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L. G. Albrigo

In Florida, pounds soluble solids per box (% soluble solids × % juice × weight) can be 60% higher in some years compared to the lowest years. Pounds solids, soluble solids and juice content data were obtained for the different citrus growing districts in Florida for a 20-year period from the USDA and Florida Agricultural Statistics Service. Weather data for each district was obtained from US National Weather Service records. Total rainfall and average daily temperature were calculated for 2–month periods from prior to the normal bloom period until harvest. Juice data was regressed against weather data and the previous years pounds solids using a stepwise multiple regression program. R2 values for early oranges, `Marsh Seedless' grapefruit and `Valencia' were 0.48, 0.48 and 0.72, respectively. Prebloom and bloom rainfall and temperatures were frequently positively correlated, while summer rainfall often was a negatively correlated independent variable to final pounds solids. Additional data and physiological implications will be discussed.

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David G. Hall and L.G. Albrigo

Attention to the management of insects that feed on foliar flush growth has increased in Florida citrus as a result of the establishment of invasive plant diseases associated with insects that develop exclusively on flush. Citrus can be monitored to identify peak periods of flush abundance to time insecticide applications for these insects; however, guidelines for quantifying flush abundance are lacking. We therefore investigated sampling procedures for estimating flush abundance. A sampling method was devised to enable a quantitative estimation of flush shoots, defined as any shoot with immature leaves. A sample unit was the area within a 15 × 15 × 15-cm frame slipped into the outer edge of a tree with the end of a branch inside the frame. The number of flush shoots originating within the sample unit was counted. Three sample units were examined per tree in 45 randomly selected trees weekly during 2005 in each of two blocks of trees, one containing young ‘Marsh’ grapefruit (Citrus paradise Macf.) and one containing mature ‘Temple’ orange (tangors) [C. reticulate Blanco × C. sinensis (L.) Osbeck]. A pronounced abundance of flush was generally indicated by means of one or more flush shoots per sample in the particular trees studied. Variation in numbers of flush shoots per sample was similar within and among trees, differed significantly among sample dates, and did not differ significantly between the two blocks of trees overall sample dates. Taylor's power law coefficients indicated that, over all sample weeks, flush shoots were randomly distributed within the young grapefruit trees and only weakly aggregated within the block of mature oranges. Projections indicated that a sampling plan consisting of 40 trees (one sample per tree) would provide density estimates acceptable enough for general estimates at mean densities of one or more shoots per sample. An index of pest abundance based on mean pest density per flush shoot and mean density of flush shoots per sample is proposed.

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D.S. Achor and L.G. Albrigo

Permanent chlorosis of leaves on plants fertilized with urea containing high levels of the contaminant biuret has been observed in several crops including citrus. Little has been reported as to the cellular changes that result from such chlorosis. Branches from `Ruby Red' grapefruit (Citrus paradisi Macfadyn) and `Hamlin' orange [C. sinensis (L.) Osbeck] were sprayed with urea solutions containing 1.05% biuret. As visible symptoms developed, leaf tissue samples were prepared for transmission electron microscopy. For comparison purposes, leaves from similar trees showing chlorosis from age-related senescence and Zn deficiency were also sampled. The progressive development of chlorosis in biuret-affected leaves was characterized by: the loss of starch, thylakoidal and granal membranes in chloroplasts along with the enlargement and increase in number of plastoglobuli or lipid bodies. The lipid bodies were liberated alone or in association with membrane vesicles to the cytoplasm and vacuoles. The number and volume of the individual chloroplasts became smaller. Concurrent loss of cytoplasmic content and the enlargement of the vacuolar space were also observed in the biuret affected leaf tissue. Similar findings were observed in the cells of senescent leaves. In cells of leaves showing nutritional deficiency, losses in cytoplasmic content and vacuolar enlargement were observed but there was neither complete loss of thylakoidal or granal membranes nor the release of lipids from the plastids. It was concluded that 1) the cytological characteristics of the biuret-affected samples were more similar to age-related senescent samples than to chlorosis from Zn deficiency and 2) that complete loss of the lipid bodies from the chromoplasts to the cytoplasm and vacuole in the biuret-affected samples and in age-related senescence in citrus leaves was responsible for the permanent nature of the chlorosis.

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D.S. Achor, L.G. Albrigo, and C.W. McCoy

Upper surface leaf lesions on `Sunburst' mandarin [(Citrus reticulata Blanco × (C. paradisi Macf. × C. reticulate)] associated with feeding by the citrus rust mite [Phyllocoptruta oleivora (Ashm.)] are more severe than those on other citrus cultivars. Development of leaf lesions on `Sunburst' mandarin and two other cultivars were examined by light and electron microscopy. Damaged leaves treated with a fungicide confirmed that the anatomical changes on `Sunburst' are an enhanced wound periderm response to feeding injury by rust mite and not the result of fungal invasion.

