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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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Gerard Krewer, D. Scott NeSmith, and Ben Mullinix

`Climax' blueberry is a major cultivar in Georgia, but because of its excessively low chilling requirement and early blooming habit, it has a poor cropping history in recent years due to spring freezes. Research was initiated to explore the potential for ethephon to delay bloom, without delaying ripening too much. In 1997-1998 a treatment of 200 ppm ethephon applied on 3 Nov. or 400 ppm applied on 17 Nov. delayed bloom 5 to 7 days compared to the control. There was no significant difference between the control and the ethephon treatment in flower bud density or fruit density in the spring. In 1998-1999 ethephon applications at 200 and 400 ppm were applied once or twice 2 weeks apart starting on 5 Oct. and ending 19 Nov. A bloom delay of about 7 days was achieved with most ethephon applications. However, an application of 400 ppm on 19 Oct. and 2 Nov. delayed bloom about 14 days compared to the control. There was a trend toward delayed fruit ripening with the most-effective bloom delay treatments, but the extent of delayed ripening was minimal. Berry weight was not effected by ethephon treatments.

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Paul Lyrene

The best time to harvest fresh blueberries in Florida is 1 April to 15 May. Weather during this period is normally favorable for harvest: low rainfall, low humidity, warm, sunny days, and cool nights, and supplies of fresh blueberries from other producing areas are low. To ripen high-quality blueberries in April, the plants must flower in February and must have a full canopy of leaves to support the developing crop in March and April. Observations of thousands of blueberry seedlings and selections over the past 25 years in Florida have indicated that blooming and leafing time are affected by the chilling requirement and heat requirement of the variety and also by environmental factors. Factors that increase plant vigor (high soil fertility, ample moisture, and young plants) cause the plants to flower earlier in the spring. Flower buds that do not open by 15 Mar. in north Florida frequently abort. The timing and extent of this physiological bud abortion varies with cultivar. Some southern highbush cultivars leaf before they flower. Others flower before they leaf. The ideal blueberry variety for north Florida would have a very low chill requirement, a high heat requirement to prevent January flowering, and a short flowering-to-ripening interval.

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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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Gregory A. Lang and Joshua Tao

Plant dormancy research has long been stifled by the lack of appropriate biochemical markers to characterize the changing physiological status of dormant vegetative or reproductive buds. Two sets of experiments were conducted in an attempt to identify changes in soluble protein profiles during endodormancy of peach and blueberry reproductive apices. Bud samples from the peach cultivars `La Festival' (low chilling requirement) and `La White' (moderate chilling requirement) were taken every 15 days in the orchard during December and January, extracted for soluble proteins, and analyzed by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Outshoots were forced at 25C in a growth chamber to determine the intensity of endodormancy. A further experiment utilized potted `Bluechip' and `Meader' (troth high chilling requirement) blueberry plants given varying periods of cold (4.5C) chamber treatment, followed by forcing at 25C in a growth chamber. Bud samples were taken following cold treatment for extraction and SDS-PAGE. The relationship of the resulting protein profiles to chilling unit accumulation and intensity of endodormancy will be discussed.

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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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Guadalupe Osorio-Acosta and Jorge Siller-Cepeda

Table grapes growing under desert conditions present a short and shallow rest mainly due to low chilling and high daily temperatures. Results using Evaporative Cooling (EC) have shown that rest is modified, and the opening of primary buds and number of clusters per plant depended on pruning date and cyanamide rate. From Oct 26 until Dec 30, rest depth was assessed under lab conditions on `Flame Seedless' canes from EC-treated and control vines. We found that rest depth was shallow and final budbreak was higher in EC-treated plants at all sampling dates. However, hydrogen cyanamide treated canes under both conditions showed no difference on final budbreak, although the opening of primary buds was higher on the EC-treated plants. Field trials were established to quantify the effect of pruning date associated with those treatments (EC and Control) on the number of clusters per plant. Plants were pruned on Dec 14, Dec 22, and Dec 30, and cyanamide (5% Dormex) was applied immediately. Plants under EC conditions and pruned on the earliest dates enhanced the number of cluster per vine by 40 and 21.7% respectively, as compared with control plants.

