Bud Hardiness Research Articles

PHENOLOGICAL PROCESSES OF DORMANCY IN APRICOT GENOTYPES IN THE CENTRAL PART OF THE CARPATHIAN BASIN

The flower bud development of traditional Hungarian and foreign apricot cultivars has been examined by the evaluation of microsporogenesis and the lengthwise growth of pistils in the central part of Hungary in the last 15 years. Substantial differences were observed between the long and short shoots both in the rate of microsporogenesis and pistil elongation. In the process of microsporogenesis six developmental stages were distinguished, while pistil development could be divided into four phases. Significant differences were found between varieties and years in the rate of microsporogenesis, indicating considerable variability in bud development within the apricot species and the sensitive response of apricot to climate changes. Parallel with the warming of climate, a delay in the end of endodormancy could be observed; in mild winters a longer period was required for the buds to saturate their cold requirements. At the same time, the pollen development and the flowering of the trees took place earlier due to warmer springs. Parallel with the development processes, the frost hardiness of the flower buds was also tested by means of artificial freezing. No close correlation could be demonstrated between frost hardiness and the development stage due to differences in the genetic regulation, but genotypes with a slower rate of development exhibited the best absolute frost resistance in the course of the winter.

2010年の茶芽の生育と耐凍性（静岡県茶業研究センター）

2010年の茶芽の生育と耐凍性（静岡県茶業研究センター）

Cold Hardiness of Floral Buds of Deciduous Azaleas: Dehardening, Rehardening, and Endodormancy in Late Winter

Dehardening resistance and rehardening capacity in late winter and spring are important factors contributing to the winter survival of woody perennials. Previously the authors determined the midwinter hardiness, dehardening resistance, and rehardening capacities in deciduous azalea (Rhododendron L.) floral buds in early winter. The purpose of this study was to investigate how these parameters changed as winter progressed and to compare rehardening response at three treatment temperatures. Experiments were also conducted to measure bud water content during dehardening and chilling accumulation of 10 azalea genotypes. Buds of R. arborescens (Pursh) Torr., R. canadense (L.) Torr., R. canescens (Michx.) Sweet, and R. viscosum (L.) Torr. var. montanum Rehd. were acclimated in the field and were dehardened in the laboratory at controlled warm temperatures for various durations. Dehardened buds were rehardened for 24 hours at 2 to 4 °C, 0 °C, or –2 °C. Bud hardiness (LT50) was determined from visual estimates of freeze injury during a controlled freeze–thaw regime. The midwinter bud hardiness in the current study was ≈4 to 8 °C greater than in early winter. R. canadense and R. viscosum var. montanum dehardened to a larger extent in late winter than in the early winter study whereas R. arborescens and R. canescens did not. The rehardening capacities were larger in early than in late winter. Even though rehardening occurred throughout the first 8 days of dehardening (DOD) in early winter in the previous study, in the current study it was only observed after 10 DOD (R. viscosum var. montanum) or 15 DOD (R. arborescens). There was no difference among the rehardening capacities at the three rehardening temperatures for any genotype. Water content decreased throughout dehardening in all four genotypes examined. R. canadense had the lowest chilling requirement (CR) [450 chilling units (CU)], followed by R. atlanticum (Ashe) Rehd., R. austrinum (Small) Rehd., R. canescens, and R. calendulaceum (Michx.) Torr. with intermediate CR [820, 830, 830, and 1000 CU respectively). The CR of R. arborescens, R. prinophyllum (Small) Millais, R. prunifolium (Small) Millais, R. viscosum var. montanum, and R. viscosum var. serrulatum (Small) Millais exceeded 1180 CU. Results of this study indicate that the dehardening kinetics (magnitude and rate) and the rehardening capacity of azalea buds are influenced by the progression of winter and the depth of endodormancy.