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B.R. Bondada, J.P. Syvertsen, and L.G. Albrigo

Foliar-applied urea nitrogen (N) has potential to become an important component in fertilizer programs for citrus in Florida and other citrus growing areas as it can reduce nitrate leaching into ground water. We evaluated seasonal absorption characteristics of three urea formulations, Triazone-urea, liquid urea, and spray grade urea by citrus leaves that were from 2 weeks to 6 months old. The effect of leaf age on 15N absorption by N-deficient and N-sufficient leaves, together with urea absorption over an eight-week period were studied using greenhouse-grown and field-grown plants. All foliar N applications were based on a recommended rate of 34 kg N/ha in 469 L of water. In the field studies, leaf N was increased similarly by the three urea formulations one week after three weekly applications. Young leaves (0.25 month and 1 month old) absorbed a greater percentage of N than the older leaves (3 month and 6 month old). Epicuticular wax concentration increased and 15N absorption declined with leaf age. Nitrogen deficient leaves (1.80% N) had greater wax concentration and lower N absorption than N sufficient leaves (2.60% N). Four to 8 weeks after urea applications, Triazone-urea sprayed leaves had significantly greater leaf N concentration than leaves sprayed with liquid urea or nonsprayed control leaves. The greenhouse studies revealed that the 15N absorption was greater through abaxial leaf surfaces than through adaxial surfaces regardless of leaf N level and application time. Applying foliar 15N-urea during night (2000 hr to 2200 hr) resulted in greater absorption of 15N than in the morning (0800 hr to 1000 hr) or afternoon (1200 hr to 1400 hr). It is clear that maximum N absorption from foliar urea sprays occurred at night through the abaxial surfaces of young leaves with sufficient N. Triazone-urea acted as a slow-release N source that could be exploited in supplying N over an extended period of time.

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D.S. Achor, H. Browning, and L.G. Albrigo

Young expanding leaves of `Ambersweet' [Citrus reticulata Blanco × C. paradisi Macf. × C. reticulata) × C. sinensis (L) Osb.] with feeding injury by third larval stage of citrus leafminer (Phyllocnistis citrella) were examined by light and electron microscopy for extent of injury and tissue recovery over time. Results confirmed that injury is confined to the epidermal layer, leaving a thin covering over the mine tunnel that consisted of the cuticle and outer cell wall. Wound recovery consisted of two possible responses: the production of callus tissue or the formation of wound periderm. The production of callus tissue developed within 3 days of injury when the uninjured palisade or spongy parenchyma below the injured epidermis produced callus tissue through periclinal or diagonal cell divisions. After 1 month, the entire epidermis was replaced by callus tissue. In the absence of secondary microbial invasion, this callus tissue developed a thick cuticle, followed by development of a covering of platelet wax after 4 months. Alternatively, wound periderm formed if the outer cuticular covering was torn before the cuticle had developed sufficiently to prevent the exposed cells from being desiccated or invaded by fungi, bacteria, or other insects. The wound periderm consisted of a lignified layer of collapsed callus cells, a suberized phellem layer, and a multilayered phelloderm-phellogen. Since there were always cellular collapse or fungi and bacteria associated with wound periderm formation, it was determined to be a secondary effect, not a direct effect of leafminer feeding.

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L.G. Albrigo, R. Russ, R. Rouseff, and R.A. Bazemore

Except for `Orlando' and `Minneola' tangelos, most citrus hybrids grown in Florida are small-flowered and produced less than half the nectar of large-flowered cultivars. Sugar contents in large- and small-flowered cultivars were not different in 1997, but the concentration of sugars doubled in 1998 over 1997 for small-flowered hybrids, while nectar volume was about one-half of that in the previous year. Nectar volume of large-flowered cultivars increased slightly in 1998 compared to 1997. Of four aromatic volatiles measured from headspace over flowers, `Robinson' and `Ambersweet' were lowest in total while other cultivars had only some specific differences. Grapefruit flowers produced high limonene levels, while `Sunburst', `Fallglo', and `Valencia' gave off the most myrcene. Bees were tested for flower preference in a round, white screenhouse using a mini-hive and duplicated fresh-cut flower bouquets each day. Bees preferred large-flowered cultivars with more nectar. Some other preferences also were observed. In the field, hedge-rows limited cross movement of bees in mature blocks of hybrids, which limits the number of contiguous rows of the preferred cultivar for good pollinization. Timing of bee hive placement was also critical to get bee movement into the intended block because flowering times differ for some cultivars and bees develop inital preferences.

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Yong-Soo Hwang, D.J. Huber, and L.G. Albrigo

Cell wall composition and structure were examined in visually normal (N), granulated (G), and collapsed (VC) juice vesicles of `Marsh Seedless' grapefruit (Citrus paradisi) Macf.). According to gel-filtration data, VC appeared to be associated with a modification of water-soluble (WSP) and chelate-soluble (CSP) pectin molecular weight (Mr); small-Mr pectins increased, whereas large-J4. pectins decreased. The difference in M = of pectins did not appear to be mediated by polygalacturonases. Molecular weight of hemicelluloses did not differ. Granulated vesicles contained about two times more structural polysaccharides (pectins, hemicelhdose, and cellulose) than N vesicles, although hemicellulose and pectin M = modification were absent. Ion-exchange profiles of WSP, CSP, and hemicelhrlose fractions of VC and G vesicles were not different from those of N vesicles. Individual cells in vesicles with G and these vesicles themselves were much larger than those of N vesicles, whereas cells in VC were partially or completely collapsed.

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K.C. Taylor, C.D. Chase, L.G. Albrigo, and J. W. Grosser

Citrus blight is an extremely complex decline disorder of unknown etiology, Zinc accumulates in the phloem of the tree 40-50 cm above the bud union 1-3 years prior to visible symptoms of blight (foliage wilt and twig dieback). This is accompanied by Zn deficits in the leaves. A Zn-binding peptide (ZBP) purified from citrus phloem tissue accounts for a symptomatic redistribution of Zn from the canopy to the trunk phloem. ZBP is found in blight and healthy trees and is therefore a normal component of cellular metabolism. To further understand ZBP's role in metabolism two citrus cell culture lines which were selected based on their susceptibility to blight have been characterized as to their growth under Zn treatments as well as Cu and Cd. In addition, their complement of metal-binding constituents is being determined.