Chilling Accumulation Research Articles

A biologically based approach to modeling spring phenology in temperate deciduous trees

Prediction of the timing of spring phenological events such as bloom and leaf-out has important uses in agricultural and ecological management and modeling. However, after decades of model comparison there remains no consensus model to predict the date of spring phenological events in perennial temperate trees across species and locations. This lack of consensus may be due to over-fitting resulting from high model complexity, use of parameters that have not been adequately validated, or omission of parameters that are sound biological indicators of dormancy breaking. The aim of this study was to construct spring phenology candidate models with biologically-based parameters and starting values to test hypotheses regarding chill accumulation duration and the impact of pre-bloom conditions. Bloom data for three cultivars of Prunus dulcis (almond) from three decades in California were analyzed. Across all three cultivars, models which accumulated chill until approximately 75% of the heat requirement had been met, and did not integrate pre-bloom conditions, were substantially supported by the data. This suggests cold temperatures affect dormancy breaking well after the chilling requirement has been met and bud break timing is not substantially impacted by environmental conditions just prior to bud break. Fitting spring phenology using biologically based starting values estimated from bud break records may allow for the development of improved predictive models and improved approximations of critical phenological thresholds.

Roles of endoplasmic reticulum stress and unfolded protein response associated genes in seed stratification and bud endodormancy during chilling accumulation in Prunus persica.

Dormancy mechanisms in seeds and buds arrest growth until environmental conditions are optimal for development. A genotype-specific period of chilling is usually required to release dormancy, but the underlying molecular mechanisms are still not fully understood. To discover transcriptional pathways associated with dormancy release common to seed stratification and bud endodormancy, we explored the chilling-dependent expression of 11 genes involved in endoplasmic reticulum stress and the unfolded protein response signal pathways. We propose that endoplasmic reticulum stress and the unfolded protein response impact on seed as well as bud germination and development by chilling-dependent mechanisms. The emerging discovery of similarities between seed stratification and bud endodormancy status indicate that these two processes are probably regulated by common endoplasmic reticulum stress and unfolded protein response signalling pathways. Clarification of regulatory pathways common to both seed and bud dormancy may enhance understanding of the mechanisms underlying dormancy and breeding programs may benefit from earlier prediction of chilling requirements for uniform blooming of novel genotypes of deciduous fruit tree species.

Winter fog is decreasing in the fruit growing region of the Central Valley of California

The Central Valley of California is home to a variety of fruit and nut trees. These trees account for 95% of the U.S. production, but they need a sufficient amount of winter chill to achieve rest and quiescence for the next season's buds and flowers. In prior work, we reported that the accumulation of winter chill is declining in the Central Valley. We hypothesize that a reduction in winter fog is cooccurring and is contributing to the reduction in winter chill. We examined a 33 year record of satellite remote sensing to develop a fog climatology for the Central Valley. We find that the number of winter fog events, integrated spatially, decreased 46%, on average, over 32 winters, with much year to year variability. Less fog means warmer air and an increase in the energy balance on buds, which amplifies their warming, reducing their chill accumulation more.

Climatic Characterization and Phenology of Local Peach Genotypes in the Udzungwa Uplands of Tanzania

The phenology of two local peach genotypes was studied to verify whether peaches can be produced in the Tanzanian uplands. Observations were conducted on trees of one early- and one late-ripening peach genotype in 2010 and 2011. Temperature trends were reconstructed and three chilling estimation models were tested. The two peach genotypes differed for timing of flower and leaf bud-break. Common symptoms of insufficient chilling (presence of numerous stages at a specific date and about 20% of undeveloped flower buds) were observed in the late genotype. Both Utah and Dynamic models indicated low chilling accumulation, lower than the Mean Temperature model in most cases. This is inconsistent with the actual phenology, suggesting that (1) both models show some limitations under our conditions, (2) the two genotypes partly adapted to the local climate, and (3) mechanisms other than chilling (seasonal rainfall fluctuations) play a role in rest control.