Cold-Hardiness Evaluation of Grapevine Buds and Cane Tissues

A system for differential thermal analysis (DTA) was constructed to assess cold hardiness of grapevine buds and cane tissues. This updated system incorporated a sample chamber of our own design with a commercially available programmable freezer and data acquisition system (DAS). Thermoelectric modules (TEM) were used to sense exotherms that are produced when water or tissues freeze. The TEM signals recorded by the DAS at 15 sec intervals were downloaded directly to an Excel spreadsheet. The DTA system was designed to test up to 35 samples of five buds or three canes per TEM simultaneously. Bud and cane low temperature exotherms (LTE) recorded by this system correlated very closely with those of a standard system, and the extent of cane phloem and xylem injury, based on tissue browning, corresponded well with expected injury based on LTE analysis. The LTEs of moist buds were 3°C to 4°C higher and those of moist canes 2°C higher than LTEs of corresponding dry tissues, indicating that surface moisture increases the susceptibility to cold injury. Cold hardiness of eight grape cultivars increased from late fall through mid-January, after which buds and canes began to deacclimate. Riesling was the hardiest of all cultivars tested. Chardonnay reached similar levels in midwinter, but was considerably less hardy in late fall and late winter. Pinot gris and Viognier were the least hardy among the white winegrape cultivars. Among red winegrape cultivars, Cabernet Sauvignon was generally the hardiest and Merlot the least hardy, with Malbec and Syrah being intermediate.

Evaluation of Midwinter Cold Hardiness among 25 Rabbiteye Blueberry Cultivars

The midwinter cold hardiness of 25 rabbiteye (V. ashei) blueberry cultivars was assayed across 2 years using a shoot freezing assay. LT50values (i.e. temperature at which 50% of buds are damaged) for the cultivars ranged from –24.9 °C for `Pearl River' (a 50% V. ashei derivative) to –13.7 °C for `Chaucer'. Under New Jersey conditions, numerous cultivars were observed to exhibit dimorphism for dormant floral bud size. Comparisons of bud dimorphism with LT50 values, found dimorphism more common in cultivars with lower floral bud hardiness. LT50 values generally supported empirical observations of winter hardiness, but exceptions suggest that additional factors contribute to observed winter hardiness under field conditions.

Frost Hardiness of some Turkish Apricot Cultivars during the Bloom Period

The frost hardiness of five apricot (Prunus armeniaca L.) cultivars used for drying and/or the fresh market was investigated under controlled conditions and in the orchard. The hardiness of flower buds at three different development stages, such as first white, first bloom, and full bloom, was tested at –4 °C for 1 hour and 3 hours in the laboratory. The flower buds of `Kabaasi', `Sekerpare', and `Alyanak' were hardier. The field observations obtained from the apricot orchard where the late frost occurred at night on 3 to 4 Apr. 2004 supported this result, and the temperatures at frost date varied from –2 to –9 °C.

Effect of Thawing Time, Cooling Rate and Boron Nutrition on Freezing Point of the Primordial Shoot in Norway Spruce Buds

Effects of cooling rates on bud frost hardiness have been studied but there is little information on bud responses to thawing. Since the cell wall pore size has been found to increase with boron (B) deficiency, B deficiency may affect the supercooling ability of buds in winter. The effects of duration of thawing time and rate of cooling on bud frost hardiness of Norway spruce (Picea abies) were studied in a B fertilization trial in February 2003 and March 2005. Frost hardiness of apical buds was determined by differential thermal analysis (DTA) and visual scoring of damage. In 2003, the freezing point of primordial shoots of buds (T(f)), i.e. the low-temperature exotherm (LTE), was, on average, -39 degrees C when buds were thawed for less than 3 h and the T(f) increased to -21 degrees C after 18 h of thawing. During the first 4 h of thawing, the rate of dehardening was 6 degrees C h(-1). In 2005, buds dehardened linearly from -39 degrees C to -35 degrees C at a rate of 0.7 degrees C h(-1). In 2003, different cooling rates of 1-5 degrees C h(-1) had a minor effect on T(f) but in 2005 with slow cooling rates T(f) decreased. In both samplings, at cooling rates of 2 and 1 degrees C h(-1), T(f) was slightly higher in B-fertilized than in non-fertilized trees. By contrast, at very short thawing times in 2003, T(f) was somewhat lower in B-fertilized trees. There was little evidence of reduced frost hardiness in trees with low B status. This study showed that buds deharden rapidly when exposed to above-freezing temperatures in winter, but if cooled again they reharden more slowly. According to this study, rapid dehardening of buds has to be taken into account in assessments of frost hardiness.

