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  • Author or Editor: C. Biles x
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Xylem fluid and cotyledon, stem, and leaf tissue of eight watermelon [Citrullus lanatus Thunb. (Matsum. and Nakai)] cultivars differentially suspectible to races 0, 1, and 2 of Fusarium oxysporum f. sp. niveum, (E.F. Sm.) Synd. and Hans., causal agent of fusarium wilt, were assayed for general proteins and specific enzymes using SDS and IEF-PAGE and starch gel electrophoresis (SGE). SGE detected no variant isozymes among the watermelon cultivars in the six enzyme systems examined (GOT, MDH, PGI, IDH, PGM, PER); however, electrophoretic variants between tissue types were observed. Cotyledon tissue expressed an additional peroxidase band not seen in stem tissue. When xylem fluid samples were applied to IEF and SDS-PAGE and silver-stained, variant protein banding patterns were observed between the cultivars. The fusarium wilt-susceptible cultivar Black Diamond lacked the protein bands at pI = 5.1, 5.2, and 5.6 that were present in other cultivars. In addition, wilt-resistant ‘Dixielee’ possessed a differential band at pI = 6.0. We believe this to be the first report of electrophoretic differences among commercial watermelon cultivars.

Open Access

Phomopsis cucurbitae is a latent infecting pathogen that infects unripe muskmelon fruit, but causes decay after harvest. This fungus causes severe losses during muskmelon fruit storage and marketing in the U.S., Japan, and some Central American countries. Previous studies showed that the fungus produced the cell wall-degrading enzyme polygalacturonase (PG) in both culture and muskmelon fruit tissue. The role of P. cucurbitae PG in the fruit decay process and its relation to latent infection is not well-understood. A prominent PG isozyme produced by the fungus in decayed fruit was purified to homogeneity by a sequence of extraction, ultrafiltration, preparative isoelectric focusing, anion exchange, and gel filtration chromatography. This isozyme exhibited endo-activity, a molecular weight of 54 kDa according to SDS-PAGE, and a pI of 4.2 based on IEF-PAGE. Isozyme activity was optimal at 40–45°C and pH 5.0. It had a Km of 44.7 g/ml and a Vmax of 0.313. The purified isozyme also effectively macerated mature muskmelon fruit tissues. This isozyme was the most prominent of the PG isozymes produced by P. cucurbitae in decaying fruit, and may play an important role in postharvest decay.

Free access

Increased peroxidase activity is used to predict development of off-flavor in frozen sweet corn. However, peroxidase activity was not indicative of flavor changes in frozen supersweet (sh2) or sugar enhanced (sul/se) sweet corn genotypes. These results suggested an inactivation or absence of certain peroxidase isozymes. Frozen `Florida Staysweet' (sh2), `Merit' (sul), and `Bodacious' (sul/se) kernels were cut from cobs after 0 and 12 months of storage. Proteins extracted from acetone powders were separated by isoelectric focusing (IEF) and Native-PAGE. Banding patterns differed according to cultivar and storage duration. All cultivars contained a peroxidase isozyme having a molecular weight of 99 kD and pI of 4.5. The sul/se and su2 cultivars expressed an additional peroxidase band of 17.9 kD. An additional peroxidase isozyme (pI 5.0) appeared after 12 months of storage in the sul cultivar. This isozyme did not appear in sul/se or sh2 and is a possible marker for predicting off-flavor in corn. This isozyme may also catalyze off-flavor reactions in sul corn genotypes. Although changes in total peroxidase activity may not predict flavor loss in all genotypes, certain peroxidase isozymes may be useful in predicting and catalyzing off-flavor reactions in sul corn cultivars.

Free access

Senescence and levels of minerals, sugars, and proteins were determined in stalk internodes of corn (Zea mays L.) cv. Illini Gold, a shrunken2 hybrid, from from mid-whorl (V9; internodes completely juvenile) to fresh-market maturity (FM; internodes exhibiting stages of senescence). Senescence was rated in internodes near the base of the stalk (I7), below the ear (I9), and between the ear and tassel (I11). Tissues were extracted and analyzed by carbon-nuclear magnetic resonance spectroscopy (C-nmr) and SDS-PAGE electrophoresis. Senescence rating increased from V9 to FM. Through silk emergence (R1) C-nmr carbohydrate spectra were similar, regardless of internode, with chemical shifts between 61 and 104 ppm, mostly of glucose, fructose, and sucrose. At FM, additional lines were found that were not associated with a saccaride. The highest concentration of sucrose was at R1, fructose at tasseling (VT), and R1, and for glucose from VT to FM. The protein profile present through R1 in I7 was not present at FM. In I9, the protein profile was similar throughout. In I11, numbers, or density, of protein bands decreased through FM. Mineral concentrations did not change, decreased, or fluctuated. Levels of N, Cl, or Na at VT, R1, and FM, respectively, were negatively correlated with senescence. In I7 and I9, senescence ratings were negatively correlated with levels of Mg, NO 3, or SO2– 4. Senescence appears to be associated with concentrations of some minerals, a reduction in levels of sucrose, and with the presence or absence of some proteins; however, cause and effect remains to be established. This research was hosted by USDA/SCARL at Lane, Okla., and made use of NMR equipment provided through USAF/AFOSR Grant F49620-95-1-0316 and NIH/NIGMS Grant GM 08003.

Free access

Studies were conducted with raspberry fruit to isolate and identify naturally occurring yeasts for possible biological control activity against postharvest fungi. The yeasts identified were heterothallic or asexual Ascomycetes and Basidiomycetes. Biocontrol activity was compared with an isolate of Pichia guillermondi and a water-treated control. After 16 days at 4C, 30% of the water-treated fruit were diseased, while there was no disease on fruit treated with Pichia sp. or one of the collected yeasts. Treated raspberry fruits were packaged in heat-sealed low density polyethylene bags and evaluated for % soluble solids, pH and internal gas concentrations as well as disease incidence. After 14 days in storage the fruit started to develop off-flavors. Soluble solids and pH did not change. Disease incidence increased as storage time increased. The yeasts collected and the Pichia sp. isolate showed potential as biocontrol agents for reducing disease in packaged raspberries.

Free access