Climate, Winter Chill, and Decision-making in Sweet Cherry Production

Growers naturally adapt and manage production for unpredictable and variable seasonal climates; however, the projected changes in climate introduce a new concern with increased variability in the frequency and severity of extreme climatic events and sustained changes in temperature. This study addresses the complexity of providing clear winter chill information in a form that facilitates grower assimilation and application of that information, ultimately to assist decision-making around variety selection and management. This study established the chill requirement for two commonly grown sweet cherry (Prunus avium L.) varieties: ‘Kordia’ and ‘Sweetheart’, and explored the uniformity of bud burst in each. It was found that varieties required different chill hours and that ‘Sweetheart’ reached mean time to, and maximum total bud burst, faster than ‘Kordia’, but no subsequent impact on uniformity of maturity at harvest. There was no significant difference in uniformity of burst between the two varieties. The results obtained by matching chill accumulation to tree phenology showed that cherry-producing regions in Australia will experience sufficient chill to support the production of the variety ‘Sweetheart’ with an increase in mean winter temperature of 1 °C. Regions in Western Australia and Queensland will become marginal, or not suitable, for ‘Kordia’.

The behaviour of peach cultivars under warm climatic conditions in the Mediterranean area

Peach production will eventually suffer from the temperature changes initiated by global warming. This study investigates the response of peach cultivars to exceptional warm winter conditions. Fifteen peach cultivars from three production areas in the north and the centre of Tunisia were surveyed during 2006 and 2007. Flowering date and period, vegetative growth, yield and fruit quality were monitored. During a warmer year, chill accumulation represents less than 50% of the average chilling accumulation for the three production areas and less than 35% of the previous year’s accumulation. The lack of chilling had variable consequences on peach species, depending on cultivars. Delayed flowering, extended flowering period, weak vegetative growth and high heterogeneity of fruit size were observed. Bud break and flowering were delayed by 6–17 days, and the flowering period was extended by 4–10 days. Early cultivars seem to be in general less sensitive to low chill accumulation. Nevertheless, some early, mid-season and late ripening cultivars were severely affected and seem to be highly sensitive to warm conditions. With the increasing frequency of warmer winters, planting early ripening and low chilling cultivars and adopting new practices that increase assimilation could compensate for the effect of the observed anomalies.

Chill unit accumulation and necessity of rest breaking agents in South African table grape production regions

Cultivation of table grapes in warm regions of South Africa is complicated by problems associated with delayed and uneven bud break. Rest breaking agents, mainly hydrogen cyanamide, are applied to overcome these problems. The chill unit accumulation of the five major table grape production regions in South Africa was compared with the minimum chilling required to obtain a high bud break percentage (>80%) and even bud break, defined as 200 hours between 0 and 10 ∘ C, or 400 hours at 3 ∘ C. The effect of chilling accumulation (0, 50, 100, 200, 400 and 800 hours at 3 ∘ C) on bud break of dormant cuttings of three table grape cultivars was investigated in controlled studies in a glass house over a three year period. Cuttings used in the study were collected from commercial vineyards in Saron in the Berg River Valley (representing a warm region) and the Hex River Valley (representing a cool region). Sultanina and Sugraone cuttings from Saron required at least 200 hours at 3 ∘ C to obtain a final bud break % of 80%+. A final bud break % of 80%+ was obtained with all Sultanina cuttings from Hex River Valley. Treatments receiving 400 and 800 hours at 3 ∘ C, showed the most rapid rate of bud break. Prime cuttings collected from Saron, receiving 0, 50, 100 and 400 hours at 3 ∘ C, showed a more rapid bud break process, as well as a higher final bud break % compared to Sugraone and Sultanina. It seems as if Prime has a lower chilling requirement than the other two cultivars. The model of Dokoozlian for quantifying chilling status is applicable to Saron and the Hex River Valley. The necessity of using rest breaking chemicals in the Saron area, for obtaining a high bud break % and even bud break, was confirmed. The Hex River Valley receives sufficient chilling temperatures to obtain a high bud break %, but rest breaking chemicals can be used to obtain more even bud break. The use of hydrogen cyanamide for improved and even bud break is a general practice in regions that receive less than 400 chill units. In regions receiving more than 400 chill units, several producers still use hydrogen cyanamide for even bud break. It is recommended that the chilling requirement of economic important cultivars should be determined simultaneously in field trials and glass house trials for the major table grape production regions of South Africa, to verify the necessity of using rest breaking chemicals for specific cultivars in specific regions.

