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Mikal E. Saltveit and Mary E. Mangrich

The density of excised 2-cm celery (Apium graveolens L.) petiole segments was highly correlated with a subjective evaluation of pithiness. Loss of density and the appearance of pithiness was stimulated by lengthening the duration of storage, raising the storage temperatures above 0C, and excising petiole segments. Segments excised from the upper two-thirds of the petiole lost less density during storage than segments excised from the bottom third of the petiole. Segments with initial high densities lost slightly less density during storage at 5C for 5 weeks than segments that were initially less dense. The extent of pithiness development varied significantly among six cultivars held at 5C for 2 weeks. Treating whole petioles with 1 μm abscisic acid for 4 days significantly increased density loss. Exposing petiole segments to up to 100 μl·liter-1 ethylene in humidified air for up to 2 weeks at 5C did not significantly change density over air controls. The loss of density and the development of pithiness in lightly processed celery petioles could be reduced by selecting resistant cultivars, monitoring water stress during growth, using only segments excised from the upper two-thirds of the petiole, and selecting segments with initial high densities.

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Mary E. Mangrich and Mikal E. Saltveit

Crops with origins in tropics and subtropics undergo physiological injury when subjected to nonfreezing temperatures below 12°C. Application of heat and chemical shocks to tissue prior to chilling induces chilling tolerance. This study was undertaken to investigate the effects of low oxygen and high carbon dioxide atmospheres on subsequent chilling tolerance. Cucumber seedlings (Cucumis sativus L., cv. Poinsett 76) with radicles 8 to 12 mm long were subjected to 0% to 21% oxygen and/or 0% to 20% CO2 atmospheres for 0 to 72 hr at 2.5 or 15°C. After chilling, they were placed at 25°C for three additional days. Radicle growth was used to assess chilling injury. Modification of the individual germination plates was necessary to ensure seedling exposure to the desired atmospheres. Chilling injury was reduced by exposure to oxygen levels below 1% and to carbon dioxide levels above 5%. Effects of brief exposures were small in comparison to prolonged exposures during chilling. Seed to seed variability was high and obscured some results. The effects of the various atmospheres were greater with the less vigorous seedlings.

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Mary E. Mangrich and Mikal E. Saltveit Jr.

Ethylene induces arenchyma formation in corn roots and other plant tissues, and abscisic acid (ABA) induces arenchyma in celery petioles. Pithiness (i.e., arenchyma) in celery can be measured as a decrease in density. Density was calculated for two cm long petiole segments by dividing their weight by their volume as calculated from the weight of water displaced upon immersion. The relationship between density (g/ml) and subjective pithiness rating (1 = none, 9 = severe) was linear (r2 = 0.87). Petiole segments exposed to 0 to 200 ppm ethylene in air at 5C for two weeks did not exhibit any significant differences (p = 0.05) in density among the treatments. Entire petioles were treated with 0, 1, 10, and 100 μM ABA in water for 96 h at 25C. The petioles were cut into thirds and the center 2 cm from each portion was excised and the density measured. Although density decreased in the top to the bottom portions over all ABA conc, the differences were not significant. Density was significantly reduced in segments excised from the bottom and middle of petioles treated with 10 and 100 μM ABA, compared to 0 and 1 μM ABA. There also was a decrease in density with ABA conc in the top portion, but the decrease was only significant for the 100 μM ABA conc.

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Mary E. Mangrich and Mikal E. Saltveit

Chilling 10-mm cucumber (Cucumis sativus L. `Poinsett 76') radicles at 2.5 °C reduced their subsequent growth during 3 days at 25 °C. The reduction in radicle growth was linear for 1 to 3 days of chilling but then increased substantially until subsequent radicle growth was all but eliminated by 6 days of chilling. Heat shocks of 40 °C applied for 4 to 12 min increased chilling tolerance such that 4 days of chilling caused only a 36% decrease in radicle growth compared to 66% for seedlings not heat shocked, which brought the response in line with the responses of the non-heat-shocked seeds chilled for 1 to 3 days. Eight-minute heat shocks applied before 5 days of chilling resulted in a 45% inhibition of subsequent growth, compared to 82% for chilled non-heat-shocked controls. Heat shocks applied before 3 days of chilling did not result in a significant increase in subsequent growth compared to the non-heat-shocked controls chilled for 3 days. Heat shocks were only able to protect that part of radicle growth that was in excess of the linear decrease in radicle growth. There appears to be two effects of chilling on radicle growth. The first is linear and cannot be affected by heat shocks. The second is much more severe and can be prevented by heat shocks. Seeds were selected for three categories of vigor according to the rate at which their radicles grew to 10 mm. Seeds classified with different vigors neither responded significantly differently to 3 days exposure to 2.5 °C nor did they respond differently to chilling stress following application of heat shocks.

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Mary E. Mangrich and Mikal E. Saltveit

Seeds of cotton (Gossypium hirsutum L.), kenaf (Hibiscus cannabinus L.), okra [Abelmoschus esculentus (L.) Moench. `Clemson Spineless' (syn. Hibiscus esculentus L.)], rice (Oryza sativa L.), and wheat (Triticum sativum (L.) Lam.) were germinated and grown at 25 °C until their radicles reached 10 mm in length. They were then exposed to chilling temperatures for 0 to 5 days followed by 3 days at 25 °C. Radicle length was measured periodically and inhibition of elongation was used as an indicator of the severity of chilling injury. Exposure to chilling reduced radicle elongation in all species except chilling insensitive wheat. When seedlings were heat-shocked at 45 °C for 1 to 12 min before being chilled, radicles of the chilling sensitive okra, kenaf, cotton, and rice seedlings elongated more than seedlings not heat-shocked before chilling. The method of heat-shock application and the stringency (i.e., time× temperature) of the heat-shock and chilling treatments all affected the response of the tissue. In comparison to nonheat-shocked wheat seedlings, the radicles of chilling insensitive wheat seedlings did not elongate more than seedlings in which the heat shocks were applied before chilling. A brief heat shock ameliorates chilling injury in these chilling sensitive species.

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Mikal E. Saltveit Jr., Mark Ritenour, Mary E. Mangrich, and John C. Beaulieu

Exogenous application of ethanol (EtOH) vapor to whole tomato fruit or excised pericarp discs inhibits ripening without affecting subsequent quality. Inhibitory EtOH levels are induced in whole tomatoes by a 72 h exposure to anaerobic atmospheres at 20C. In contrast to tomatoes, exposure to EtOH vapor (0 to 6 ml EtOH/kg FW, for 3 to 6 h at 20C) did not retard ripening (e.g., changes in external color, flesh firmness, and soluble solids) of avocado, banana, cucumber, melon, peach, or plum fruit. When the blocked replicates for nectarines were sorted by the firmness of the control fruit, higher levels of EtOH vapor appeared to delay softening of the firmer fruit. From 0 to 4 ml EtOH/kg FW was injected as 95% EtOH into the seed cavity of melon fruit through a surface sterilized area near the equator of the fruit with a plastic syringe fitted with a 7.5 cm long hypodermic needle. Injection of 1 to 4 ml EtOH/kg FW inhibited the softening of `Honey Dew' and muskmelons. Slight tissue necrosis near the site of injection was noted in a few fruit. Unlike the ripening inhibition of tomatoes which is relatively insensitive to the stage of maturity, the inhibition of melon ripening by EtOH appeared to be significantly affected by the maturity of the fruit.