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Smit le Roux and Graham H. Barry

As part of a larger study to improve rind color of citrus (Citrus spp.) fruit, this initial study was conducted to determine the concentration of various gibberellin-biosynthesis inhibitors required to elicit a biological response in citrus trees, as measured by vegetative growth. Paclobutrazol and GA3 were included as control treatments at concentrations known to elicit growth-retarding or growth-promoting effects, respectively. Repeated (×4) foliar applications of GA3 (at 64 ppm) increased growth of ‘Eureka’ lemon (Citrus limon) shoots by 63%, with no significant effect on rootstock and scion diameters. Repeated foliar applications of prohexadione-calcium (ProCa) at various concentrations (100, 200, 400, or 800 ppm) as well as uniconazole (at 500 or 1000 ppm) and paclobutrazol (at 0.25%) had no effect on rootstock or scion diameters 8 months after the first application. The high concentrations of ProCa (800 ppm) and uniconazole (1000 ppm), and the paclobutrazol treatment (0.25%) reduced shoot length compared with the control. Uniconazole at 1000 ppm resulted in the most growth retardation, which resulted in 34% shorter shoot length than the control. Although the number of nodes on the longest shoot did not differ from the untreated control, internode length differed significantly among treatments. ProCa at 400 and 800 ppm, uniconazole at 1000 ppm, and paclobutrazol at 0.25% significantly reduced internode length relative to the control by 31%, 56%, 50%, and 28%, respectively. Vegetative growth of ‘Eureka’ lemon nursery trees was retarded following the repeated (×4) foliar application of gibberellin-biosynthesis inhibitors. ProCa at 400 to 800 ppm and uniconazole at 1000 ppm were identified as prospective treatments for further field studies to test their effects on rind color enhancement of citrus fruit.

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Graham H. Barry and Smit le Roux

Rind color is an important cosmetic preference of consumers when purchasing citrus fruit. As citrus fruit mature, a decrease in chlorophyll concentration unmasks the presence of carotenoid pigments followed by further synthesis of carotenoids, resulting in the first appearance of the characteristic orange color of mandarins and sweet oranges. Factors contributing to invigorating growing conditions are antagonistic to optimal rind color development and tree vegetative vigor as well as high gibberellin and cytokinin levels are also thought to adversely affect rind color. Thus, a method to increase preharvest rind color by moderating vegetative vigor using a growth retardant was investigated. Prohexadione–calcium (ProCa; Regalis®), a gibberellin-biosynthesis inhibitor with growth retardant activity, was applied to ‘Nules Clementine’ mandarin (Citrus reticulata Blanco), ‘Navelina Navel’ orange [C. sinensis (L.) Osbeck], and ‘Eureka’ lemon [C. limon (L.) Burm. f.] during the 2005 and 2006 seasons at 200 and 400 mg·L−1 a.i. Rind color rating, colorimeter measurements, and pigment analyses were conducted directly after harvest, after ethylene degreening, and 3 weeks after cold storage. In the 2005 season, ProCa significantly increased rind color of ‘Nules Clementine’ mandarin and ‘Navelina Navel’ orange directly after harvest and after ethylene degreening by decreasing chlorophyll and increasing carotenoid concentrations in the flavedo of fruit but did not affect the pigment concentration of ‘Eureka’ lemon despite an improvement in rind color rating. After cold storage, however, rind color was not significantly different among treatments. In the 2006 season, rind color was significantly increased directly after harvest, and chlorophyll degradation together with carotenoid synthesis of all Citrus spp. tested were stimulated by the late 400 mg·L−1 ProCa application. Therefore, foliar spray application of ProCa at a concentration of 400 mg·L−1 applied 6 plus 3 weeks before anticipated harvest has the potential to increase preharvest rind color of early-maturing citrus cultivars as a result of increased carotenoid-to-chlorophyll ratio. This treatment provides a novel approach to manipulate chlorophyll degradation and carotenoid synthesis in citrus fruit, and these results support the hypothesis that there may be an inverse relationship between vegetative vigor and rind color development of citrus fruit. Therefore, by moderating vegetative vigor through the use of growth retardants, rind color of citrus fruit can be enhanced.

