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  • Author or Editor: T.A. Wheaton x
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Four decades ago, irrigation in much of the southeastern U.S. was considered not sensible economically because of normal rainfall in excess of 1200 mm in some areas. More-recent research has shown that irrigation makes definite economic sense because it can increase production substantially. This is especially true in Florida citrus, where irrigation can increase yield by up to 60%. Drip and microsprinkler irrigation have become popular, and these methods of partial root-zone coverage affect tree water potential and yield. Growing environmental concerns about possible nitrate and pesticide leaching to the groundwater have led to greater emphasis on irrigation management in an area of highly variable rainfall. Rapidly growing population has brought about increased competition for water and greater restrictions on agricultural water use. Reclaimed water, once considered a disposal problem, is now being promoted as a partial solution for periodic water shortages. Discussion will focus on tree response to different irrigation management systems and how agriculture is dealing with greater irrigation restrictions.

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Abstract

Substantial improvement in the external color of harvested citrus fruit was achieved by holding at optimum temperature and ethylene concentration. Under specific conditions, marked improvement in orange and red pigmentation occurred. Initial color development was most rapid at 30°C but carotenoid accumulation ceased after a few days. Best color was obtained with fruit held at 15-25°C for longer periods of time. Optimal ethylene concentration decreased as temperature decreased. At the lowest temperature, high ethylene concentration inhibited color development.

Open Access

Abstract

The time required to degreen Florida ‘Bearss’ lemons can be greatly reduced by the use of 1 to 10 ppm ethylene at 25 or 30°C. Degreening was accomplished in 2 to 3 days instead of the 2 to 3 weeks required with the current commercial practice of cool coloring at 15° without ethylene. Applications of a benzimidazole fungicide (thiabendazole, benomyl) prior to degreening adequately controlled decay. Rapid degreening with ethylene at higher temperatures (25° or 30°) eliminates the need for cool coloring storage and brings lemon availability more nearly in phase with consumer demand.

Open Access

Growth and nitrogen (N) accumulation relationships based on tree size, rather than age, may provide more generic information that could be used to improve sweet orange [Citrus sinensis (L.) Osbeck] N management. The objectives of this study were to determine how orange trees accumulate and distribute biomass and N as they grow, investigate yearly biomass and N changes in mature orange trees, determine rootstock effect on biomass and N distribution, and to develop simple mathematical models describing these relationships. Eighteen orange trees with canopy volumes ranging between 2 and 43 m3 were dissected into leaf, twig, branch, and root components, and the dry weight and N concentration of each were measured. The N content of each tree part was calculated, and biomass and N distribution throughout each tree were determined. The total dry biomass of large (mature) trees averaged 94 kg and contained 0.79 kg N. Biomass allocation was 13% in leaves, 7% in twigs, 50% in branches/trunk, and 30% in roots. N allocation was 38% in leaves, 8% in twigs, 27% in branches/trunk, and 27% in roots. For the smallest tree, above-/below-ground distribution ratios for biomass and N were 60/40 and 75/25, respectively. All tree components accumulated biomass and N linearly as tree size increased, with the above-ground portion accumulating biomass about 2.5 times faster than the below-ground portion due mostly to branch growth. The growth models developed are currently being integrated in a decision support system for improving fertilizer use efficiency for orange trees, which will provide growers with a management tool to improve long-term N use efficiency in orange orchards.

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Improving our understanding of processes that control and limit nitrogen uptake by citrus can provide a scientific basis for enhancing nitrogen fertilizer use efficiency. Nitrogen uptake dynamics of two rootstock seedlings will be compared to those of young budded trees. Three-month old Swingle citrumelo [Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.] and Volkamer lemon (C. volkameriana Ten. & Pasq.) trees were planted in PVC columns filled with a Candler fine sand. Field experiments were conducted using 4-year-old `Hamlin' orange trees [Citrus sinensis (L.) Osb.] grafted on `Carrizo' [C. sinensis × Poncirus trifoliata (L.) Raf.] or on Swingle citrumelo. Trees were either grown in solution culture using 120-L PVC containers or in 900-L PVC tubs filled with a Candler fine sand. Additional trees were planted in the field during Spring 1998. Two lateral roots per tree were trained to grow in slanted, partly burried, 20-L PVC columns filled with a Candler fine sand. Nitrogen uptake from the soil was determined by comparing the residual N extracted by intensive leaching from planted units with that of non-planted (reference) units. With the application of dilute N solutions (7 mg N/L), plants reduced N concentrations to near-zero N concentrations within days. Applying N at higher concentrations (70 or 210 mg N/L) resulted in higher initial uptake rates, increased residual soil N levels, and reduced nitrogen uptake efficiency. Contributions of passive uptake to total nitrogen uptake ranged from less than 5% at soil solution concentrations around 3 ppm N to 20% to 30% at concentrations of 60 ppm N.

