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Lisa Tang, Shweta Chhajed, Tripti Vashisth, Mercy A. Olmstead, James W. Olmstead, and Thomas A. Colquhoun

During the fall in temperate climates, buds of deciduous woody perennials, including peach, enter dormancy, which is a state in which the visible growth of a plant structure containing a meristem is absent ( Lang et al., 1987 ). Bud dormancy has two

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L.H. Fuchigami and M. Wisniewski

The purpose of this presentation is to discuss the value of identifying growth stages of bud dormancy numerically. The Degree Growth Stage Model (°GS Model) will be used to quantify the annual growth stages and the various developmental stages of endo-, eco-, and paradormancy. The model is divided into 360°GS's, illustrated either as a sine curve or a circle, that serve as a timeline for the cyclical passage of temperate woody plants, through five distinct point events (growth stages). The sine curve illustrates the relative degree of development of the segment events between the point events. This paper will focus on the °GS model as a relative method of quantifying the various segment events and improving our communication of the annual physiological processes of temperate woody plants. In addition, recent evidence on altering dormancy, and its impact on dormancy models, will be presented.

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Miklos Faust

At the beginning and near to the end of the endodormant period, cytokinin-type growth regulators are effective to end dormancy in apple. The same growth regulators are not effective during the middle of this period. Terminal buds require less chilling than lateral buds to emerge from the dormant period. Lateral buds on decapitated shoots also require less chilling, indicating that auxin may be involved in dormancy. Replacing the terminal with IAA keeps water in bound state in the lateral buds, indicating the effect of IAA in dormancy. We have developed the theory that the beginning and the end of the winter-dormant period is governed by apical dominance. It appears that only this period can be manipulated either with dormancy avoidance methods or with dormancy-breaking chemicals. The central portion of the dormant period is not subject to manipulation. Therefore, it is important that the depth of the dormancy is quantified. Certain growth regulators can be used for determining the state of bud dormancy. Thidiazuron gives results within 2 to 4 days.

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Naohiro Kubota and Mayumi Miyamuki

Four dormant grapevine (Vitis spp.) cultivars grown under forced conditions were treated immediately after pruning with a paste of fresh garlic, the supernatant of a 20% suspension of CaCN2, and 50% `Merit' solution a foliar fertilizer, to break bud dormancy. Garlic paste significantly accelerated budbreak and increased the rate of budbreak in three cultivars, but for `Delaware,' CaCN2 was more effective. Garlic also affected budbreak of cuttings with a single bud of `Kyoho,' `Neo Muscat,' or `Muscat Bailey A' grapes in a deep stage of dormancy. Garlic paste applied to cool `Muscat of Alexandria' vines immediately after they were pruned in various stages of dormancy (from November to March) accelerated budbreak in the spring when application was made by January. The rate of budbreak was steady, but earlier CaCN2 treatments resulted in a rate of budbreak that was not uniform, although the first budbreak was accelerated. Low concentrations of garlic juice did not promote budbreak of cuttings of `Muscat of Alexandria'. When garlic paste was applied to various parts of cuttings with two buds, budbreak was accelerated when the upper cross-section was treated, but the overall rate of budbreak was highest when the lower half of the cutting was treated. Chemical name used: calcium cyanamide (CaCN2).

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Roberto Hauagge and James N. Cummins

Dormancy patterns throughout the season were studied in more than 90 apple (Malus ×domestica Borkh.) cultivars and related Malus spp. The seasonal apple bud dormancy pattern resembles a normal curve: it starts to intensify soon after bud formation and reaches maximum intensity by the time of leaf fall/senescence. Genotypes were grouped into three general classes based on maximum dormancy intensity. Maximum intensity of bud dormancy measured in cold winters is inversely related to adaptation to the subtropics. Low-chilling requirement (CR) cultivars have a shallow depth of dormancy with very little alteration throughout the year. High-CR cultivars have intense bud dormancy, the first stage of which can be induced by growing these cultivars at temperatures above 20C. Genotypes differed in their rates of dormancy dissipation. The efficiency of chilling unit (CU) accumulation to break dormancy was negatively correlated with CR, which indicates the importance of factors other than CU accumulation in terminating bud dormancy in low-CR cultivars. The inherent length of bud dormancy plays a major role in determining the time of budbreak in the spring. Deviations may be related to the genotypic efficiency in which chilling modifies dormancy and possibly the basal temperatures to which buds respond. Chill unit requirement and heat unit requirement are dependent factors. Heat requirement comparisons may be meaningless if the dormancy intensities of the genotypes are not taken into consideration.

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Anne Fennell and Emily Hoover

The grape species Vitis labruscana Bailey and V. riparia Michx. were subjected to a decreasing photoperiod at constant moderate temperatures to determine whether acclimation occurred in response to a shortening photoperiod. Cane growth, periderm development, killing temperature of the primary bud, and bud dormancy were measured in vines receiving a natural photoperiod (ND), a simulated long photoperiod of 15 hours (LD), and shorter photoperiods of 14, 13, or 12 hours (SD). The LD treatment was effective at maintaining growth and inhibiting periderm development and the onset of bud dormancy in V. labruscana. Cane growth rate with all SD treatments decreased as compared to the LD regime. A significant increase in periderm development occurred with the 12-hour SD treatment. Similarly, the onset of bud dormancy was promoted by the 12-hour SD in V. labruscana. The primary bud killing temperature was 1C lower in V. labruscana under the 12-hour SD than under the LD treatment. In contrast, the LD treatment neither maintained growth nor fully inhibited periderm development and the onset of dormancy in V. riparia. The decrease in the cane growth rate upon exposure to SD was significantly greater in V. riparia than V. labruscana. Periderm development was observed in both the SD and its respective LD-treated V. riparia vines. However, the rate of periderm development was significantly greater in the SD vines than in the LD vines. The onset of bud dormancy was promoted by 13-hour SD in V. riparia. Similarly, the primary bud killing temperature was 2 to 3C lower in V. riparia upon exposure to SD. Vitis riparia has a longer critical photoperiod than V. labruscana and appears to be more sensitive to changes in light intensity or light quality. While the change in freezing tolerance in response to short photoperiods is small, the photoperiod response at a longer critical photoperiod, when combined with lower temperatures, will promote an earlier and possibly more rapid cold acclimation in V. riparia than in V. labruscana.

