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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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Catalina M. Anderson, William S. Castle, and Gloria A. Moore

Isozyme analysis was the basis for determining the frequency of occurrence and the characteristics of zygotic plants in Swingle citrumelo seedling populations from various sources of open-pollinated seeds, in a commercial nursery of Swingle citrumelo before and after roguing, and in commercial orchards and rootstock trials where this rootstock was used. Most zygotic seedlings identified by isozyme analysis could be distinguished by careful examination of morphological characteristics. Frequencies of zygotic seedlings varied among seedling populations, but were in the range (≈5% to 10%) found in previous studies. Roguing based primarily on size and growth habit of seedlings was effective in removing some, but not all, zygotic seedlings. Most of the remaining zygotic plants in the rogued population were found among the smaller seedlings. Trees budded on zygotic rootstock seedlings were found in two of the three groves studied, and in some instances an apparent incompatibility was developing in young trees.

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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.

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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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William S. Castle, James Nunnallee, and John A. Manthey

A broad range of plant selections across the orange subfamily Aurantioideae were screened in solution and soil culture for their tolerance to low iron (Fe) stress. Young seedlings grown in soil were transferred to tubs of +Fe nutrient solution, which was later replaced after a brief period with a –Fe solution. Over several trials, ≈20 white root tips were harvested periodically from the plants in each tub and assayed for their ability to reduce Fe3+. The procedure was miniaturized to determine if a fewer number of root tips could be assayed to screen individual plants and to estimate the required sample size. For solution screening, seven root tips were estimated to be adequate for representing a single plant. Seedlings of a few selections were also grown in small containers of soil amended with 0% to 5.9% CaCO3. The results in solution and soil culture were consistent with each other and with previous assessments of the various selections. Based on a summary of the solution and soil responses, the citrus selections were grouped in descending order of Fe3+ reduction rates as Volkamer lemon/Rangpur/sour orange selections/Citrus macrophylla > mandarins and mandarin hybrids > citranges > citrumelos > trifoliate orange. Of the citrus relatives tested in solution culture only, those in the genera Glycosmis, Citropsis, Clausena, and Murraya had high Fe reduction rates with good seedling growth and new leaves developed a light yellow color or showed no loss of greenness. Other citrus relatives in the genera Severinia, Atalantia, and Fortunella and most somatic hybrids had low seedling vigor and produced too few root tips to be properly assessed. The results are useful because of the breadth of selections screened, the identification of various citrus relatives as potential sources of low-Fe stress tolerance in breeding new rootstocks, and the apparent positive relationship between the Fe3+ reduction responses, soil screening responses, and field experiences with carbonate-induced Fe chlorosis responses.

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

Casuarina cunninghamiana Miq. is an introduced species to Florida that has potential as a windbreak plant to help manage canker in citrus groves; however, only Florida sources can be used for that purpose. Local sources of Casuarina are generally adequate seed producers, but germination percentages are frequently poor. Thus, the causes of low seed germination and methods to improve germination were investigated using C. cunninghamiana and a local hybrid (C. equisetifolia L. × C. glauca Sieb. ex Spreng.). Seeds of the hybrid were larger and heavier (88 mg/100 seeds) than those of C. cunninghamiana (mean wt. 67 mg/100 seeds). Shrunken, insect-damaged, and empty seeds, present in all unsorted seed lots, were responsible for poor seed germination of the four seed sources studied. Petroleum ether separation improved germination by dividing seeds into floaters and sinkers. The floater fraction consisted of 47.5% to 93% insect-damaged seeds compared with 9.0% to 43.5% among sinkers. More than 50% of the sinkers were filled seeds and less than 21% in floaters. No empty seeds were sinkers except for one source of C. cunninghamiana. In sorted hybrid seeds, petroleum ether separation eliminated a large proportion of ungerminable seeds (floaters) and seed germination among sinkers was faster with a higher germination percentage than floaters. Cumulative germination of hybrid seeds in a trial involving two temperatures was 23.0% for sunken seeds at 30 °C at the end of 8 weeks compared with 1% of unsorted seeds. Temperature had no significant effect on seed germination. The germination percentage of hybrid seeds with seedcoats removed was 91.0% in the first week of culture compared with only 1.2% in the first week and 12.6% seed germination at the end of 8 weeks' culture of intact seeds.