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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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Paul J.R. Cronje, Graham H. Barry, and Marius Huysamer

Because photosynthesis provides the required carbohydrates for fruit development and respiration releases the stored energy from these carbon compounds, interalia during postharvest storage, it is therefore important that fruit tissues have an adequate carbohydrate concentration at the start of the postharvest period to ensure optimal storage life. In addition to photosynthate supply from leaves, the chlorophyll-containing flavedo of citrus (Citrus sp.) fruit (outer, colored part of the rind) has the ability to fix CO2 through its own photosynthetic system. In this experiment, spanning three seasons, the three main sugars (sucrose, glucose, and fructose) were quantified in the flavedo of ‘Nules Clementine’ mandarin (Citrus reticulata) fruit during Stages II and III of fruit development. Flavedo was sampled from fruit borne on the inside (low light intensity) or outside (high light intensity) of the tree’s canopy. In one season, the photosynthetic and respiration rates of fruit borne in the two canopy positions were measured pre- and post-color break (March and April, respectively). Sucrose concentration increased constantly from initial sampling in February until harvest (May), whereas glucose and fructose concentrations increased significantly only during the last month of fruit development. The flavedo of inside fruit, developing under low-light conditions, was less well colored (higher hue angle) and had a lower sugar concentration compared with outside fruit developing under conditions of high light levels. This response could be attributed to the higher pigment concentration leading to a higher photosynthetic rate as well as greater sink strength of the outside fruit. The inside fruit had an increased susceptibility to the progressive postharvest physiological disorder, rind breakdown. The lower carbohydrate and pigment concentrations of the rind from fruit borne inside the canopy compared with those from the outside of the canopy could be indicative of a weaker rind condition at the time of harvest.

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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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Ockert P.J. Stander, Graham H. Barry, and Paul J.R. Cronjé

The objectives of this study were to improve the understanding of the mechanism of alternate bearing and the role of carbohydrates in ‘Nadorcott’ mandarin (Citrus reticulata) trees. Selected phenological responses were measured in natural heavy- (“on”) and low-fruiting (“off”) ‘Nadorcott’ mandarin trees grown under commercial South African production conditions. The relationships with seasonal leaf and root carbohydrate concentrations were evaluated at the shoot-, branch- and tree level over two seasons. Fruit load [R 2 = (−)0.80 and R 2 = (−)0.73 in seasons 1 and 2, respectively; (P < 0.01)] and the number of newly developed vegetative shoots [R 2 = 0.81 and R 2 = 0.78 in seasons 1 and 2, respectively; (P < 0.01)] were the most important determinants of return bloom. Sprouting of a higher number of new vegetative shoots from “off” trees compared with “on” trees (“off” = 863 and 1439 vs. “on” = 306 and 766) was not related to leaf carbohydrate concentration. Root sugar concentration peaked during full bloom and higher root growth activity was observed before a higher number of new vegetative shoots developing in “off” trees during summer. The root sugar concentration early in the season was ≈3-fold lower, and root and shoot growth were absent, or lower in “on” trees compared with “off” trees. These results concur with previous research and confirm that fruit load in “on” trees inhibits summer vegetative shoot development, which manifests in poor flowering and an “off” year. This study shows that fruit are the major carbohydrate sink and probably disturb the balance between vegetative shoot development and root growth by limiting carbohydrate allocation to roots.

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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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Ockert P.J. Stander, Graham H. Barry, and Paul J.R. Cronjé

The significance of macronutrients nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in leaves was studied in relation with their possible roles in alternate bearing of ‘Nadorcott’ mandarin (Citrus reticulata) trees over a period of three seasons. Fruit load (“on,” a heavy fruit load, vs. “off,” a light fruit load) affected the leaf macronutrient concentrations, and the amount of macronutrients removed through the harvest of fruit, i.e., the crop removal factor (g·kg−1), was consistent in both seasons. The crop removal factors were higher for each macronutrient in “off” trees—harvest of 1 kg fruit removed ≈2.3 g·kg−1 N, 0.3 g·kg−1 P, 3.1 g·kg−1 K, 1.0 g·kg−1 Ca, and 0.4 g·kg−1 Mg, compared with 1.3 g·kg−1 N, 0.2 g·kg−1 P, 1.7 g·kg−1 K, 0.6 g·kg−1 Ca, and 0.2 g·kg−1 Mg in “on” trees. Fruit load per tree (kg/tree) of 84, 110, and 52 kg/tree in “on” trees, however, removed ≈217 g/tree N, 28 g/tree P, 296 g/tree K, 100 g/tree Ca, and 35 g/tree Mg, which was 1.5–6 times more than that of fruit loads of 14, 71, and 16 kg/tree in “off” trees. In “off” trees, N, P, and K, and in “on” trees, Ca accumulated in leaves to between 20% and 30% higher concentrations in season 1, but the higher macronutrient status did not manifest in or consistently correlate with intensity of summer vegetative shoot development in the current season, or intensity of flowering in the next season, the two main determinants of fruit load in ‘Nadorcott’ mandarin. Apart from some anomalies, the concentrations of macronutrients in leaves were unaffected by de-fruiting and foliar spray applications of N and K to “on” trees, and showed no consistent relationship with treatment effects on parameters of vegetative shoot development and flowering. Leaf macronutrients in alternate bearing ‘Nadorcott’ mandarin trees, fertilized according to grower standard practice, are not related to differences in flowering and vegetative shoot development, and appear to be a consequence of fruit load and not a determinant thereof.