Autumn frost hardiness in Norway spruce plus tree progeny and trees of the local and transferred provenances in central Sweden

Reforestation with provenances from locations remote from the planting site (transferred provenances) or the progeny of trees of local provenances selected for superior form and vigor (plus trees) offer alternative means to increase yield over that obtained by the use of seed from unselected trees of the local provenance. Under Swedish conditions, Norway spruce (Picea abies (L.) Karst.) of certain transferred provenances generally has an advantage in productivity relative to the local provenance comparable to that of progeny of plus trees. The aim of this study was to explore the extent to which productivity gains achieved by provenance transfer or the use of plus tree progeny are associated with reductions in autumn frost hardiness, relative to that of trees of the local provenance. In a field trial with 19-year-old trees in central Sweden, bud hardiness was tested on four occasions during the autumn of 2002. Trees of the local provenance were compared with trees of a south Swedish provenance originating 3 degrees of latitude to the south, a Belarusian provenance and the progeny of plus trees of local origin. The Belarusian provenance was the least hardy and the local provenance the most hardy, with plus tree progeny and the south Swedish provenance being intermediate in hardiness. Both the Belarusian provenance and the plus tree progeny were significantly taller than trees of the other populations. Within provenances, tree height was negatively correlated with autumn frost hardiness. Among the plus tree progeny, however, no such correlation between tree height and autumn frost hardiness was found. It is concluded that although the gain in productivity achieved by provenance transfer from Belarus was comparable to that achieved by using the progeny of plus trees of the local provenance, the use of trees of the Belarus provenance involved an increased risk of autumn frost damage because of later hardening.

(435) Physiological Study of Deacclimation and Reacclimation in Deciduous Azalea (Rhododendron subgenus Pentanthera) Species

Winter survival of temperate-zone woody perennials requires them to resist loss of frost hardiness (deacclimation) during winter and early spring thaws. However, little is known about deacclimation response in woody landscape plants. Moreover, what impact, if any, the degree of deacclimation has on reacclimation capacity has not been systematically studied. We used nine genotypes of deciduous azaleas (Rhododendron subgenus Pentanthera) to investigate effects of deacclimating conditions on bud cold hardiness and reacclimation ability. Dormant floral buds, with 3–5 cm stem attached, were collected in late December from field-grown plants, and placed in constant warm [22 °C 15 °C (D/N)] and humid conditions for increasing durations (0-day to 14-day) to stimulate deacclimation. Bud cold hardiness (lt50) was determined (using logistic regressions) by evaluating immature flower survival at subfreezing treatment temperatures. Results indicated that azalea genotypes from colder provenances showed greater initial frost hardiness. Typically northern genotypes had slow to intermediate deacclimation rates, while rates of southern genotypes were intermediate to rapid. High initial frost hardiness was frequently associated with slow deacclimation. Buds retained the capacity to reacclimate upon cold exposure [2 °C/–2 °C; (12 h/12 h)] even after 8 days of deacclimation. Distinct differences were observed between the two latitudinal ecotypes of R. viscosum with respect to their initial bud hardiness, deacclimation rates, and reacclimation capacities. We suggest that the three attributes, i.e., high initial hardiness, slow deacclimation, and high reacclimation capacity, together may be important for winter-survival of azalea buds.

Cold Hardiness, Deacclimation Kinetics, and Bud Development among 12 Diverse Blueberry Genotypes under Field Conditions

