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Schuyler D. Seeley

Forcing plant material has long been used to determine dormancy intensity (DI) in woody species. Forcing with growth regulators may enhance this ability. Some forcing with naturally occurring hormones may be showing us the actual DI of certain materials. But, measurements of DI that use caustic, near-lethal treatments, or metabolic agents may be all or nothing breaking indicators acting on mechanisms other than the dormancy mechanism and thus not as useful in determining DI. It is possible to cause a meristem to break without completely breaking dormancy. Measurement of normal post-dormancy growth is necessary to determine the effect of a DI agent. DI breaking treatments that act on the dormancy mechanism can cause a temporary growth flush, but, unless the extent of that growth flush is measured and compared with the growth flush of the same normally broken plant material, its true effect remains unknown. In some plant material, the safest way to determine DI is to determine the chilling required to produce normal growth. This assumes that the vernalization requirement and temperature response curves are known for the plant in question. In peach, for instance, vernalization at 2C will cause seeds to germinate, but the resulting seedlings will be physiologically dwarfed. Vernalization at 6C or at 2C cycled with higher temperatures within the vernalization range results in normal seedlings. This indicates that, for chilling to progress normally, vernalization per se must be interspersed or concomitant with growth heat units. Vernalization, therefore, has a low temperature driven component and a heat requiring development and/or growth component. Vernalization driving conditions are slowly being elucidated. Each clarification requires modification of dormancy models. DI does not equal dormancy status!

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Schuyler D. Seeley

The effect of thermal accumulation on anthesis rate in apricot, apple, peach, and tart cherry flowers during dormancy, dormancy release, and normal anthesis was determined. Data from several studies in warm and cold climates have indicated that thermally driven anthesis has an early low-temperature optimum that rises during anthesis. This is not true. Erroneous interpretation of results may have been due to inadequate measurements of the endodormancy status of seeds and buds. After endodormancy, flower-bud development temperature responses follow a normal sigmoidal curve with small but significant contributions at temperatures as low as 2C. The grand phase of the growth curve occurs between 16 and 20C in tart cherry. Asymptotic growth vs. temperature responses occurred at <10 and >22C, with minima near 0 and optima >24C. These data indicate that asymmetric curvilinear anthesis models need to be fitted to each species.

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Schuyler D. Seeley

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James W. Frisby and Schuyler D. Seeley

We compared peach [Prunus persica (L.) Batsch cv. Johnson Elberta] seed germination (G) and seedling emergence (E) after various stratification (St) treatments. Treatments were arranged in factorial combinations of five St durations (20 to 60 days) at eight constant temperatures (0 to 18C) in a completely randomized design followed by repeated measures during forcing time. G and E were recorded every 5 days during forcing. Seed St at 0 to 10C and 0 to 14C promoted G and E, respectively. G and E increased with longer St treatments at promoting temperatures. There was a weak correlation between G and E averaged over the forcing measurements (r 2 = 0.54). The best correlation was between E after 15 days and G after 10 days (r 2 = 0.83). The results indicate that G and E in peach are not identical indicators of endodormancy (ED) release and should not be used interchangeably. Forcing times must be considered when making comparisons between G and E.

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James W. Frisby and Schuyler D. Seeley

We observed initial peach [Prunus persica (L.) Batsch] seedling growth after endodormant (ED) seeds (`Johnson Elberta') had been stratified for five durations (20 to 60 days) at eight constant temperatures (0 to 18C). Seedling growth increased and became more normal when seeds were stratified for longer durations at chilling temperatures. Stratification (St) at 0 to 6C (especially 2C) produced seedlings with more abnormal growth than St at higher temperatures (8 to 10C) at intermediate St durations (30 to 50 days). Growth of the primary stem increased with additional St (0 to 14C). Abnormal (epinastic) leaf development decreased following longer St treatments at 4 to 14C. Lateral shoot growth increased initially, then decreased after longer treatments at low temperatures (0, 4, and 6C) and decreased after St at higher temperatures (8 to 10C).

