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V.M. Russo and C. Biles

Cucumber (Cucumis sativus L.) seed require soil temperature to be around 20C for efficient germination. This hinders early planting in cool soils. This study was conducted to determine how germinating seed of cucumber cultivars Earlipik 14 and Arkansas Little Leaf at 13.9, 15.6, and 20C in the dark affected protein formation. Seed were removed from moist chambers at 0, 12, 24, 48, 72, 84, 96, 120, and 168 h. Germination, defined as the radicle being at least 5 mm long, was determined at each time. Germinated and ungerminated seed was prepared for polyacrylamide gel electrophoresis. At 20C, 90% to 100% of seed had germinated by 48 h. At 15.6C, 20% to 50% of seed germinated by 168 h, and at 13.9C, ≈2% of seed had germinated by 168 h. For seed incubated at 20C, concentrations of proteins at 70.1 kDa decreased, while those at 37.4, 43.4, and 50 kDa increased after 24 h, which corresponded to formation and elongation of the majority of radicles. These changes were expressed later for seed germinated at 15.6 and 13.9C. Identification of the proteins is being attempted. The importance of these proteins in germination and early development will be discussed.

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K. Roberts, J. Williamson, J. Wright, C. Biles, and V. Russo

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.

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J.X. Zhang, B.D. Bruton, and C.L. Biles

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.

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J.K. Collins, C. Biles, E.V. Wann, and P. Perkins-Veazie

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.