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The interactive effects of irrigation rate and nitrogen concentration of the irrigation water on the growth of seedlings of two citrus rootstocks were studied. Four-month old seedlings of Swingle citrumelo [Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.] and Volkamer lemon (C. volkameriana Ten. & Pasq.) were grown for ≈10 months in square citripots filled with a Candler fine sand. Plants were irrigated at 0.5, 0.75 or 1.0 times the evapotranspiration rate. Irrigation was applied using water containing 0, 7, 21, or 63 ppm nitrogen. Plant growth increased with irrigation rate and nitrogen concentration. Evapotranspiration rates, as determined from weight losses of reference plants, increased with nitrogen rate. Overall plant growth and weekly evaporation rates were greater with Volkamer than with Swingle. Leaf senescence of Swingle was more pronounced at low irrigation rates and/or low nitrogen concentrations than it was with Volkamer. Increasing nitrogen concentration of the irrigation water during the winter months reduced leaf senescence of both Swingle and Volkamer seedlings, and also promoted continuous growth in Volkamer. Leaf growth of Swingle ceased during the winter months, regardless of the nitrogen concentration of the irrigation water.

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Abstract

The ‘Murcott Honey Orange’ is grown rather extensively in Florida and is highly desired because of its fine flavor and dark orange colored flesh. Its origin is unknown, but the ‘Murcott’ is probably a hybrid of mandarin and sweet orange parentage. This variety bears very heavy crops with much of the fruit set in clusters. These extremely heavy crops result in a collapse of the trees near the time of fruit maturity. Depending on the severity of this decline, the trees may require from one to several years to recover (Figs. 1,2,3) or in extreme cases, die. The leaves and fruit from severely affected trees turn yellow and drop followed by dieback of the branches. These symptoms appear in December and January and become progressively worse as the fruit matures. Fruit on less severely affected trees are usually small and fail to develop the dark orange color characteristic of the variety. The problem of ‘Murcott’ collapse is probably the main reason this variety is not more widely grown. Knorr and Collins (1) previously described this condition and reported that in some cases, severe root deterioration occurs. The problem apparently is not limited to ‘Murcotts’ since similar symptoms have been reported in California for the ‘Wilking’ and ‘Kinnow’ mandarins (3).

Open Access

Abstract

The uptake, translocation, and metabolism of 14C-gibberellic acid (14C-GA3) was studied in 3-year-old container-grown ‘Marsh’ grapefruit trees (Citrus paradisi Macf.). A total of 1.65 × 105 disintegrations per min (dpm) in 200 μl of solution was applied evenly over the entire fruit surface, or, on both surfaces of 3 to 5 subtending leaves of a fruit. Absorption of 14C-GA3 by leaves and peel began within 1 hr of application and continued for 8 hr. Translocation of labeled material from leaves to peel and the reverse began 4 to 8 hr after application and continued for 4 weeks. No labeled material was recovered from juice or seeds. Labeled material persisted in albedo, flavedo, and leaves for 8 weeks with the highest accumulations in the peel. Separation of 14C-GA3 metabolites from the 95% EtOH extract by reversed-phase HPLC produced 2 14C-labeled peaks. Analysis of these 2 peaks by β-D-glucosidase hydrolysis, n-butanol partitioning, and cochromatography with 14C-GA3 standards suggested that the major component was 14C-GA3 and the other a polar metabolite.

Open Access

Abstract

Chilling injury (CI) of ‘Marsh Seedless’ grapefruit was reduced by a preharvest spray of benomyl (Benlate) but not of Vapor Gard (VG) pinolene antitranspirant Postharvest treatments with benomyl and thiabendazole (TBZ) also reduced CI. The effect of high CO2 atmospheres (up to 20%) induced under 0.0254 mm (1 mil) PVC film varied sharply with picking date. CI was almost eliminated in such atmospheres in early and midseason pickings but accentuated in the late (postbloom) picking. Modified atmospheres tended to increase decay regardless of picking date.

Open Access

`Hamlin' and `Valencia' oranges [Citrus sinensis (L.) Osb.], `Murcott' tangor (C. reticulata Blanco × C. sinensis), and `Redblush' grapefruit (C. paradisi Macf.) on 15 rootstock and own-rooted cuttings were planted at a 1.5 × 3.3-m spacing providing a density of 2020 trees/ha. Growth rate, productivity, and fruit quality varied among the scion and stock combinations. Combinations of moderate vigor and precocious fruiting performed better than very vigorous or dwarfing materials. Several freezes slowed canopy development and delayed production. Most trees had filled their allocated canopy space 7 years after planting. At that age, the orange trees yielded 23 to 75 t·ha-1. Scion and stock combinations with desirable vigor and fruiting characteristics were satisfactory in this high-density planting. However, there appears to be little advantage of high tree density under Florida conditions, and moderate densities of fewer than 1000 trees/ha may be preferable.

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