Overhead irrigation increased winter chilling and floral bud production in Eucalyptus nitens

Eucalyptus nitens requires a sufficiently cold winter to produce flower buds. In areas in South Africa where E. nitens commercial plantations as well as breeding and production seed orchards are located, winter chilling is often insufficient for floral bud initiation. Hence, under such conditions, E. nitens floral bud and seed crops are poor and inconsistent. The local industry is almost entirely dependent on paclobutrazol (PBZ) applications for encouraging flowering in E. nitens seed orchards. Between 2008 and 2010, an experiment was conducted to investigate the potential of overhead irrigation (sprinkling) as a means of supplementing winter chilling to improve floral bud production in E. nitens. The treatments included three levels of sprinkling (nil, 10 weeks and 16 weeks duration), two levels of PBZ (nil, 0.025 g a.i. per mm basal stem circumference) and two grafted clones (prolific flowerer and shy-flowerer). Sprinkling reduced E. nitens daytime bud temperatures by as much as 16.2 °C on warm, dry winter days. In 2009 (cold winter) and 2010 (warm winter), sprinkling increased chilling accumulation by 44% and 72% (nil versus maximum sprinkling), respectively. In 2009, in the absence of PBZ, sprinkling resulted in a higher percentage of trees of either clone producing umbels (flower buds) compared with the control. In the warmer 2010 winter, sprinkling again increased flowering, with the number of flowering shoots and umbels per tree being significantly higher than the control at p < 0.05. In both 2009 and 2010, PBZ showed a strong additive effect to winter chilling on E. nitens floral bud production. The E. nitens clone × chilling × PBZ flowering interaction was complex and warrants more detailed investigation in future. Overhead sprinkling offers a practical method of supplementing winter chilling and improving floral bud production in high-chill-requiring temperate eucalypt species such as E. nitens.

English

Blueberry is an economically and nutritionally important small fruit crop, native to North America. As with many crops, extreme low temperature can affect blueberry crop yield negatively and cause major losses to growers. For this reason, blueberry breeding programs have focused on developing improved cultivars with broader climatic adaptation. To help achieve this goal, the blueberry genomic database (BBGD454) was developed to provide the research community with valuable resources to identify genes that play an important role in flower bud and fruit development, cold acclimation and chilling accumulation in blueberry. The database was developed using SQLServer2008 to house 454 transcript sequences, annotations and gene expression profiles of blueberry genes. BBGD454 can be accessed publically from a web-based interface; this website provides search and browse functionalities to allow scientists to access and search the data in order to correlate gene expression with gene function in different stages of blueberry fruit ripening, at different stages of cold acclimation of flower buds, and in leaves.AvailabilityIt can be accessed from http://bioinformatics.towson.edu/BBGD454/

Chilling and heat requirements for flowering in temperate fruit trees

Climate change has affected the rates of chilling and heat accumulation, which are vital for flowering and production, in temperate fruit trees, but few studies have been conducted in the cold-winter climates of East Asia. To evaluate tree responses to variation in chill and heat accumulation rates, partial least squares regression was used to correlate first flowering dates of chestnut (Castanea mollissima Blume) and jujube (Zizyphus jujube Mill.) in Beijing, China, with daily chill and heat accumulation between 1963 and 2008. The Dynamic Model and the Growing Degree Hour Model were used to convert daily records of minimum and maximum temperature into horticulturally meaningful metrics. Regression analyses identified the chilling and forcing periods for chestnut and jujube. The forcing periods started when half the chilling requirements were fulfilled. Over the past 50 years, heat accumulation during tree dormancy increased significantly, while chill accumulation remained relatively stable for both species. Heat accumulation was the main driver of bloom timing, with effects of variation in chill accumulation negligible in Beijing’s cold-winter climate. It does not seem likely that reductions in chill will have a major effect on the studied species in Beijing in the near future. Such problems are much more likely for trees grown in locations that are substantially warmer than their native habitats, such as temperate species in the subtropics and tropics.

