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J.G. Williamson and B.E. Maust

Two experiments were conducted to determine the effects of rootstock and bud-forcing treatment on scion budbreak and nursery tree growth of `Hamlin' orange. In Expt. 1, `Carrizo' citrange, `Swingle' citrumelo, and `Cleopatra' mandarin were budded with `Hamlin' orange and forced by one of the following methods: cutting off (purning away the rootstock top about 2 cm above the inserted scion bud); lopping (cutting half to two-thirds of the way through the rootstock stem 2 cm above the bud union, and breaking over the stem but leaving it attached); or bending (bending the rootstock shoot above the inserted scion bud and tying it to the base of the plant). For `Swingle' only, percent budbreak was less for bending or lopping compared to cutting off. For `Carrizo' and `Swingle', scion dry weights were less when plants were forced by cutting off compared to bending or lopping. For all rootstocks, whole-plant dry weights were greater for plants forced by bending and lopping than for plants forced by cutting off. In Expt. 2, scion buds on `Swingle' and `Cleopatra' plants were forced by the three methods in Expt. 1 plus combinations of bending with notching (making an inverted V incision through the bark and into the wood on the rootstock stem directly above the scion bud) and/or topping (removing the teminal 2 cm of rootstock shoot tips of plants forced by bending). Percent scion budbreak was high for `Cleopatra' plants regardless of forcing treatment. For `Swingle', scion budbreak was greater when bending was combined with notching than for bending alone. For `Cleopatra', plant dry weight was greatest for plants forced by lopping. When bending was combined with notching, or notching with topping, `Swingle' scion budbreak was comparable to cutting off, but plant dry weights were greater with these combination treatments than when cutting off was used.

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J.G. Williamson and B.E. Maust

Two experiments were conducted to determine the effects of bud forcing method and rootstock on scion budbreak and nursery tree growth of `Han-din' orange (Citrus sinensis Osb.) In Expt. 1, Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] (Ca), Swingle citrumelo [C. paradisi (L.) Osb. × P. trifoliata (L.) Raf.] (Sw), and Cleopatra mandarin (C. reticulata Blanco) (Cl) were budded with `Hamlin' orange and forced by cutting off the rootstock tops, lopping (cutting half way through the rootstock stem above the scion bud), or bending the rootstock top and tying it to the trunk). For Cl and Ca percent budbreak was high for all forcing methods. For Sw percent budbreak was greater for cutting off than for lopping or bending. For Sw and Ca, bending or lopping resulted in greater whole plant and scion dry weights than cutting off. Expt. 2 was similar to Expt. 1 except that bending was used alone, or in combination with notching (cutting an invertal v-shaped notch above the scion bud), or with topping the bent rootstock shoot. Scion budbreak of Sw plants was greater for bending + notching than for bending alone. Other effects of rootstock and forcing method for cutting off, bending or lopping were similar to those found in Expt. 1.

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B.E. Maust and J.G. Williamson

Experiments were conducted with `Hamlin' orange [Citrus sinensis (L.) Osb.] budded on Cleopatra mandarin (Citrus reticulata Blanco) or Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] seedling rootstocks to determine minimum container solution N concentrations required for optimum growth and fertilizer uptake efficiency at various growth stages. Plants were fertigated daily with 1 liter of N solution at either 0, 12.5, 25, 50, 100, or 200 mg·liter-1 from NH4NO3 or 0, 3.13, 6.25, 12.5, 25, or 50 mg·liter-1 from NH4NO3 dissolved in a complete nutrient solution, respectively. Percentage of N in the mature plant tissues increased as N concentration in the medium solution increased. Shoot length and leaf area increased as N concentrations increased up to a critical concentration of 15 to 19 mg·liter-1. The critical N concentration for root, shoot, and total plant dry weight was ≈18 mg·liter-1 for `Hamlin'-Cleopatra mandarin nursery plants and 15 mg·liter-1 for `Hamlin'-Carrizo nursery plants. The critical N concentration for relative total plant dry weight accumulation (percentage) for the two experiments was 16.8 mg·liter-1. In a separate experiment, plants were given labeled fertilizer N (FN) (15NH4 15NO3) at one of five growth stages: A) in the middle of rapid shoot extension of the third flush, B) immediately following the cessation of the third flush shoot extension but during leaf expansion, C) immediately following leaf expansion, D) before the fourth flush, or E) in the middle of rapid shoot extension of the fourth flush. Labeled FN recovery increased during rapid shoot extension of the fourth scion flush compared to the other labeling periods. FN uptake per gram of total plant dry weight was greatest during rapid shoot extension (A and E) and lowest during the intermediate labeling periods (B-D). FN supplied 21% to 22% of the N required for new growth during rapid shoot extension.

