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Paul Lyrene

Cultivated blueberries (Vaccinium section Cyanococcus species, including lowbush, highbush, and rabbiteye) normally produce flower buds at the end of the growing season; these remain dormant during the winter and give rise to flowers the following spring. However, rabbiteye and low-chill highbush cultivars that are maintained in a state of vigorous growth throughout the winter in an unheated greenhouse in Gainesville in north Florida flower and produce fruit continuously on new growth throughout December, January, and February. The regimen of cool (but not freezing) nights and short, warm days permits the plants to continue growth throughout the winter and results in rapid conversion of newly-formed axillary buds into flower buds. These do not become dormant, but sprout to produce flowers and fruit almost as quickly as they are formed. Extending the photoperiod or raising night temperatures inhibits primocane flowering by allowing the axillary buds to remain vegetative. Primocane flowering, which occurs naturally in highbush blueberry production fields south of lat. 28°N in Florida and at lat. 30°S in eastern Australia, can contribute to an extended harvest season (4 to 8 months per year) from a single cultivar.

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R.A. Neja, N.K. Dokoozlian, and N.C. Ebisuda

Field experiments conducted in 1994 (low-chill winter) and 1995 (high-chill winter) examined the effects of surfactants on the efficacy of hydrogen cyanamide (H2CN2) applied to `Perlette' grapevines (Vitis vinifera L.) in the Coachella Valley of California. In 1994, when surfactants were not used, vines treated with 1% and 2% H2CN2 exhibited similar rates of budbreak and grew more rapidly than vines treated with 0.5% H2CN2. When 1% or more of the surfactant Armobreak was used, budbreak was generally similar among all H2CN2 concentrations. The number of days after treatment required for 70% budbreak declined as H2CN2 and Armobreak concentrations were increased. Results were similar in 1995, however, budbreak was inhibited when vines were treated with 2% H2CN2 + 2% Armobreak. A separate experiment conducted in 1995 revealed that two other surfactants, Activator 90 and Agridex, had similar effects on the efficacy of H2CN2 as Armobreak. The results indicate that, when 2% surfactant is used, the standard commercial H2CN2 concentration used in California may be reduced 75% while maintaining treatment efficacy. Chemical names used: hydroxypolyoxyethylene polyoxypropylene ethyl alkylamine (Armobreak); alkyl polyoxyetheylene ether (Activator 90); paraffin petroleum oil (Agridex).

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Iwan F. Labuschagné, J.H. Louw, Karin Schmidt, and Annalene Sadie

Genetic variation in chilling requirement was investigated over three growth periods using clonal progenies of six apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] families derived from crosses of high and low chill requiring cultivars. Two quantitative measurements related to chilling requirement, viz., the time of initial budbreak (vegetative and reproductive) and the number of breaking buds over a specified time interval, were used as evaluation criteria. Genetic and environmental variances of the traits are presented as intra-class correlation coefficients for clones within and between families. For budbreak time, reproductive and vegetative, broad-sense heritability averaged around 75% and 69% respectively, indicating a high degree of genetic determination in this material. For budbreak number, moderate to low genetic determination was found with broad-sense heritabilities around 30%. Estimates of genetic components of variance between families were generally very low in comparison to the variance within families and predict potentially favorable responses to truncation selection on the traits within these progeny groups. Analysis of the data showed that distribution of budbreak time is typical of quantitative traits with means distributed closely around midparent values. Skewed distributions towards low budbreak number were obtained in varying degrees in all families.