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Roberto Hauagge and James N. Cummins

The chilling requirements (CR) to break bud dormancy in a broad range of apple cultivars (Malus ×domestica Borkh.) and related Malus spp. were assessed by periodic sampling and forcing of field-grown shoots as a function of chill unit (CU) accumulation and/or by the total growing degree hours (GDH) accumulated from leaf fall until the time of budbreak under a simulated subtropical winter. The mean number of CU required to break dormancy of field overwintered shoots varied between 218 ± 113 for `Anna' and 1516 ± 113 for `Wright #1'. However, most genotypes had CR between 800 and 1200 CU. Much wider variation for the length of bud dormancy was observed in plants growing under simulated subtropic winter conditions. Genotypes that had shown the lowest CR values under Geneva, N.Y., winters generally had the highest year-to-year variation in CR estimates. Cultivar bud CR values obtained under cold winters are related to field-observed CR estimates in a subtropical environment, but absolute values may differ markedly. Furthermore, several genotypes that show reasonable adaptation to the subtropics have similar or higher CR than apple cultivars with standard CR under Geneva conditions. In addition, enough CU accumulated under the simulated subtropic winters to break dormancy of standard apple cultivars. However, complete dormancy removal was observed only in cultivars well-adapted to a subtropical environment. This result indicates that in addition to CU accumulation, there are important interactions among cultivars and environmental factors that are responsible for terminating bud dormancy. Several cultivars and wild species have shown resistance to delayed foliation. Among the species, M. brevipes, M. rockii, M. spectabilis, and M. turesii are more tolerant than M. baccata and its hybrids, which are recognized for their adaptation to the subtropic environment.

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Guochen Yang and Paul E. Read

A certain period of cold is needed to break bud dormancy for almost all woody species. A pre-forcing bleach soak has been demonstrated to at least partially replace this requirement (Yang and Read, 1989). Therefore, new softwood growth can be produced in the off-season. Such supple softwood growth is excellent material to be used either as explants for in vitro culture, or as cuttings for macropropagation of woody species. Further studies on pre-forcing bleach soaks were conducted to investigate optimum concentration and duration of soak, and to find the most suitable depth of bleach solution soak, in order to maximize the breaking of bud dormancy. Optimum bud break was obtained by soaking the basal 1/3 of dormant stems in 10% bleach solution for 10 minutes prior to forcing. Soaking dormant woody stems in alcohol solutions prior to placing stems in the forcing solution was also studied. The alcohol soak had negative effects on bud break of spirea, although it showed positive effects for lilac and privet.

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Karen E. Burr, Stephen J. Wallner, and Richard W. Tinus

Greenhouse-cultured, container-grown seedlings of interior Douglas fir [Pseudotsuga menziesii var. glauca (Beissn.) France], Engelmann spruce [Picea engelmannii (Parry) Engelm.], and ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) were acclimated and deacclimated to cold in growth chambers over 19 weeks. Heat tolerance and cold hardiness of needles, and bud dormancy, were measured weekly. Heat tolerance of Douglas fir and Engelmann spruce needles increased with development through the first complete annual cycle: new needles on actively growing plants; mature needles, not cold-hardy, on dormant plants; cold-hardy needles on dormant and quiescent plants; and mature, needles, not cold-hardy, on actively growing plants. Heat tolerance of ponderosa pine needles differed in two respects. New needles had an intermediate tolerance level to heat, and fully cold-hardy needles were the least tolerant. Thus, the physiological changes that conferred cold hardiness were not associated with greater heat tolerance in all the conifers tested. In none of these species did the timing of changes in heat tolerance coincide consistently with changes in cold hardiness or bud dormancy.

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Aliza Benzioni, David A. Palzkill, and John M. Nelson

Flower bud dormancy and anthesis patterns, ABA concentration, and bud survival following frost were studied in eight jojoba [Simmondsia shinensis (Link) Schneider] clones grown under two irrigation regimes (water stress and well irrigated). Several clones broke dormancy in the autumn. Anthesis in the field before winter occurred only in one clone (a male) in the well-irrigated treatment. Buds on water-stressed plants broke dormancy earlier than those on well-watered plants, but anthesis in the field occurred later in the year. Buds on water-stressed plants were less affected by a severe frost than those on control plants (43% vs. 10% survival). There were large differences among clones in the amount of frost damage. ABA levels did not correlate with dormancy patterns or with the amount of frost damage. Chemical name used: S-(Z,E)-5-(1-hydroxy-2,6,6-trimethyl-4-oxo-2-cyclohexen-1-yl)-3-methyl-2,4-pentadienoic acid (ABA).