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J.G. Williamson and B.E. Maust

Two experiments were conducted to determine the effects of rootstock shoot defoliation or shading on growth during the first two scion flushes of budded, containerized, citrus nursery trees. Both experiments were conducted in a greenhouse with Cleopatra mandarin (Citrus reticulata Blanco) and Swingle citrumelo [C. paradisi (L.) Osb. × Poncirus trifoliata (L.) Raf.] seedlings budded with `Hamlin' orange [C. sinensis (L.) Osb.]. Scion buds were forced by cutting off the rootstock shoots above the bud union or by bending the rootstock shoots and tying them to the base of the plants (bending). In one experiment, shoots from both rootstocks that were forced by bending received one of four defoliation treatments: treatments were 0%, 40%, 60%, or 85% (based on leaf area) defoliation for Cleopatra and 0%, 30%, 60%, or 100% defoliation for Swingle. In the second experiment, rootstock shoots of plants forced by bending were subjected to a maximum photosynthetic photon flux (PPF) of 820, 225, 90, or 30 μmol·m–2·s–1. Growth of Cleopatra plants declined linearly with increasing levels of rootstock shoot defoliation. When rootstock shoot defoliation was ≤40%, whole-plant (minus rootstock shoot) dry weights were higher for plants forced by bending than for those forced by cutting off rootstock shoots. For Swingle, most growth responses to rootstock shoot defoliation were curvilinear. Higher levels of rootstock shoot defoliation resulted in less growth than lower defoliation levels. Low PPF reduced whole-plant (minus rootstock shoot) and root dry weights for both rootstocks compared to high PPF. For Cleopatra, whole-plant and scion dry weights were highest at the highest PPF. For Swingle, whole-plant and scion dry weights were highest at 225 μmol·m–2·s–1. For both rootstocks, plants forced by bending had higher dry weights at 820 and 225 μmol·m–2·s–1 than plants forced by cutting off the rootstock shoots. For Swingle, most of the reduction in scion growth from rootstock shoot defoliation occurred during the first scion flush. Our results suggest that recently produced rootstock shoot photosynthates are the primary source of greater plant growth achieved by bending compared to cutting off rootstock shoots.

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B.E. Maust, J.G. Williamson and R.L. Darnell

Floral budbreak and fruit set in many southern highbush blueberry (SHB) cultivars (hybrids of Vaccinium corymbosum L. with other species of Vaccinium) begin prior to vegetative budbreak. Experiments were conducted with two SHB cultivars, `Misty' and `Sharpblue', to test the hypothesis that initial flower bud density (flower buds/m cane length) affects vegetative budbreak and shoot development, which in turn affect fruit development. Flower bud density of field-grown plants was adjusted in two nonconsecutive years by removing none, one-third, or two-thirds of the flower buds during dormancy. Vegetative budbreak, new shoot dry weight, leaf area, and leaf area: fruit ratios decreased with increasing flower bud density in both cultivars. Average fruit fresh weight and fruit soluble solids decreased in both cultivars, and fruit ripening was delayed in `Misty' as leaf area: fruit ratios decreased. This study indicates that because of the inverse relationship between flower bud density and canopy establishment, decreasing the density of flower buds in SHB will increase fruit size and quality and hasten ripening.

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B.E. Maust, J.G. Williamson and R.L. Darnell

A field experiment was conducted in Gainesville, Fla., with two southern highbush blueberry cultivars, `Misty' and `Sharpblue', to investigate the influence of varying flower bud load on the timing and extent of vegetative and reproductive development. Flower bud load was adjusted on three different canes on ten plants by removing none, one-third, or two-thirds of the flower buds. Vegetative budbreak, leaf area, fruit number, and fruit fresh weight and dry weight were measured. Vegetative budbreak was delayed with increasing flower bud load. Vegetative budbreak, leaf area, and leaf area: fruit ratio decreased with increasing flower bud load. Fruit maturity was delayed and average berry fresh weight and dry weight declined with decreasing leaf area:fruit ratio. Responses were similar for both cultivars although `Misty' was more adversely affected by high flower bud load and low leaf area: fruit ratio.

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B.E. Maust, J.G. Williamson and R.L. Darnell

Vegetative budbreak, leaf area development, and fruit size in southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrids) decrease as flower bud density increases. The effect on fruit size has been attributed to both insufficient carbohydrate reserves and reductions in current photoassimilates caused by decreased vegetative growth. Experiments were conducted with two southern highbush blueberry cultivars, `Misty' and `Sharpblue', to test the hypothesis that increased carbohydrate reserve concentrations can overcome the detrimental effects of high flower bud density by increasing vegetative budbreak, shoot development, and whole-canopy net CO2 exchange rate (NCER), which in turn will increase fruit size. Fully foliated plants were placed in greenhouses with either ambient (AMB) CO2 levels (≈360 μmol·mol-1) or enriched (ENR) CO2 levels (≈700 μmol·mol-1) for 38 d during fall. Plants were then moved outdoors, hand defoliated, and flower bud density (flower buds/cm cane length) adjusted to range from 0.07 to 0.31. Root starch and whole plant carbohydrate concentrations increased in ENR compared with AMB plants of both cultivars. Vegetative budbreak (number per centimeter cane length), leaf area, and whole-canopy NCER decreased as flower bud density increased in AMB and ENR plants of both cultivars; however, ENR `Sharpblue' plants had significantly greater vegetative growth and wholecanopy NCER at a given flower bud density compared with AMB `Sharpblue'. Concomitant with this was an increase in fruit fresh weight in ENR compared to AMB `Sharpblue'. This was not the case with `Misty', where vegetative development and fruit size were similar in ENR and AMB plants. Thus, the hypothesis that increased carbohydrate reserves will increase vegetative development and subsequent fruit size may be true only in certain cultivars of southern highbush blueberry. Alternatively, the increased carbohydrate reserve concentrations in ENR compared with AMB `Misty' plants may have been insufficient to affect subsequent vegetative or reproductive development.

