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

Conversion of wastewater to reclaimed water for crop irrigation conserves water and is an effective way to handle a growing urban problem: the disposal of wastewater. Water Conserv II is a large reclaimed water project developed by Orlando and Orange County, Fla., that presently irrigates ≈1900 ha of citrus. The project includes a research component to evaluate the response of citrus to irrigation using reclaimed water. Citrus trees in an experimental planting responded well to very 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 2500-mm rate is excessive, and since disposal was of interest, this rate was used to determine if citrus could tolerate such high rates of irrigation. The effects of these treatments were compared on `Hamlin' orange [Citrus sinensis (L.) Osb.] and `Orlando' tangelo (C. paradisi Macf. × C. reticulata Blanco) combined with four rootstocks: Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.], Cleopatra mandarin (C. reticulata Blanco), sour orange (C. aurantium L.), and Swingle citrumelo (C. paradisi × P. trifoliata). Growth and fruit production were greatest at the highest irrigation rate. Concentration of soluble solids in the juice was usually lowered by the highest irrigation rate, but total soluble solids per hectare were 15.5% higher compared to the 400-mm rate, due to the greater fruit production. 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. Fruit peel color score was lower but juice color score was higher at the highest irrigation rate. Crop efficiency (fruit production per unit of canopy volume) was usually lower at the 2500-mm rate and declined as trees grew older. Weed cover increased with increasing irrigation rate, but was controllable. Irrigation with high rates of reclaimed water provided a satisfactory disposal method for treated effluent, benefited growth and production of citrus, and eliminated the need for other sources of irrigation water. Reclaimed water, once believed to be a disposal problem in Florida, is now considered to be one way to meet irrigation demands.

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Eric H.C. Chilembwe, William S. Castle, and Daniel J. Cantliffe

Commercially processed citrus seeds of Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L) Raf.], Swingle citrumelo (C. paradisi Macf. × P. trifoliata), Cleopatra mandarin (C. reticulate Blanco), and sour orange (C. aurantium L.) were used to test the effects of grading, hydrating, and priming on the rate of germination and seedling emergence. Sorting seed into groups by fresh weight or diameter did not generally improve seed performance. Seed fresh weight was highly correlated with maximum seed diameter; also, large seed weight and size were associated with a larger number of embryos. When seedlings from the extra embryos were removed, large seed produced the largest seedlings. Soaking seeds in aerated water significantly increased germination and emergence rates over unsoaked seeds. Soaking at 35C rather than 25C enhanced these differences. Priming seeds in one of three solutions of polyethylene glycol 6000 (—0.6 to—1.2 Mpa) was not successful> as germination and emergence Per centages were lower than in distilled water.

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J.G. Williamson, W.S. Castle, and K.E. Koch

Three commercially employed treatments to force scion bud growth were compared using greenhouse-grown `Carrizo' citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] seedlings budded with `Hamlin' orange [Citrus sinensis (L.) Osb.]. Scion buds were forced either by 1) cutting off (removing the rootstock shoot above the bud union); 2) lopping (cutting half-way through the rootstock shoot above the bud union and breaking over the rootstock stem); or 3) bending (bending the rootstock shoot to its base and tying it in position). Plants were harvested, dried, and weighed at various times after scion shoot emergence. Plants on which rootstock shoots remained attached (lopping or bending) had the largest scion leaf area and gained the most scion and whole-plant dry weight. Bending rootstock shoots also resulted in a longer scion, more leaves, and higher root dry weight than did removal of rootstock shoots by cutting off. Few differences in overall growth were observed among trees retaining their rootstock shoots after two scion growth flushes. Removal of rootstock shoots after the first scion growth flush reduced leaf area and dry weight gain of the second scion growth flush; however, it did not affect total scion leaf area or dry weight. Analysis of 14C-photosynthate transfer from labeled rootstock leaves showed that bending allowed the greatest movement of labeled assimilates to other plant parts during the 24 hours after 14CO2 fixation. Radiolabeled photosynthates from rootstock leaves were partitioned primarily to shoots during scion growth flushes and to roots during periods between growth flushes.

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Luiz A.B.C. Vasconcellos and William S. Castle

Wood samples were taken from healthy and blighted citrus trees on several rootstocks to describe and compare the xylem anatomy of the healthy trees and to determine if blight altered xylogenesis. Horizontal trunk xylem cores, 6 cm long, were extracted from blighted 18-year-old commercial grapefruit (Citrus paradisi Macf.) trees on rough lemon (RL) (C. jambhiri Lush.), Cleopatra mandarin (CM) (C. reshni Hort. ex Tan.), and Carrizo citrange (CC) [C. sinensis (L). Osb. × Poncirus trifoliata (L.) Raf] and from healthy trees on those rootstocks and sour orange (SO) (C. aurantium L.). Cores were taken from the eastern and western sides of the scion and rootstock of each tree. The cores were divided into 2-cm pieces and cross-sections were prepared for analysis of vessel element (VE) number and diameter in 0.5-cm increments. A sample-size study showed that tree side was not a significant source of variation and that 10 replications were sufficient to detect differences of ≈12% from the overall mean. Among the healthy trees, VE densities and diameters were similar for the trees on CC or RL and larger than those for trees on SO or CM. VEs were generally smaller and at lower densities in the scion than the rootstock. Few VE occlusions were observed in the healthy trees. In the blighted trees, to a depth of 1 cm, VE density increased and diameter decreased compared to the healthy trees. The largest change occurred in the trees on RL and in the rootstock vs. scion trunk part. The frequency of VE amorphous plugs in blighted trees ranged from 1% to 30%. Similar changes in xylem anatomy were not found in trees with citrus tristeza virus or soilborne pests. Trunk water uptake and dye movement patterns in blighted trees were typical for trees with xylem dysfunction.