height ranged from greater than 5 m [Volkamer lemon (C. volkameriana Ten. & Pasq.)] and Cleopatra mandarin (C. reshni Hort. ex Tan.) to 2.4 m {Flying Dragon trifoliate orange [Poncirus trifoliata (L.) Raf. ]}. In H2, the trees on somatic hybrid rootstocks were ≈2 m tall after 8 years and 4.4 m among those on mandarins and C-32 citrange (C. sinensis × P. trifoliata). Tree losses from citrus blight were generally low except for the trees on Carrizo and Troyer citranges (greater than 50%). Horticulturally, the highest performing trees in H1, measured by cumulative yield and soluble solids production over 10 years, were those on Carrizo, Troyer, and Benton citranges; poor performers were those on Smooth Flat Seville and Kinkoji (putative sour orange hybrids). Fruit yield and soluble solids production were directly related to tree height regardless of the difference among rootstocks in juice quality. The same relationship existed among the trees in H2 in which the best rootstocks were C-32 and Morton citranges. Trees on Swingle citrumelo (C. paradisi Macf. × P. trifoliata) ranked no. 12 of 19 rootstocks and 9 of 13 rootstocks in H1 and H2, respectively. Financial interpretation of the outcomes to include tree replacement resulting from blight losses did not substantially change the horticultural interpretations. Additional financial analyses demonstrated that the performance of trees on rootstocks with relatively low productivity/tree, like those on C-35 citrange and Kinkoji, would equal those on more vigorous rootstocks when tree vigor was properly matched with spacing. Yield determined the economic outcomes and financial analysis aided the interpretation of rootstock horticultural effects but did not greatly alter the relationship among rootstock results. Highly significant correlations between annual and cumulative data indicated that relative rootstock performance among ‘Hamlin’ orange trees in Florida could be reliably determined based on the first 4 cropping years.

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Xiuli Shen, William S. Castle, and Frederick G. Gmitter Jr

This study examined the effects of plant growth regulators, explant types, and their orientations on in vitro shoot proliferation of Casuarina cunninghamiana Miq. and also the subsequent rooting ability of shoots. Results showed that shoot proliferation occurred only in shoot tip explants cultured vertically on Murashige and Skoog (MS) medium supplemented with 2 or 4 μM thidiazuron (TDZ). Neither 6-benzylaminopurine alone nor in a combination with 1-naphthalene acetic acid (NAA) or gibberellic acid had any effect on shoot proliferation. TDZ at 4 μM resulted in the greatest percentage of axillary bud sprouting (70%) and mean number of sprouts per explant (2.3). Additionally, no shoot proliferation was observed from detipped or single-node explants or from horizontally placed shoot tip explants when cultured on the same TDZ-containing medium. The induced shoots produced adventitious roots on MS medium supplemented with 2.5, 5, or 10 μM indole-3-butyric acid (IBA), not with indole-3-acetic acid and NAA. Although the mean number of roots per explant was not significantly different between 2.5 and 5 μM IBA, the highest rooting percentage (68%) and mean length of roots per explant (0.7 cm) was achieved at 5 μM IBA. The current study provided preliminary information toward commercial in vitro propagation of Casuarina cunninghamiana male plants.

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Larry R. Parsons, T. Adair Wheaton, and William S. Castle

Citrus trees in an experimental planting responded well to high application rates of reclaimed water. Irrigation treatments included annual applications of 400 mm of well water and 400, 1250, and 2500 mm of reclaimed water. The effects of these irrigation treatments on two citrus cultivars (`Hamlin' orange and `Orlando' tangelo) combined with four rootstocks were compared. Growth and fruit production were better at the higher irrigation rates. The concentration of soluble solids in juice was diluted at the highest irrigation rate, but total soluble solids per hectare increased due to the greater fruit production. Average soluble solids/ha production was >15% higher at the 2500-mm rate than the 400-mm reclaimed water rate. While fruit soluble solids were usually lowered by higher irrigation, the reduction in fruit soluble solids observed on three of the rootstocks did not occur in trees on Carrizo citrange. Trees on Cleopatra mandarin grew similarly at the different irrigation rates, but canopy volume of trees on Swingle citrumelo was significantly smaller at the 400 mm rate than at the 2500 mm rate. Fruit peel color score was lower but juice color score was higher at the highest irrigation rate. Weed pressure increased with increasing irrigation rate, but was controllable. Both juice and fruit soluble solids were higher on Swingle citrumelo and lower on Cleopatra mandarin rootstock. Total soluble solids/ha, solids/acid ratio, and juice color were higher on Swingle rootstock. Reclaimed water, once believed to be a disposal problem in Florida, can be an acceptable source of irrigation water for citrus on well drained soils at rates up to twice the annual rainfall.