Differential responses of trees to temperature variation during the chilling and forcing phases

Temperate-zone trees must fulfill cultivar-specific chilling and heat requirements during the dormant period, in order to produce leaves and flowers in the following growing season. Timing and accumulation rate of chill and heat are understood to determine the timing of spring events, but both processes are difficult to observe in dormant tree buds. Where long-term phenological observations are available, Partial Least Squares (PLS) regression offers a statistical opportunity to delineate phases of chill and heat accumulation and determine the climatic requirements of trees. This study uses PLS regression to explore how the timing of spring events of chestnut in China, cherry in Germany and walnut in California is related to variation in the daily rates of chill and heat accumulation, as calculated with horticultural models. Dependent variables were 39 years of flowering dates for chestnuts in Beijing (China), 25 years of cherry bloom in Klein-Altendorf (Germany) and 54 years of walnut leaf emergence in Davis (California, USA). These were related to daily accumulation rates of chill, calculated with the Dynamic Model, and heat, calculated with the Growing Degree Hours Model. Compared to an earlier version of the procedure, in which phenological dates were related to unprocessed temperature data, delineation of chilling and forcing phases was much clearer when using horticultural metrics to quantify chill and heat. Chestnut bloom in the cold-winter climate of Beijing was found to depend primarily on the rate of heat accumulation, while cherry bloom in the temperate climate of Germany showed dependence on both chill and heat accumulation rates. The timing of walnut leaf emergence in the mild-winter climate of California depended much more strongly on chill accumulation rates. Chilling (in Chill Portions=CP) and heat (in Growing Degree Hours=GDH) requirements determined based on PLS regression were 79.8±5.3 CP and 13,466±1918 GDH for chestnut bloom in Beijing, 104.2±8.9 CP and 2698±1183 GDH for cherry bloom in Germany, and 37.5±5.0 CP and 11,245±1697 GDH for walnut leaf emergence in California. Spring phases of cherry in Klein-Altendorf and especially chestnut in Beijing will likely continue to advance in response to global warming, while for walnut in California, inadequate chilling may cause delays in flowering and leaf emergence. Such delays could serve as an early-warning indicator that future productivity may be threatened by climate change. The R package ‘chillR’ makes the method used in this study available for wider use.

Study on Integrated Climatic Index for Low Temperature Injury of Loquat in Putian

Based on the daily climate data in winter and relative meteorological yield of loquat for 2 meteorological stations in Putian from 1992 to 2009, this paper studied the disaster-inducing factors and integrated climatic index for low temperature injury of loquat. The results showed that the critical temperature for low temperature injury of loquat could be determined as 3.0°C. The disaster-inducing factors included extreme minimum temperature, the sum of daily numbers for less than or equal to 3.0°C, the sustained days of low temperature injury for less than or equal to 3.0°C, and harmful chilling accumulation for less than or equal to 3.0°C, and there were obvious correlations among these factors. According to the method of principal component analysis, an integrated climatic index was obtained. By the correlation analysis of integrated climatic index for low temperature injury and the relative meteorological yield of loquat in Putian, the value of integrated climatic index of low temperature injury was significantly negatively correlated with the yield of loquat, and could be used to analyze the degree of low temperature injury of loquat.

Response of chestnut phenology in China to climate variation and change

Climate change has affected the phenology of plants and animals throughout the world, but few studies have evaluated climate responses of fruit trees in East Asia. In particular, the response of tree phenology to warming during different parts of the year has not been explored. We evaluated long-term records (1963–2008) of chestnut (Castanea mollissima Blume) first flowering, leaf coloring and length of the growing season from Beijing, China. Phenological dates were related with daily temperatures (subjected to an 11-day running mean) for the 12 months leading up to the respective events, using Partial Least Squares (PLS) regression. For each phenological indicator, regression results identified two relevant phases, during which temperatures were correlated with event timing or growing season length.First flowering dates in Beijing advanced by 1.6 days per decade over the length of the record, whereas leaf coloring showed no significant trend. The growing season expanded by 4.3 days per decade. First flowering was advanced by high temperatures between January and June, but delayed by warm conditions during the chill accumulation phase (late October through early January). Leaf coloring was advanced by warm conditions during most of the growing season, but delayed by high temperatures in fall. Variation in the length of the growing season was strongly correlated to variation in spring phenology.All phenological indicators of chestnut appeared to respond to high temperatures during certain parts of the growing season in a way that ran counter to currently dominant effects. For instance, warming during the period of chill accumulation delayed rather than advanced spring phenology. These secondary temperature responses may explain responses of certain plants and ecosystems that are not in line with general trends of advanced spring and delayed fall phases. It seems possible that the importance of these effects may increase as warming continues.

