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Iwan Labuschagné, Bill Louw, Karin Schmidt, and Annalene Sadie

The variability of characteristics associated with prolonged dormancy in apple (Malus ×domestica Borkh.) progenies planted in the Western Cape region of South Africa was recorded over a three-year period. The time of initial vegetative and reproductive budbreak, the number of vegetative and reproductive budbreak, and the flowering duration were used as criteria. Data were collected on bearing trees of apple seedling families where the cultivars Anna and Golden Delicious were used as a common parent. Analysis of variance detected significant variation among seedling families for time of budbreak, number of breaking buds, and flowering duration in `Golden Delicious' families. `Braeburn' × `Golden Delicious' consistently produced seedlings with higher numbers of breaking buds and `Golden Delicious' × `Prima' showed significantly lower numbers than other `Golden Delicious' families. In `Anna' families, significant differences were found for time of budbreak. `Anna' families showed higher variability within families than `Golden Delicious' families. Comparisons of progenies of `Anna' and `Golden Delicious' showed large differences in variation for the time of budbreak and for duration of flowering. Mean budbreak of `Anna' progenies did not differ from `Golden Delicious' progenies. Associations were found between initial time of budbreak and number of breaking buds as well as time and flowering duration. Results reveal high genetic variance in prolonged dormancy symptoms among seedlings within apple families which can be directly ascribed to high levels of heterozygosity in the cultivars used, and should be further explored for the purposes of breeding and selection.

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

Absence or long delay of budbreak, also known as prolonged dormancy, is the most important symptom during incomplete dormancy. Budbreak number was evaluated to quantify seedling response to chilling and selection on excised and intact 1-year-old apple (Malus ×domestica Borkh.) seedlings under controlled and natural environmental conditions. Indices based on: 1) the number and distribution of budbreak (prolonged dormancy grade = PDG); 2) the number of buds breaking, including shoot length with increased budbreak as part of the calculation (prolonged dormancy index = PDI); and 3) budbreak number per 100-cm shoot (NB) were tested in association with budbreak time (TB). The indices expressed the effects of cold treatments that induce earlier and higher numbers of budbreak. PDI and NB, but not PDG, identified families with increased budbreak. Seedlings with high PDG and NB were also associated with families in which high chill requiring parents were used, indicating that TB as pre-selection criterion may fail to identify seedlings with increased budbreak. Response to pre-selection for increased budbreak using PDG could be verified with the PDS and NB indices in seedlings and seedling clones. The NB of intact 1-year-old shoots under natural conditions is recommended as a pre-selection criterion against prolonged dormancy in suboptimal winter conditions.

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

Significant response to selection for budbreak number (NB) based on data recorded on 1-year-old shoots of young apple (Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.) seedlings (Expt. I) and branches from adult seedling trees (Expt. II) has been demonstrated in clonally propagated seedling trees. Between family variation for NB was low and masked by year × family interaction effects. Realized heritability for NB was estimated as 40% to 60%. Correlated response in uniformity and position of budbreak, and in the number and length of side shoots, was found. Association between the time of budbreak (TB) and NB, according to midparent and cross groupings, and according to the parental means, indicate a positive genetic correlation between these traits. Where data on adult trees were used as a measure of selection response and tested on young clonal trees, significant response and genetic variation was shown, confirming the presence of utilizable genetic variance and that this procedure may be successfully applied as an early screening method for increased budbreak in adult trees. Combined selection utilizing genetic variance between crosses as well as within crosses is proposed as the best procedure to increase the frequency of seedlings with increased budbreak and to improve adaptation to low winter chilling conditions.