Deacclimation response is an important part of reproductive success in woody perennials because late winter or early spring thaws followed by hard freezes can cause severe injury to dehardened flower buds. There is a need to develop more spring-frost tolerant cultivars for the blueberry (Vaccinium L.) industry. The identification of later or slower deacclimating genotypes could be useful in breeding for more spring-frost tolerant cultivars. This study was undertaken to investigate cold hardiness and deacclimation kinetics under field conditions for 12 Vaccinium (section Cyanococcus A. Gray) genotypes (the cultivars Bluecrop, Duke, Legacy, Little Giant, Magnolia, Northcountry, Northsky, Ozarkblue, Pearl River, Tifblue, and Weymouth; and a population of V. constablaei Gray) with different germplasm compositions and expected mid-winter bud hardiness levels. Examination of bud cold hardiness (BCH) vs. weeks of deacclimation over a 7-week period in 2 consecutive years (2002 and 2003) revealed clear genotypic differences in cold hardiness and timing and rate of deacclimation. Among cultivars, `Legacy' was the least cold hardy at initial evaluation, even less so than `Tifblue'. Regarding deacclimation kinetics, the weekly intervals with the largest losses (i.e., high rates of deacclimation) also varied among genotypes. For `Duke', the largest losses in BCH were detected at weeks 2 and 3, making it the earliest deacclimator. For `Bluecrop', `Ozarkblue', `Weymouth', `Tifblue', and `Legacy', the greatest losses in BCH were observed at weeks 3 and 4. For `Little Giant', `Magnolia', `Northcountry', `Northsky', and `Pearl River', losses in BCH were greatest at weeks 4 and 5, while for V. constablaei, losses were greatest at weeks 6 and 7, making it the latest deacclimator. Deacclimation kinetics were not correlated with mid-winter hardiness or chilling requirements in any fixed pattern. On the other hand, a strong positive correlation was found between BCH and stage of bud opening (r = 0.84). A comparison of timing of deacclimation with germplasm composition indicated that V. constablaei was particularly late to deacclimate. `Little Giant', a 50:50 hybrid of V. constablaei and V. ashei Reade, was nearly as late to deacclimate as the 100% V. constablaei selections. Thus, V. constablaei may be useful in breeding programs to contribute genes for late deacclimation, which should translate into greater spring frost tolerance, in addition to genes for mid-winter hardiness.

Dehardening Kinetics, Bud Development, and Dehydrin Metabolism in Blueberry Cultivars during Deacclimation at Constant, Warm Temperatures

Loss of freeze tolerance, or deacclimation, is an integral part of winter survival in woody perennials because untimely mid-winter or spring thaws followed by a hard freeze can cause severe injury to dehardened tissues. This study was undertaken to investigate deacclimation kinetics, particularly the timing and speed, of five blueberry (Vaccinium L.) cultivars (`Bluecrop', `Weymouth', `Ozarkblue', `Tifblue', and `Legacy'), with different germplasm compositions and mid-winter bud hardiness levels, in response to an environmentally controlled temperature regime. Based upon bud cold hardiness evaluations in 2000 and 2001, `Tifblue', a Vaccinium ashei Reade cultivar, was one of the least hardy and the fastest to deacclimate; `Bluecrop', a predominantly V. corymbosum L. cultivar, was the most hardy and the slowest to deacclimate; and `Ozarkblue', a predominantly V. corymbosum cultivar but including southern species V. darrowi Camp. and V. ashei, was intermediate in speed of deacclimation. `Weymouth' (predominantly V. corymbosum) and `Legacy' (73.4% V. corymbosum and 25% V. darrowi) were slow to intermediate deacclimators. Deacclimation rates did not correlate strictly with mid-winter bud hardiness. Data suggest that the southern germplasm component V. ashei may be responsible for the observed faster deacclimation whereas both southern species, V. darrowi and V. ashei, may contribute genes for cold sensitivity. Strong positive correlations between stage of bud opening and bud cold hardiness existed in both years (r = 0.90 and 0.82 in 2000 and 2001 study, respectively). Previously identified major blueberry dehydrins, 65-, 60-, and 14-kDa, progressively decreased in their abundance during incremental dehardening in `Bluecrop', `Weymouth', and `Tifblue'. However, down-regulation of the 14-kDa dehydrin most closely mirrored the loss in cold hardiness during deacclimation, and, therefore, may be involved in regulation of bud dehardening. Because differences in deacclimation rate were clearly evident among the genotypes studied, rate of deacclimation of the flower buds of blueberry should be an important consideration in breeding to improve winter survival.