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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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Roberto Hauagge and James N. Cummins

In a study of chilling requirement in Malus, broad-sense heritability estimates for the length of vegetative bud dormancy in 43 clones growing under simulated subtropical winter conditions were 0.76 ± 0.04 in 1986 and 0.81 ± 0.04 in 1987. Narrow-sense heritability estimates were 0.66 ± 0.13 in 1986 and 0.69 ± 0.13 in 1987. Seedlings with low chilling requirements (CR) were not observed in crosses where both parents had high bud-chilling requirements. `Koningszuur' did not transmit its long CR to its seedlings. Open-pollinated (OP) seedling populations from the Malus × domestics Borkh. cultivars Anna, Dorsett Golden, Ein Shemer, Khashabi, Winter Banana, and Zabaoani, and the species and interspecific hybrids M. baccata L. DE#98, M. brevipes Rehd., M. ×robusta (Carr.) Rehd. DE#485, M. × robusta No. 5 (`R5'), M. rockii, M. turesi Rehd. PI 34143, and `Rosedale' had at least 5% of their descendants in the lower CR classes. In all but one instance, 50% or more of `Anna' descendants had low CR. Many of these seedlings were within a few classes of the extreme low CR. It is postulated that the low-CR character present in `Anna' is controlled by at least one major dominant gene and that minor genes interact to modulate its effects. Very low-CR cultivars have a shallow bud dormancy. This highly heritable component for low bud CR is related to a failure to develop a deep dormancy state, rather than to acceleration of the termination of the dormancy process.

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Sorkel A. Kadir and Edward L. Proebsting

Flower buds of 20 Prunus species showed quite different strategies to cope with low temperatures. Buds of most species deep supercooled. The two hardiest species, both from the subgenus Padus (P. padus L. and P. virginiana L.), did not supercool and survived -33C with no bud kill. Prunus serotina J.F. Ehrh., also in Padus, did supercool. Prunus nigra Ait., P. americana Marsh, P. fruticosa Pall., and P. besseyi L.H. Bailey had a low minimum hardiness level (MHL), small buds, and a low water content. Exotherms were no longer detectable from the buds of these species after 2 days at -7C and some buds survived -33C. Prunus triloba Lindl. and P. japonica Thunb. were similar to that group, but no buds survived -33C. Prunus davidiana (Carriere) Franch., P. avium L., and P. domestica L. had a relatively high MHL but hardened rapidly when the buds were frozen. Prunus persica (L.) Batsch., P. subhirtella Miq., P. dulcis (Mill) D. A. Webb, and P. emarginata (Dougl. ex Hook) Walp. deep supercooled, had large flower buds and a high MHL, and were killed in the Dec. 1990 freeze. Prunus salicina Lindl., P. hortulana L.H. Bailey, P. armeniaca L., and P. tomentosa Thunb. were in an intermediate group with a moderately low MHL and a moderate rate of hardiness increase while frozen. Prunus dulcis and P. davidiana had a low chilling requirement and bloomed early, whereas P. virginiana, P. fruticosa, P. nigra, and P. domestica had high chilling requirements and bloomed late.

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Nicholi Vorsa and Richard Novy

Vaccinium darrowi (D) is a wild blueberry species with low chilling requirements for budbreak, and heat and drought tolerance. Breeding efforts to incorporate these desirable traits into cultivated blueberry (V. corymbosum) (C) would be facilitated with a better understanding of the genomic homology between the two species. An interspecific tetraploid hybrid (CCDD, 2n=4x=48) was used to evaluate genome homology and interspecific recombination. Pollen mother cells examined at diakinesis and early metaphase I exhibited an average of 4.6 chain bivalents, 11.4 ring bivalents, 1.0 chain quadrivalent, and 3.0 ring quadrivalents. This data most closely fits a chromosome pairing model in which there is a greater pairing affinity between homologues than homoeologues. An analysis of the inheritance of 14 RAPD markers unique to V. darrowi in 72 backcross progeny of the V. darrowi–corymbosum hybrid also supported the pairing model: Seven of the 14 markers deviated significantly from tetrasomic inheritance ratios, expected if chromosome pairing was totally random. On the basis of the cytogenetic and RAPD analyses, the genomes of V. darrowi and V. corymbosum are divergent from one another, with preferential pairing within genomes. This outcome suggests there may be difficulty in breaking undesirable linkages when introgressing desirable traits from V. darrowi to V. corymbosum.