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Graham H. Barry, William S. Castle, Frederick S. Davies, and Ramon C. Littell

Variability in fruit quality of citrus occurs among and within trees due to an interaction of several factors, e.g., fruit position, leaf: fruit ratio, and fruit size. By determining variability in fruit quality among i) fruit, ii) trees, iii) orchards, and iv) geographic locations where citrus is produced in Florida, optimal sample size for fruit quality experiments can be estimated. To estimate within-tree variability, five trees were randomly selected from each of three `Valencia' orange orchards in four geographic locations in Florida. Six fruit were harvested from each of two tree canopy positions, southwest top and northeast bottom; fruit were not selected or graded according to fruit size. °Brix and titratable acidity of juice samples were determined, and the °Brix: acid ratio was calculated. Statistical analysis of fruit quality variables was done using a crossed-nested design. The number of trees to sample and the number of fruit per sample were calculated. To estimate between-tree variability, 10 trees were randomly selected from each of three `Valencia' orange orchards from four geographic locations in Florida. Fifty-fruit composite samples were picked from around the tree canopy (0.9 to 1.8 m). Juice content, SSC, acid content, and ratio were determined. Using a nested design, the number of orchards and number of trees to sample were determined. There was greater variability in fruit quality among trees than within trees for a given canopy position; the optimal sample size when taking individual fruit samples from a given location and canopy position is four fruit from 20 trees. There was less variability in fruit quality when 50-fruit composite samples were used, resulting in an optimal sample size of five samples from three orchards within each location.

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Graham H. Barry, William S. Castle, Frederick S. Davies, and Ramon C. Littell

Sources of variation in juice quality of `Valencia'sweet orange [Citrus sinensis(L.) Osb.] were quantified and their relative contributions to variability in juice quality were determined, from which sample sizes were estimated. Commercial orchards of `Valencia' sweet orange trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] rootstock were selected at four geographic locations representing the major citrus-producing regions in Florida. Within- and between-tree variation in soluble solids concentration (SSC) and titratable acidity (TA) were estimated in two experiments over two or three seasons, respectively. Variance components for all treatment effects were estimated to partition total variation into all possible component sources of variation. Seasonal variation in SSC and TA was relatively small, but larger for TA than SSC. Variation in SSC among blocks within a location was intermediate to low, and was less than variation among locations. In contrast, tree-to-tree variation in SSC and TA was large, in spite of sampling from trees of similar vigor and crop load, and variation in SSC and TA among fruit was relatively large. Based on results of this study, samples consisting of 35 fruit are required to detect differences (P ≤ 0.05) of 0.3% SSC and 0.06% TA, whereas 20-fruit samples can be used to detect differences of 0.4% SSC and 0.08% TA. Seven replications are required to detect differences of 0.5% SSC and 0.1% TA, with small gains in precision when tree numbers exceed 10.

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Laura Guazzelli, Frederick S. Davies, James J. Ferguson, and William S. Castle

Two experiments were conducted with container-grown `Hamlin' orange trees [Citrus sinensis (L.) Osb.] on `Swingle' citrumelo [C. paradisi Macf. × Poncirus trifoliata (L.) Raf.] rootstock to study the effects of N rate on plant growth in the nursery. Treatments consisted of 12, 50, 100, or 200 mg N/liter per tree applied once a week by drip irrigation. Commercial media was used and soil water content was maintained at container capacity. In Expt. 1, fertilization at 200 mg·liter−1 resulted in greater scion growth, trunk diameter, and total leaf dry weight compared to the other rates. In Expt. 2, fertilization at 100 and 200 mg·liter−1 resulted in greater scion growth,” trunk diameter, and leaf and stem dry weights compared to lower rates, but no differences were observed between the two highest rates. Trees that received 12 and 50 mg·liter−1 were stunted and leaves were chlorotic. Therefore, the optimum calculated N rate for `Hamlin' nursery trees on `Swingle' citrumelo rootstock, based on critical level analysis, is 155 to 165 mg·liter-1.

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

Water Conserv II is a municipal reclaimed water project operated by the city of Orlando and Orange county, FL. The Water Conserv II project has been supplying high-quality reclaimed water for irrigation of citrus orchards, nurseries, greenhouse operations, golf courses, and residential landscapes in Orange and Lake counties since 1986. Selected commercial citrus orchards in the Water Conserv II service area receiving either groundwater or reclaimed water have been monitored quarterly since the project began. This yearly monitoring was undertaken to determine any adverse long-term effects on citrus tree growth or production associated with irrigation using this reclaimed water. Citrus blocks were rated for horticultural condition quarterly, fruit quality was determined before harvest, and soil and leaf samples were analyzed yearly from 1994 to 2004. Citrus growers irrigating with reclaimed water were encouraged to use higher-than-recommended amounts of water as a means of disposal of this reclaimed water resulting in increased weed growth and dilution of juice solids per box of fruit. Leaf boron and magnesium were significantly higher after irrigation with reclaimed water. Calcium and boron from the reclaimed water have eliminated the need in orchards receiving reclaimed water for liming of the soil and applying annual foliar sprays containing boron.