Supercooling and Cold Hardiness in Sour Cherry Germplasm: Flower Buds

Flower buds of two sweet cherry (Prunus avium L.), 12 sour cherry (Prunus cerasus L.) and one ground cherry (P. fruticosa Pall.) were collected monthly from Aug. 1990 to Mar. 1991, and subjected to freeze tests to determine the level of cold hardiness. LT50 values (temperatures at which 50% of the flower buds were killed) summed over all months were significantly correlated (r = 0.6844, P ≤ 0.01) to the flower bud low temperature exotherms (LTEs). Correlation of LTEs to LT50 values was highest, r = 0.85, P ≤ 0.01 for the acclimation and midwinter period, November to February collections. During this period the average LT50 occurred before and within 2.5 °C of the LTE, indicating tissue injury before the LTE occurrence. During deacclimation, represented by the March collection, the LT50 began within 2.0 °C, on average, of the LTE, and in 11 of 12 cultivars and seedlings preceded the LTE. In March, the correlation of LTEs to LT50 values was less, r = 0.69, P ≤ 0.05, indicating possible changes flower bud deep supercooling. LTE values were selected as a measure of flower bud hardiness in sour cherry. Exotherms were not detected in the flower buds of all germplasm tested on all evaluation dates, but were the best means of separating selections. While LTE analyses expressed significant differences in November, December, and March at P ≤ 0.01, the LT50 analyses expressed differences only in December and February at P ≤ 0.05. The relationship between ambient temperatures and floral tissue hardiness indicated that November and March are two critical times for flower bud injury. November injury would occur in years when sudden low temperatures occur without sufficient pre-exposure to freezing temperatures. March injury would occur in years when sudden freezing temperatures follow warm days. This type of injury would be most pronounced in southern genotypes. Spring freeze injury could be significantly reduced by the selection of cultivars and seedlings that have delayed deacclimation. Exotherm occurrence and bud volume were correlated (r = 0.95, P ≤ 0.05). In January, when exotherms were least prevalent, they were generally present only in the five cultivars and seedlings with large bud volumes. The LTEs in midwinter, occurred within 3 °C of the reported average annual minimum temperature for the northern range of Prunus commercial production (Zone 6). The results of the principal component analysis of flower bud LTEs indicated that other selection criteria as flowering time might have played a more significant role in the hardiness range of sour cherry than simply geographic origin. The first principal component (PC1), which accounted for 77% of the total variance was used to separate among cultivars and seedlings. Selections at the positive end of PC1 had flower buds that were more cold susceptible than selections at the negative end of PC. This concurs with other research showing that flower bud hardiness is related more to commercial range (i.e., the range of commercial production) than to geographic distribution.

BUD HARDINESS AND DEACCLIMATION IN BLUEBERRY CULTIVARS WITH VARYING SPECIES ANCESTRY: FLOWERING TIME MAY NOT BE A GOOD INDICATOR OF DEACCLIMATION

ISHS XXVI International Horticultural Congress: Berry Crop Breeding, Production and Utilization for a New Century BUD HARDINESS AND DEACCLIMATION IN BLUEBERRY CULTIVARS WITH VARYING SPECIES ANCESTRY: FLOWERING TIME MAY NOT BE A GOOD INDICATOR OF DEACCLIMATION

CONNECTION BETWEEN THE DEVELOPMENTAL STAGE AND THE COLD HARDINESS OF PEACH CULTIVARS

The frost tolerance of 12 peach (Prunus persica (L.) Batsch) varieties was determined in artificial freezing tests, while the developmental rate was examined through microscopic study of microsporogenesis. The frost tolerance of flower buds continuously increased from autumn till December, then started to decrease. There was a high correlation between the rate of microsporogenesis and the change of cold hardiness of the flower buds during winter. The slower the rate of microsporogenesis the better the cold hardiness of a variety. 'Piroska', a native Hungarian variety had slow microsporogenesis and the best frost tolerance. Of the foreign varieties, the frost tolerance of 'Champion' was the best. In addition 'Early Redhaven', 'Harko', 'Redhaven' and 'Babygold 6' possessed a suitable level of frost tolerance. The floral bud developmental rate was quick for the varieties 'Fairlane', 'Red June' and 'Michelini', which was paralleled by quick decrease of their frost tolerance in the second half of winter. 'Venus', 'Springcrest' and 'Mayfire' are not recommended for Hungary, due to their poor frost tolerance.

