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Paul H. Jennings

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Paul H. Jennings

We have previously shown that both temperature and chemical shocks are capable of inducing chilling tolerance in 24h germinated cucumber seedlings. Using a heat shock temperature of 50°C. it has been demonstrated that a 2 min treatment is most effective in inducing chilling tolerance as measured by root survival growth. However the induced chilling tolerance is transient and disappears if the heat shocked seedlings are held at 25°C for 12h before chilling at 2°C. Older seedlings (36h of germination) are more sensitive to chilling but are still capable of chilling tolerance induction by heat shock. Using chilling as a selective pressure, it is possible to increase chilling tolerance of 24h germinated seedlings.

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Eleazar Reyes and Paul H. Jennings

The effect of chilling stress on induction of the cyanide-resistant pathway was investigated using roots of 3-day-old cucumber (Cucumis sativus L.) grown at 26C and then chilled at 2C, 10C, or 15C for 24, 48, 72, and 96 h. A 24-h post-chilling treatment was imposed on different sets of chilled cucumber roots at 26C. Exposing seedlings to 2C, 10C, and 15C, as well as to a post-chilling treatment, induced differential responses in the activity of the cyanide-resistant pathway. Cucumber seedling roots exhibited an increase in the cyanide-resistant pathway after a 96-h chilling treatment at 2C. The involvement of the cyanide-resistant pathway in the chilling stress response will be discussed.

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Hua Zhang and Paul H. Jennings

The effects of heat shock duration and persistence on the induction of chilling tolerance in cucumber roots were studied using total root growth, electrolyte leakage, and membrane peroxidation as injury indices after chilling. Heat shock reduced the chilling induced electrolyte leakage, decreased membrane peroxidation as measured by MDA content, and resulted in a greater total root growth after chilling compared to the control. Heat shocks at 40°C, applied to 36 hr germinated seedlings for time periods from 1 to 15 hr, all resulted in an increase in chilling tolerance in a time-dependent manner. The heat shock induction of chilling tolerance is most effective when heat shock was imposed immediately before chilling, but the effect is persistent even 32 hr after heat shock when seedlings are held at 25°C before chilling. The possible mechanism of heat shock effect and its persistence will be discussed in relation to heat shock proteins and antioxidant enzyme systems.

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Paul H. Jennings and Ann Fitzpatrick

Four cucumber seed lines obtained from the Inst. of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Beijing, China, were tested for chilling tolerance. Comparisons were made with `Poinsett 76', a commercially available cultivar from the United States. Seeds germinated at 25°C were exposed to 2°C for time periods up to 108 hr. Root injury was assessed by measuring subsequent root growth at 25°C at 72 hr after the chill. Electrolyte leakage measurements were taken on roots excised immediately after the chill. Total seedling root length and electrolyte leakage studies showed significant tolerance to chilling in the selections from China as compared to `Poinsett 76'. `Poinsett 76' seedling roots began to show stress after 72 hr of chill and were irreversibly damaged, with abortion of root tips, after 96 hr at 2°C. The China seed selections were more tolerant to a 96-hr chill and even at exposure times up to 108 hr only began to approximate chilling effects exhibited by `Poinsett 76' at 72 hr of treatment.

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Ann Fitzpatrick and Paul H. Jennings

Preliminary studies on the effects of chilling stress during the early stages of germination and radicle emergence in cucumber have suggested that damage may be due to the accumulation of active oxygen species. Several methods are available which are capable of assessing various types of oxidative damage and were selected for use in determining the involvement of active oxygen species in the chilling response of emerging cucumber roots. Oxygen radicals have been shown to interact with cellular proteins resulting in the formation of carbonyl derivatives. A procedure using 2,4-dinitrophenylhydrazine was tested to determine carbonyl protein content. Cellular lipids are also subject to peroxidation by active oxygen species resulting in the production of malondialdehyde and can be quantified by reaction with thiobarbituric acid. Sulfydryl groups may also be attacked by oxygen free radicals and changes monitored by a procedure using 5,5'-dithio-(2-nitrobenzoic acid). A comparison of these three methods for detecting oxidative damage associated with chilling stress in radicles of germinating cucumber seed will be discussed.