Evaluation and fitting of models for determining peach phenological stages at a regional scale

Phenological models provide an assessment on the adaptation of species and cultivars to current conditions as well as the on the impact climate change may have on their performance in the future. The aim of this study was to obtain phenological models that predict accurately peach bud development for reference cultivars in temperate growing areas. Model testing and validation were done using phenological observations from orchards in Ebro and Guadiana Valleys, two of the main peach growing areas in Spain, where chilling accumulation ranges from medium to high. Data were acquired during 6 years for three widely grown cultivars (Big Top, O’Henry, and Andross). Two different types of models were tested: Critical Date models (CD), which consider only the action of forcing temperatures accumulated from a fitted date, and Critical Chilling models (CC), which also take into account the action of chilling temperatures during the dormancy period. CC models were always at least a 40% more efficient than CD ones and the version which uses the Dynamic method to estimate chill (CCDY) had also the best accuracy with prediction errors generally around four days, so it was chosen as the best model. The validation process showed adequate accuracy and no significant predictive bias. The fitted values for CCDY parameters also had physiological sense, as the chilling values at which forcing heat began to accumulate were in accordance with the limit in which additional chilling is known to have little effect on heat requirements; and fitted threshold temperature for forcing heat accumulation was nearly the same considered to be the physiological base temperature for this species (4.7°C). On the contrary, CD models fitted threshold temperatures for bud development inconsistent with peach physiology (between −6 and 0°C) and were poorly adapted to predict phenological stages at a regional scale. All in all, the results showed that phenological evolution of peach cultivars can be simulated with good accuracy using simple sequential statistically fitted models that include chilling evaluated by Dynamic method and forcing heat accumulation evaluated by growing degree sums. These models could be used to estimate the phenological evolution of peach under a wide range of climatic situations, including the evaluation of the impact climate change scenarios may have on the phenology of the crop.

Effects of dormancy progression and low-temperature response on changes in the sorbitol concentration in xylem sap of Japanese pear during winter season

In order to elucidate which physiological event(s) are involved in the seasonal changes of carbohydrate dynamics during winter, we examined the effects of different low temperatures on the carbohydrate concentrations of Japanese pear (Pyrus pyrifolia (Burm.) Nakai). For four winter seasons, large increases in the sorbitol concentration of shoot xylem sap occurred during mid- to late December, possibly due to the endodormancy completion and low-temperature responses. When trees were kept at 15 °C from 3 November to 3 December in order to postpone the initiation and completion of chilling accumulation that would break endodormancy, sorbitol accumulation in xylem sap was always higher from trees with sufficient chilling accumulation than from trees that received insufficient chilling. However, an additional increase in xylem sap sorbitol occurred around late December in trees regardless of whether their chilling accumulation naturally progressed or was postponed. To examine different temperature effects more closely, we compared the carbohydrate concentrations of trees subjected to either 6 or 0 °C treatment. The sorbitol concentration in xylem sap tremendously increased at 0 °C treatment compared with 6 °C treatment. However, an additional increase in xylem sap sorbitol occurred at both the temperatures when sufficient chilling accumulated with a peak coinciding with the peak expression in shoots of the sorbitol transporter gene (PpSOT2). Interestingly, the total carbohydrate concentration of shoots tremendously increased with exposure to 0 °C compared with exposure to 6 °C, but was not affected by the amount of accumulated chilling. Instead, as chilling accumulated the ratio of sorbitol to total soluble sugars in shoots increased. We presumed that carbohydrates in the shoot tissues may be converted to sorbitol and loaded into the xylem sap so that the sorbitol accumulation patterns were synchronized with the progression of dormancy, whereas the total carbohydrate transported into shoots from other storage organs may be related to freezing tolerance acquisition independent of dormancy progression. We thus propose that there are different effects of dormancy progression and low-temperature responses on carbohydrate dynamics in Japanese pear.

