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  • Author or Editor: Kathryn C. Taylor x
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To discern how the packing process influences pesticide residue loads on peach (Prunus persica L. Batsch) fruit; postharvest, post hydrocooled, and post brushed fruit were assessed for levels of several pesticides. The packing house process reduced pesticide residue levels on fresh peaches to levels that were generally below detection limits of our assays in 1998. Carbaryl and captan residues from field packed fruit were 32.2× and 21.9×, respectively, of that found in the peel of fruit processed in the packing house in 1998. Carbaryl levels were not reduced by hydrocooling but postharvest brushing reduced pesticide residues up to 94% in fruit peel. Across processing operations and cultivars assessed in 1999, hydrocooling, hydrocooling plus brushing, and brushing alone removed 37%, 62%, and 53%, respectively, of the encapsulated methyl parathion residues from fruit peel. Hydrocooling had the greatest impact on phosmet removal from peel, reducing levels by 72.5%. After hydrocooling, phosmet was 5.7× following brushing in one-half of the subsequent samples. This increase occurred at all three farms, suggesting that periodic cleaning of brushes may be necessary to prevent later contamination of peach peel with pesticides. In the only example in which propiconazole residue remained on peaches at picking, it was removed most effectively (69%) by the brushing operation. Nearly 31% of the propiconazole was removed in the hydrocooler. The packing process before shipment to retail outlets was generally effective in the removal of pesticides that may be present on peel at the time of harvest. Assessment of pesticide residue levels in peach flesh was uniformly below the levels of detection in our assays, suggesting that the classes of pesticide analyzed in peaches were not transepidermal.

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A 22-kDa Zn-binding protein (ZBP) was isolated from the phloem tissue and evacuated xylem sap of `Valencia' sweet orange [Citrus sinensis (L.) Osbeck] on rough lemon [C. jambhiri (L.)], as well as Valencia on Rangpur lime [Citrus limonia Osbeck]. Phloem and xylem Zn was associated with the 22 kDa ZBP. The Mr value of this ZBP was estimated to be 19,500 by size exclusion chromatography and 22,800 by SDS-PAGE. This protein was isolated with an isoelectric point of 7.5. Ion exchange chromatography demonstrated that 22-kDa ZBP was highly anionic, requiring 0.43 M NaCl for elution from QAE Sepharose. The 22-kDa ZBP appears unique to citrus, having no cross reaction with protein from several tissues from a range of plant species. Accumulation decreased under Zn-deficient conditions, was enhanced by osmotic stress, and the protein completely disappeared with wounding. Amino acid composition demonstrated that the protein was rich in aspartate, and glutamate; and contained 6 cysteine, and 4 histidine residues. These amino acids may be involved in metal binding. N-terminal amino acid sequencing demonstrated that the 22-kDa ZBP had identity with sporamin A&B precursors, Kunitz-type trypsin inhibitors, and miraculin. It is suggested that the genes that encode these proteins are derived from a common ancestral gene.

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Citrus blight is a decline disorder of unknown etiology. Its earliest symptom is redistribution of Zn within the tree. The canopy exhibits symptoms of Zn deficiency while Zn accumulates in trunk phloem just above the bud union. Phloem Zn was associated with complexing agents. Zn-binding proteins (ZBPs) were isolated from phloem tissue extracts of mature healthy and blight citrus trees. After purification by ion exchange chromatography (IEC) and size exclusion chromatography, ZBP's from healthy and blight-affected citrus trees {`Valencia' sweet orange [Citrus sinensis (L.) Osbeck] on rough lemon [C. jambhiri (L.)]) were compared for relative abundance. Size exclusion chromatography indicated that the proteins were and 19.5 kD Mr. IEC demonstrated that the 5 kD ZBP was slightly anionic and that the 19.5 kD ZBP was highly anionic. There were 2.5 × times more of the 5 kD ZBP in blight material than in healthy (purified on an equal prot basis). Levels 19.5 kD ZBP were equivalent in blight and healthy phloem tissue extracts, on both fresh weight and total protein bases. The amino acid composition of the 5 kD ZBP was much more complex than would be expected for phytochelatins. Altered phloem structure also was apparent in blight versus healthy trees, with phloem fresh and dry weights increased in the blight-affected citrus. Phloem specific expression of the ZBPs was also apparent.

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Macrophylla decline was observed primarily in lemons on the macrophylla rootstock (Citrus macrophylla, Wester). This disorder is characterized by foliar zinc deficiency symptoms with zinc accumulation in trunk phloem above the bud union. Trees show symptoms of twig dieback and are less thrifty in appearance compared to the same cultivars on other rootstocks. These characteristics are very similar to a citrus disorder of humid citrus production regions, citrus blight. Citrus blight affects the more vigorous rootstocks (a characteristic of Macrophylla) such as the extremely blight susceptible rough lemon rootstock. In the humid regions, macrophylla is considered somewhat susceptible to blight. Experiments are currently underway to assess the water conductivity of xylem in trees on macrophylla and to screen for the presence of citrus blight specific proteins.

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A field trial conducted at Yuma, Ariz., examined the effect of foliar boron application on fruit yield and quality of Citrus sinensis cv. Hamlin. Boron was applied to 5-year-old trees at five treatment levels (0, 500, 1000, 2000, and 3000 ppm) before or after flowering in a split plot design. At harvest, fruit number, size and quality were determined. Yield (P = 0.01) and average fruit number per tree (P = 0.02) were different among treatments. The highest yield was obtained with the 500 ppm treatment. In this first year of the trial there was no difference in average fruit weight, fruit pH, titratable acidity, peel thickness, juice volume, or soluble solid content of fruit between the treatments. Previous studies indicate that boron influenced in vivo and in vitro pollen germination in many crops. Increased fruit yield may have occurred because boron was transported to the flowers where it exerted its influence on increased fruit set through an effect on pollen viability or pollen tube growth. Further investigation of this hypothesis is underway.

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