Effect of temperature and exposure time on cold hardiness of primary buds during the dormant season in `Concord', `Norton', `Vignoles' and `St. Vincent' grapevines

SummaryPrimary bud cold hardiness in `Concord', `Norton', `Vignoles', and `St. Vincent' grapevines was studied at Southwest Missouri State University Research Campus in Mountain Grove, Missouri, USA. Canes from mature vines were exposed to 210, 25, 0, 10 or 20°C for 2, 4, 6 or 8.d in early December, late January, and early March during the 1998–99 winter season. Cold hardiness of primary buds was in¯uenced by temperature, exposure time, and dormant period. Temperatures of 10 or 20°C had similar deacclimating effects on primary bud cold hardiness of all cultivars during the entire winter. Cold hardiness of primary buds exposed to 0¼ was greater than that of buds exposed to 10 or 20°C only in early March for all cultivars and late January for `Concord'. Exposure to 0°C was as deacclimating as that to 10 or 20°C otherwise. Cold hardiness of primary buds becomes more sensitive to temperature as dormancy progresses. Longer exposure was advantageous for cold hardiness of primary buds only at 25 or 210°C. `Concord' and `Norton' were generally more sensitive to temperature than were `Vignoles' and `St. Vincent'. The greater cold hardiness of the tested non-vinifera cultivars is due to both their greater cold hardiness at high temperature and their sensitivity to temperature change, acclimating faster and deeper at lower temperature. During the early and middle dormant season, exposure to 10 to 20°C for 2.d was sufficient to deacclimate primary buds to their minimum cold hardiness. However, exposure to 210°C even for 8.d was not always sufficient to acclimate them to their maximum cold hardiness.

Acclimation and deacclimation of primary bud cold hardiness in ‘Norton’, ‘Vignoles’ and ‘St. Vincent’ grapevines

SummaryAcclimation and deacclimation of primary bud cold hardiness in ‘Norton’, ‘Vignoles’, and ‘St. Vincent’ grapevines were studied at Southwest Missouri State University Research Campus in Mountain Grove, Missouri, USA. Canes from mature vines were acclimated at -10°C for 7 d and then subjected to deacclimation at 20°C for 1,2,3,5 and 7 d in January and February 1997.During 1997-98 winter season, they were acclimated at -7°C and deacclimated at 20°C for 1, 2, 3, 5, and 7 d in December, February, and March. Cold hardiness of primary buds under field temperature conditions was also evaluated at the same time as the buds treated under laboratory conditions. Cold hardiness of primary buds was influenced by temperature regimes, and the length of exposure time to acclimating and deacclimating temperatures. ‘Norton’ was the most cold hardy and ‘St Vincent’ was the least cold hardy cultivar, while ‘Vignoles’ was least sensitive to acclimating and deacclimating temperatures. Cold hardiness of primary buds decreased gradually while their sensitivity to acclimating and deacclimating temperatures increased, from December to March. Exposure to -7 or -10°C increased cold hardiness and exposure to 20°C decreased cold hardiness during certain sampling periods but not always so. It seems that there is a maximum level of cold hardiness above which primary buds will not acclimate and there is a minimum level of cold hardiness below which primary buds will not deacclimate.

Provenances and families show different patterns of relationship between bud set and frost hardiness in Picea abies

We have compared bud set and frost hardiness among Norway spruce (Piceas abies (L.) Karst.) provenances and families in two cold-acclimation regimes in a phytotron; low light intensity and high night temperatures (LL-HNT), and high light intensity and low night temperatures (HL-LNT) under shortening day lengths. Nine provenances from 59-66°N and altitude 100-700 m within Norway, and nine open-pollinated families from a single stand (61°N, 270 m elevation) were used. Both provenances and families started bud set and frost hardening earlier in LL-HNT than in HL-LNT. Correlations between the same trait expressed in two regimes were high for both bud set and hardiness at the provenance level and slightly lower at the family level. The variation among family means in bud set and hardiness was large. The differences found between the family extremes were up to 75% of those found between provenance extremes. The relationship between bud set and frost hardiness was strong among the provenance means within both environments (r = 0.92) but weak for the families (r = 0.22-0.44). Causal factors influencing phenotypic variation within traits and covariation among traits may differ for provenances and families within stands. The strong relationships among traits that are found at the provenance level cannot be generalized to the levels of families or clones.