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James W. Frisby and Schuyler D. Seeley

We studied the response of physiologically dwarfed (PD) to near normal peach [Prunus persica (L.) Batsch] seedlings (`Johnson Elberta' seeds) to various chilling treatments. Peach seedlings were obtained by forcing seeds that had been subjected to a brief stratification treatment. Seedlings were divided into four types (groups) according to the length of the primary stem and the presence and size of lateral branches. The seedlings were used in a chilling study with treatments of five durations (20 to 60 days) at four temperatures (2 to 14C). Terminal shoot growth and lateral budbreak were recorded 17 days after forcing. Shoot and leaf dry weight were obtained after seedlings had grown for 64 days. Budbreak and growth improved with the duration of the chilling treatment. Generally, 7C was the best chilling temperature, with 2 or 10C only slightly less effective. Treatment at 14C did not promote budbreak or growth. Budbreak and growth had significant interactions between treatment duration and temperature. The seedling type and treatment duration interaction was significant for terminal shoot length, lateral budbreak, and leaf dry weight, but were probably the result of differences between the seedling types before treatment and not true interactions with the length of the treatment. There was a significant interaction between the seedling type and treatment temperature on terminal shoot growth. Subsequent shoot growth did not differ significantly between the seedling types after similar chilling treatments. Thus, shoot growth was the best indicator of the chilling process of `Johnson Elberta' peach seedlings. Indicators of dormancy removal such as lateral budbreak or terminal shoot growth after 17 days forcing were not good predictors of subsequent seedling growth.

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James W. Frisby and Schuyler D. Seeley

Cuttings from peach (Prunus persica (L.) Batch cv. Johnson Elberta) trees were subjected to five chilling durations (20 to 76 days) at seven temperatures (0 to 14C) and five concentrations of gibberellic acid [GA, (0.0 to 1 mm)]. There was a significant increase in terminal shoot length with longer chilling treatments. Cuttings treated at 2 to SC had the longest shoots, and shoot length decreased, in order, following treatment at 10, 0, and 14C. Treatment with the highest concentration of GA, resulted in the longest terminal shoots. Interactions between GA and chilling durations indicated that either higher concentrations of GA, or longer chilling treatments increased terminal shoot growth. Thus, endogenous promoters, like GA, are evidently produced or released during chilling. Sensitivity to GA, was also important. Chemical name used: Gibberellic acid (GA3).

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James W. Frisby and Schuyler D. Seeley

We determined whether the chilling process (endodormancy release) was similar in peach [Prunus persica (L.) Batch cv. Johnson Elberta] seeds, seedlings (near normal to physiologically dwarfed), and mature plants (cuttings) by comparing correlation coefficients of various growth measurements following similar chilling treatments. Seed germination (10 days after forcing at 20C) and seedling emergence (15 days after forcing in the greenhouse) correlated highly with leaf and shoot growth (56 days of growth) of seedlings and terminal shoot growth of cuttings (13 days after forcing). The correlations were higher for germination than for emergence. Initial (first season) seedling growth correlated poorly with germination, emergence, budbreak, and growth of seedlings (second season) and shoot growth of cuttings. Budbreak and growth of seedlings correlated highly with shoot growth of cuttings. The abnormal leaf problem, which can cause apex abortion (common with initial seedling growth), confounded correlations with initial seedling growth. Yet, the abnormal leaf problem did not hinder correlations with the second seasons growth. Good relationships between the chilling mechanisms that promoted germination, emergence, budbreak, and growth of seedlings and shoot growth of cuttings existed, but were dependent on what was measured and when the measurement was taken. Germination (forced at 20C) was the most accurate indicator of the seed chilling status for comparisons with the responses of the other propagules.

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James W. Frisby and Schuyler D. Seeley

Correlations were made between the responses of seeds, physiologically dwarfed seedlings and dormant cuttings to similar chilling treatments. Seed germination correlated highly with growth of physiologically dwarfed seedlings and shoot growth of dormant cuttings. Emergence and seedling growth correlated poorly with germination, growth of physiologically dwarfed seedlings and shoot growth of dormant cuttings. Thus, germination was a better seed predictor of the mature peach chilling response than emergence or seedling growth. Growth of dwarfed seedlings correlated highly with shoot growth of dormant cuttings. The anomalous leaf condition of peach seedlings may have confounded seedling growth after seed chilling, but was not a problem when the chilling treatment was provided to physiologically dwarfed seedlings. The dormancy release mechanisms that promoted seed germination, growth of physiologically dwarfed seedlings and growth of dormant cuttings were similar.