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Mustafa Ozden and Paul H. Jennings

The effects of calcium salts and concentrations from 25 to 200 mM on the induction of chilling tolerance in cucumber roots were studied using total root growth, electrolyte leakage, lipid peroxidation, and activities of antioxidant enzymes as indicies of chilling injury. Cucumber seeds `Poinsett 76' germinated at 25 °C for 36 h were treated with calcium sulfate, calcium nitrate, and calcium chloride for 2 h at 25 °C. After incubation, treated seedlings were rinsed with distilled H2O and chilled at 2 °C for 72 or 96 h with or without re-warming at 25 °C. Roots of CaSO4-treated cucumber seedlings exhibited less chilling injury at all concentrations, when exposed to 72- or 96-h chilling periods with a 72-h re-warming period as shown by greater root growth compared to the chilled control. Concentrations of CaCl2 and Ca(NO3)2 above 100 mM resulted in significant root growth inhibition. Electrolyte leakage (EL) was significantly reduced by CaSO4 up to 150 mM under chilling conditions and all calcium salt treatments reduced malondialdehyde (MDA) levels in seedling roots up to 150 mM. However, at 150 mM CaSO4 both EL and MDA values of 72 h chilled and re-warmed roots were at their lowest levels compared to the control and other treatments. Both superoxide dismutase and catalase activities of seedling roots decreased under chilling conditions compared to the nonchilled control, although the reduction was less in the presence of CaSO4. Peroxidase and glutathione reductase activities increased under chilling conditions and were generally reduced in the presence of calcium salts compared to the chilled control.

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Paul Jennings and Mikal E. Saltveit

Unlike horticulturally mature fruit of `Dasher II' and `Poinsett 76' cucumbers (Cucumis sativus L.), two cultivars that differ significantly in their level of chilling tolerance, imbibing and germinating seeds of these two cultivars responded similarly to chilling temperatures (e.g., increases in fresh weight, time to radicle emergence, and root growth). `Dasher II' and `Poinsett 76' seeds were imbibed and germinated at 10 to 30C, and seeds germinated at 25C for 24 h were chilled at 2.5C for various durations. In comparison, seeds from an aged lot of `Poinsett 76' seed (1989) responded very differently from the 1992 seed lots in all experiments. The chilling tolerance level of germinating `Poinsett 76' seed varied with the seedling age as measured by resumption of root growth. Our results suggest that some factor that confers chilling tolerance is gradually lost during the early stages of germination following imbibition.

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Eleazar Reyes and Paul H. Jennings

The effect of chilling stress on induction of the cyanide-resistant pathway was investigated using roots of 3-day-old cucumber (Cucumis sativus L.) and 5-day-old pea (Pisum sativum L.) grown at 26°C, and then chilled at 2°C for 48 or 96 hours for cucumber, and 72 or 192 hours for pea. A 24-hour post-chilling treatment at 26°C was imposed on different sets of chilled roots from both crops. Carbohydrate status was determined by gas chromatography with an autosampler using a 12.5-m cross-linked methyl silicone capillary column (0.1 mm). Exposing seedlings to 2°C, as well as to a postchilling treatment, induced differential responses in the activity of the cyanide-resistant pathway. Cucumber seedling roots exhibited an accumulation of fructose, glucose and sucrose during chilling, with a rapid decline observed during the post-chilling treatment at 26°C. Pea seedling roots maintained a constant level of carbohydrates throughout the chilling period, and exhibited a slight decrease by the end of 192 hours at 2°C. There was an increase in carbohydrate levels during the post-chilling treatment. The involvement of the cyanide resistant pathway and carbohydrate changes will be discussed.

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Paul H. Jennings and Cecil Stushnoff

Various carbohydrates have been shown to be associated with stress tolerance in some plant species. Specifically, the content of soluble sugars have been correlated with desiccation tolerance and winter hardiness. We have previously demonstrated that radicles of cucumber seed become progressively more sensitive to chilling injury during the early stages of germination and that cultivar differences exist. Sucrose, raffinose, and stachyose contents of `Poinsett 76' and `Ashley' seed were determined in dry seed during imbibition and at three stages of radicle emergence. The more chilling-tolerant cultivar (Ashley) contained lower raffinose and higher stachyose contents than the less chilling-tolerant `Poinsett 76'. In both cultivars, the contents of raffinose and stachyose declined dramatically between the 1-mm and 5- to 7-mm stage of radicle emergence. At the 1-mm stage, when cultivar chilling-tolerance differences are most pronounced, `Ashley' appears to have a higher content of stachyose and lower raffinose content.