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Yang Fang, Jeffrey Williamson, Rebecca Darnell, Yuncong Li, and Guodong Liu

Southern highbush blueberry (SHB, Vaccinium corymbosum L. interspecific hybrid) is the major species planted in Florida because of the low-chilling requirement and early ripening. The growth pattern and nitrogen (N) demand of SHB may differ from those of northern highbush blueberry (NHB, V. corymbosum L.). Thus, the effect of plant growth stage on N uptake and allocation was studied with containerized 1-year-old SHB grown in pine-bark amended soil. Five ‘Emerald’ plants were each treated with 6 g 10% 15N labeled (NH4)2SO4 at each of 12 dates over 2 years. In the first year, plants were treated once in late winter, four times during the growing season, and once in the fall. In the second year, treatment dates were based on phenological stages. After a 14-day chase period following each 15N treatment, plants were destructively harvested for dry weight (DW) measurements, atom% of 15N, and N content of each of the plant tissues. Total DW increased continuously from mid-May 2015 to Oct. 2015 and from Mar. 2016 to late Sept. 2016. From August to October of both years, external N demand was the greatest and plants absorbed more N during the 2-week chase period, about 0.53 g/plant in year 1 and 0.67 g/plant in year 2, than in chase periods earlier in the season. During March and April, N uptake was as low as 0.03 g/plant/2 weeks in year 1 and 0.21 g/plant/2 weeks in year 2. Nitrogen allocation to each of the tissues varied throughout the season. About half of the N derived from the applied fertilizer was allocated to leaves at all labeling times except the early bloom stage in 2016. These results suggest that young SHB plants absorb greater amounts of N during summer and early fall than in spring.

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Jeffrey G. Williamson, Gerard Krewer, Brian E. Maust, and E. Paul Miller

Experiments were conducted in north Florida and south Georgia to determine the effects of H2CN2 sprays on vegetative and reproductive growth of blueberry. In Florida, mature, field-grown `Misty' southern highbush (Vaccinium corymbosum L. hybrid) blueberry plants were sprayed to drip with 0, 10.2, or 20.4 g·L-1 of H2CN2 [hereafter referred to as 0%, 1.0%, and 2.0% (v/v) H2CN2] on 20 Dec. 1996 and 7 Jan. 1997. During the following winter, mature `Misty' southern highbush and `Climax' rabbiteye (V. ashei Reade) plants were sprayed to drip with 0, 7.6, or 15.3 g·L-1 of H2CN2 [hereafter referred to as 0%, 0.75%, and 1.5% (v/v) H2CN2] on 17 Dec. 1997 and 6 Jan. 1998. For all experiments, plants were dormant and leafless, with slightly swollen flower buds, at the time of spray applications. Generally, H2CN2 sprays increased the extent and earliness of vegetative budbreak and canopy establishment and advanced flowering slightly. The number of vegetative budbreaks usually increased linearly with increasing spray concentrations. In Florida, H2CN2 [0.75% to 1.0% (v/v)] sprays increased mean fruit fresh weight and yield, and shortened the fruit development period (FDP) compared to controls. However, H2CN2 sprays ranging in concentration from 1.5% to 2.0% (v/v) resulted in significant flower bud injury and reduced total fruit yield compared to controls. In south Georgia, 27 of 37 field trials conducted between 1991 and 1998 on several rabbiteye and southern highbush cultivars indicated that leaf development was significantly enhanced by H2CN2. H2CN2 shows potential for increasing early fruit maturity, fruit size, and yield of southern highbush and rabbiteye blueberry cultivars with poor leaf development characteristics in low-chill production regions. Chemical name used: hydrogen cyanamide (H2CN2).