Provenances and families show different patterns of relationship between bud set and frost hardiness in <i>Picea abies</i>

We have compared bud set and frost hardiness among Norway spruce (Piceas abies (L.) Karst.) provenances and families in two cold-acclimation regimes in a phytotron; low light intensity and high night temperatures (LL-HNT), and high light intensity and low night temperatures (HL-LNT) under shortening day lengths. Nine provenances from 59-66°N and altitude 100-700 m within Norway, and nine open-pollinated families from a single stand (61°N, 270 m elevation) were used. Both provenances and families started bud set and frost hardening earlier in LL-HNT than in HL-LNT. Correlations between the same trait expressed in two regimes were high for both bud set and hardiness at the provenance level and slightly lower at the family level. The variation among family means in bud set and hardiness was large. The differences found between the family extremes were up to 75% of those found between provenance extremes. The relationship between bud set and frost hardiness was strong among the provenance means within both en...

Dormancy, Cold Hardiness, Dehardening, and Rehardening in Selected Red Raspberry Cultivars

Canes and flower buds of selected red raspberry cultivars (Rubus idaeus L. `Maurin Makea', `Muskoka', and `Ottawa') were sampled from a field (latitude, 61 °20'N; longitude, 24 °13'E) at 1-month intervals during Winter 1996-97 to study the interaction of dormancy and cold hardiness, hardiness retention, and rehardening capacity. One set of canes was subjected to dehardening (3 days) and two sets to dehardening + rehardening (3 and 7 days) treatments before cold hardiness determination. Maximum midwinter hardiness occurred in January, after breaking of endodormancy. Cold hardiness of canes and buds reached -28.6 to -37.2 °C and -24.2 to -31.6 °C, respectively. Throughout the winter, raspberry canes were hardier than buds. Endodormancy had a greater influence on dehardening and rehardening in buds than in canes, and cultivars differed in their response. Dehardening of `Maurin Makea' canes and buds, and `Muskoka' buds was slightly enhanced by breaking of dormancy, whereas dehardening in `Ottawa' was not affected by dormancy. Raspberry canes and buds could reharden even after dormancy release. Rehardening capacity was affected by the state of dormancy only in `Maurin Makea' buds. Changes in dormancy status failed to explain cultivar differences regarding dehardening and the capacity to reharden suggesting other factors may be involved.

Supercooling in Dormant Flower Buds of Forsythia, and the Correlation between Pistil Size and Bud Hardiness

Abstract Experiments were conducted to determine if dormant buds of Forsythia taxa exhibit the deep supercooling characteristic. Specimens were collected from thirteen Forsythia taxa including: F. suspensa (Thunb.) Vahl, F. x intermedia cv. Spectabilis (Koehne), F. x intermedia cv. Lynwood (G.E. Peterson), F. europaea (Degen and Baldacci), F. giraldiana (Lingelsh), F. japonica (Makino) var. saxatilis (Nakai), F. mandshurica (Uyeki), F. ovata (Nakai), F. suspensa var. fortunei (Lindl.), F. viridissima (Lindl.), F. x intermedia cv. Arnold Giant (Sax), F. cv. Arnold's Dwarf, and F. cv. Meadowlark (Flint). Buds and attached stem segments, were cooled at 2C (3.6F) per hour, and the temperature at which freezing occurred was determined by thermal analysis. Typically, two distinct freezing events were detected within Forsythia buds. The first freezing event, or high temperature exotherm, occurred just below 0C (32F), while the second freezing event, or low temperature exotherm, occurred between −16C (3.2F) and −28C (−18.4F). The low temperature exotherm corresponded to the freezing of supercooled water within dormant buds, and the detection of low temperature exotherms in buds of all 13 Forsythia taxa indicated that deep supercooling is common among members of this genus. In nine of the 13 Forsythia taxa, the temperature of the low temperature exotherm was an accurate indicator of bud freeze-tolerance (LT50), as determined by a laboratory freeze-stress protocol. The discrepancies noted in the other four taxa were apparently due to the occurrence of field freezing injury prior to conducting these laboratory studies. Evidence indicated a relationship between the extent of supercooling and the size of the pistil in dormant Forsythia buds.