Cultivar and areal differences in the breaking period of bud endodormancy in pear plants

We investigated the occurrence of bud break in Japanese pear [Pyrus pyrifolia (Burm.f.) Nakai] cultivars and wild pear species to estimate the chilling requirement (CR) for breaking bud endodormancy. The leaf buds of the ‘Hosui’ and ‘Kosui’ cultivars showed a higher CR than their counterpart floral buds. Each CR values of the ‘Kosui’ and ‘Niitaka’ cultivars in three areas, namely Tottori (cold-winter area), Saga (moderate-winter area), and Kumamoto (warm-winter area), were completely dependent on the chilling accumulation in each cultivar and area. Among these areas, a chill unit (CU) value of 1566 was observed in Kumamoto in mid- or late February. The results suggest that pear cultivars showing less than 1500CU should be selected for commercial production in that specific area, south of Kumamoto. Among the Japanese pear cultivars observed, ‘Akizuki,’ ‘Natsusoyoka,’ ‘Ryougetsu,’ and ‘Shinkansen’ showed the lowest CR, whereas ‘Syuugyoku’ showed the highest. The CR of Japanese pears ranged from 800 to 1800CU. Among the examined wild pear plants, a bud break incidence of at least 60% was observed in 14 plants during the period from January 7 to February 4. Among these, P. calleryana (No. 12) showed the lowest CR and was thus identified as a useful rootstock for warm-winter regions.

Chemicals Applied in Fall and Defoliation on Dormancy Evolution and Release in Low-chill Peach ‘Flordaking’

The aim of this work was to study the effect of fall defoliation and chemical application on the progression and release of dormancy, and phenology, of low-chill peach ‘Flordaking’ under temperate climate conditions. At the onset of leaf fall, ‘Flordaking’ peach (Prunus persica L. Batsch) trees were defoliated or treated with hydrogen cyanamide (2.5 g L-1 a.i), norflurazon (46 g L-1 a.i.) or ethephon (20 mg L-1 a.i.). Untreated trees were used as the control. The rate of budbreak and the mean time to budbreak (MTB) was tested on stem isolates in a phytotron, whereas tree phenology and vegetative and reproductive traits were evaluated in a field experiment. Defoliation and chemical treatments significantly affected the rate of budbreak evolution of floral, but not of vegetative, buds. Treatments also significantly affected the evolution of the MTB of both vegetative and floral buds, but with a greater effect on the latter. In the field, the phenology of Flordaking was more affected by treatments that modified the depth of dormancy than those which affected the percentage of budbreak in excised shoots. Defoliation and hydrogen cyanamide treatments advanced sprouting (15 and ten days, respectively) and blooming (16 and four days, respectively), whereas ethephon delayed flowering and fruit set by three days each. Fall defoliation at the beginning of leaf abscission appears to be a strong tool to manipulate the evolution of dormancy and the time of spring bloom of Flordaking, mainly when insufficient chilling accumulation is forecasted.

Identification of chilling and heat requirements of cherry trees—a statistical approach

Most trees from temperate climates require the accumulation of winter chill and subsequent heat during their dormant phase to resume growth and initiate flowering in the following spring. Global warming could reduce chill and hence hamper the cultivation of high-chill species such as cherries. Yet determining chilling and heat requirements requires large-scale controlled-forcing experiments, and estimates are thus often unavailable. Where long-term phenology datasets exist, partial least squares (PLS) regression can be used as an alternative, to determine climatic requirements statistically. Bloom dates of cherry cv. ‘Schneiders späte Knorpelkirsche’ trees in Klein-Altendorf, Germany, from 24 growing seasons were correlated with 11-day running means of daily mean temperature. Based on the output of the PLS regression, five candidate chilling periods ranging in length from 17 to 102 days, and one forcing phase of 66 days were delineated. Among three common chill models used to quantify chill, the Dynamic Model showed the lowest variation in chill, indicating that it may be more accurate than the Utah and Chilling Hours Models. Based on the longest candidate chilling phase with the earliest starting date, cv. ‘Schneiders späte Knorpelkirsche’ cherries at Bonn exhibited a chilling requirement of 68.6 ± 5.7 chill portions (or 1,375 ± 178 chilling hours or 1,410 ± 238 Utah chill units) and a heat requirement of 3,473 ± 1,236 growing degree hours. Closer investigation of the distinct chilling phases detected by PLS regression could contribute to our understanding of dormancy processes and thus help fruit and nut growers identify suitable tree cultivars for a future in which static climatic conditions can no longer be assumed. All procedures used in this study were bundled in an R package (‘chillR’) and are provided as Supplementary materials. The procedure was also applied to leaf emergence dates of walnut (cv. ‘Payne’) at Davis, California.Electronic supplementary materialThe online version of this article (doi:10.1007/s00484-012-0594-y) contains supplementary material, which is available to authorized users.