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J.G. Williamson, B.E. Maust and D.S. NeSmith

The effects of hydrogen cyanamide (H2CN2) sprays on vegetative and reproductive bud growth and development were evaluated for `Climax' rabbiteye (Vaccinium ashei Reade) and `Misty' southern highbush blueberry (V. corymbosum L. hybrid). `Climax' plants were sprayed with 0% or 1% H2CN2 (v/v) at each of several time intervals or flower bud growth stages following either 270 or 600 hours of artificial chilling. `Misty' plants were sprayed with 0%, 1%, or 2% H2CN2 (v/v) immediately after exposure to 0, 150, or 300 hours of artificial chilling. H2CN2 application to `Climax' plants at 3 days after forcing (DAF) and at 10% to 30% stage 3 flower bud development dramatically accelerated leafing, and only minimal flower bud damage was observed at these application times. For `Misty', vegetative budbreak was increased and advanced by both H2CN2 spray concentrations, regardless of pretreatment chilling levels; the number of vegetative budbreaks per plant increased with increased concentration. Timing of anthesis did not appear to be affected by H2CN2, but fruit maturity was hastened. Increased pretreatment chilling also hastened fruit development. This effect on maturity appears to be due primarily to increased and accelerated vegetative budbreak, which probably increased leaf: fruit ratios. Greater flower bud mortality from H2CN2 occurred in nonchilled plants than in those chilled for 150 or 300 hours, especially at 2% H2CN2. These results indicate that H2CN2 has potential value in stimulating vegetative bud development, which potentially hastens maturity in blueberries grown under the mild winter conditions of the Southeast. However, spray concentration and timing of application will be critical to successful use of this compound.

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B.E. Maust, J.G. Williamson and R.L. Darnell

Two southern highbush blueberry cultivars, `Sharpblue' and `Misty', were used to investigate the influence of varying flower bud density and fruit load on vegetative development, whole-plant canopy CO2 exchange rate (CER), and leaf CER. Plants were grown in pots and flower buds were removed so that initial flower bud density (fl ower bud number/total cane length) on a whole-plant basis ranged from 0.05–0.35 flower buds/cm. Vegetative budbreak number, leaf area, and leaf area: fruit ratio decreased with increasing flower bud density. In `Sharpblue', whole-plant canopy CER measured at fruit ripening decreased with increasing flower and fruit load and decreasing leaf area:fruit ratio, while leaf CER increased with increasing fruit load and decreasing leaf area:fruit ratio. In `Misty', whole-plant canopy CER measured 4 weeks after full bloom decreased with increasing flower and fruit load, but whole-plant canopy and leaf CER at fruit ripening were similar among the different fruit loads. Average fruit fresh and dry weights increased and the fruit development period decreased with increased leaf area:fruit ratio in both cultivars. These data suggest that carbohydrate source limitations from reduced leaf area development and whole-plant canopy CER lead to decreased fruit fresh and dry weights and delayed ripening in some southern highbush blueberry cultivars.

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B.E. Maust, J.G. Williamson and R.L. Darnell

Vegetative budbreak and subsequent canopy development in some southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid) cultivars are delayed and/or reduced as flower bud density increases. This delay/reduction in vegetative growth has been correlated with decreased weight and soluble solids of individual fruit. In the present study, the effects of flower bud density (FBD) on vegetative budbreak and canopy development, starch reserves, and whole-canopy net CO2 exchange rate (NCER) were assessed to determine how FBD affected the source supply for fruit development. A range of flower bud densities was established in two cultivars of containerized southern highbush blueberry during dormancy. Vegetative budbreak was delayed and vegetative budbreak, leaf area, and leaf area to fruit ratios decreased as FBD increased. In general, increasing FBD increased the rate of root and cane starch depletion during the first four weeks after bloom. Whole-canopy NCER was similar across the range of FBD during early fruit development, but between four weeks after bloom and fruit ripening, NCER decreased as FBD increased. Although FBD explained only a small proportion of the variability observed in carbohydrate concentration and NCER, the data suggest that both the rapid depletion of starch reserves early in fruit development and the decrease in whole canopy NCER later in fruit development contribute to the detrimental effects of increased FBD on fruit development.