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Raul Leonel Grijalva-Contreras and Arturo Lopez-Carvajal

Almond production in hot climate areas of Mexico uses low-chilling cultivars. One problem in young almond trees is that timely leaf drop does not occur; therefore, budbreak is late and uneven. With the objective of chemical defoliation, foliar applications of different compounds [urea (5%), ZnSO4 (5%), CuSO4 (5%), NH4NO3 (5%), ZnSO4 (2.5%) + urea (1.5%)], hand defoliation, and a nondefoliation control were made on `R-633' young almond trees (2 years old). The percent defoliation was high (77% to 86%) after 6 days of the application in the majority of treatments, except for NH4NO3 (5%), urea (5%), and the control; but 3 days later, all treatments showed >80% defoliation. Nondefoliated trees had an uneven budbreak and occurred 3 and 6 days later. The yield was greater for ZnSO4, with 435 g/tree and only 55.6 g/tree for the control. Fruit quality was the same for all treatments. No injury to branches were observed with any compounds.

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Aroldo Isudro Rumayor Flores*, Jose Antonio Vázquez Ramos, Martínez Cano Andres, and Borrego Escalante Fernando

In hybrids of apple (Malus × domestica Bork.) subjected to study phenological in Aguanueva, Coahuila, Mexico, their requirements of chill hours (CH), heat units (HU), bud breaking flower and vegetative % (BB) for good adaptation to warm milder climate, bloom period (BP), and vegetative period (VP), were determined using the Methodology of Identification of New Cultivars of Fruit Breeding (Ploudiv 1983). They were material with requirements of cold from 200 up to 650 (CH) when they underwent a test of controlled conditions of (CH). These materials are; AR-109 (200 CH), AR-106 (300 CH), AR-108 (300 CH), AR-147 (300 CH), AR-144 (550 CH), and AR-a60 (650 CH), while the control Mutant Aguanueva II (500 CH). Under winter conditions of the year 2000 with so slone 168.76 (CH), some materials showed a bud break superior to the control. The bud break dates understand between 30 days before the witness Aguanueva II, as the hybrid AR-147 and 34 days later in the case of the hybrid AR-151, location this way to the materials as: Early with regard to the control; AR-16-S (24 days), AR-130 (14 days) and AR-147 (30 days). Similar to the control; AR-144, AR-103 and AR-127. Later than the control; AR-111 and AR-103-B. since they don't require spray bud breaking res compounds for their bud break and they have bloom period (BP) of 8 to 21 days. And when presenting low chill requirements they will be set fruit in a microclimate frost-free and growing and have their cultivation in a mild winter climate.

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Hsin-Shan Lin and Jia-Shing Lin

Taiwan, located in subtropic regions, naturally is not an ideal region for temperate-zone fruit trees' production due to the supra-optimum temperature, heavy rainfall, and higher relative humidity in summer and insufficient chilling in winter. Higher relative humidity and temperature in summer and autumn months cause excessive vegetative growth, resulting in poor flowerbud initiation and formation. Typhoon invasions result in the severe damage of twigs as well as the loss of quality and yield of fruits. In order to overcome these natural barriers, Hengshan (Pyrus serotina Rehd.) pear has been selected as a major cultivar for lowlands in Taiwan. It has low-chilling requirement and higher temperature tolerance. Branches of Hengshan are pulled and tied to a horizontal wire net to adapt to the environmental status. This trellis system enhances flowerbud initiation through the retardation of vegetative growth. It also induces numerous water shoots. Scions from high-chilling cultivars grown at a high altitude on mountains are grafted onto water shoots of Hengshan pear trees. The system has been successful in the production of both high-chilling pears in June and the Hengshan pears in August, and has made production of both pears an important industry in Taiwan. Heavy load and trellis systems, however, result in hastening the senescence of Hengshan trees. Vitality of trees could be restored by grafting scions from a vigorous cultivar, P. koehnei, onto the terminal position of the branches. The practice resulted in several advantages including: 1) uniform growth of branches, 2) redistribution of water shoots, 3) inducing formation of calluses on old damaged trunks, 4) quick recovery of mealybug-damaged branches, 5) rejuvenation of branches, and 6) termination of dormancy.