MUME CLONAL ROOTSTOCKS FOR 'AURORA-1' PEACH IN SÃO PAULO STATE, BRAZIL

In spite of significant results achieved with scion genetic improvement in stone fruits, the peach culture in Brazil still needs studies and new technologies regarding the use of rootstocks. A wide research project has being developed at the Faculdade de Ciencias Agrarias e Veterinarias (FCAV/UNESP), Jaboticabal, Brazil, dealing with the use of mume clones (Prunus mume) as rootstocks for peach trees, which has produced promising results. In this research, two mume genotypes propagated by herbaceous cuttings were tested as rootstocks for peach cultivar Aurora-1. Three different tree spacing were used: 6 x 2 m, 6 x 3 m and 6 x 4 m. The experiment was carried out at Vista Alegre do Alto (21°10’14” S, 48°37’45” W, 700 m of altitude), Sao Paulo State, Brazil. Growing field conditions included Hapludalfs soil with medium sandy texture and using micro sprinkler irrigation. The region has an average chilling accumulation 17.9 hours per year. The evaluations were taken in 2005 and 2006 (2 nd and 3 rd year after planting, respectively). The trunk diameter was evaluated every three months, from the 24 th to the 41 st month after planting, totalizing seven evaluations. Plants on ‘Rigitano’ had higher trunk diameter on the 33 rd , 39 th and 41 st month after planting (May/06, November/06 and February/07, respectively). No significant differences were observed in the other evaluations. The diameter at 5 cm above to the graft point was larger than below, but no incompatibility symptoms were observed between rootstocks and scion. Spacing tested did not influence trunk diameter, phenology and flower bud production in ‘Aurora-1’ scion. In conclusion, ‘Rigitano’ and ‘Clone 15’ are recommended for high density plantings of peach ‘Aurora-1’ in Brazil, and the 6 x 2 m spacing can be recommended, with productivity advantages for peach under low air relative humidity and mild winter conditions.

Dormancy release and flowering of Paeonia lactiflora ‘Taebaek’ by natural cumulative chilling and GA3 treatment

Dormancy breaking and flowering of Paeonia lactiflora ‘Taebaek’ were controlled by natural cumulative chilling and GA3 treatments according to the transfer date in the central region of Suwon, Korea. Shoot emergence and flowering did not occur throughout the experimental period when dormant rootstocks were transferred to a glasshouse between September 10 and October 29, in which the natural cumulative chill (NCU) unit was nearly 0 h. The number of days from the transfer date to sprouting and flowering was shortened as the transfer date was delayed. All the plants flowered normally with shoot growth when they were transferred after December 31 (1,222 h NCU). All dormant buds that were treated with GA3 sprouted, regardless of transfer date, but failed to flower due to shoot blasting and flower bud abortion when they were transferred between September 10 (0 h NCU) and November 12 (185 h NCU). Shoot blasting was 38% when they were transferred on November 26 (429 h NCU). When GA3 was applied to the plants after December 17 (876 h NCU), they flowered without blasting. Plant height and stem diameter were not affected by GA3 treatment. GA3 treatment decreased the number of days to sprouting and flowering, and increased the number of flowers, irrespective of insufficient chilling accumulation, as compared to 1,222 h NCU on December 31. According to the above results, at least 1,222 h NCU could be recommended as a practical forcing method for dormancy release, subsequent growth, and normal flowering of dormant P. lactiflora ‘Taebaek’ in a temperate climate region. The GA3 treatment played an important role in breaking dormancy and significantly increased the percentage of sprouting, accelerated the days to sprouting and promoted the flowering of P. lactiflora ‘Taebaek’ when the plants had undergone insufficient chilling accumulation.