No abstract

In the Western Cape region of South Africa dormancy release and the onset of growth does not occur normally in apple (Malus x domestica Borkh.) trees during spring due to the mild winter conditions experienced and fluctuations in temperatures experienced during and between winters. In this region the application of chemicals to induce the release of dormancy forms part of standard orchard management. Increasing awareness of the environmental impact of chemical sprays and global warming has led to the demand for new apple cultivars better adapted to local climatic conditions. We report the construction of framework genetic maps in two F1 crosses using the low chilling cultivar 'Anna' as common male parent and the higher chill requiring cultivars 'Golden Delicious' and 'Sharpe's Early' as female parents. The maps were constructed using 320 simple sequence repeats (SSR), including 116 new markers developed from expressed sequence tags (ESTs). These maps were used to identify quantitative trait loci (QTLs) for time of initial vegetative budbreak (IVB), a dormancy related characteristic. Time of IVB was assessed 4 times over a 6-year period in 'Golden Delicious' x 'Anna' seedlings kept in seedling bags under shade in the nursery. The trait was assessed for 3 years on adult full-sib trees derived from a cross between 'Sharpe's Early' and 'Anna' as well as for 3 years on replicates of these seedlings obtained by clonal propagation onto rootstocks. A single major QTL for time of IVB was identified on linkage group (LG) 9. This QTL remained consistent in different genetic backgrounds and at different developmental stages. The QTL may co-localize with a QTL for leaf break identified on LG 3 by Conner et al. (1998), a LG that was, after the implementation of transferable microsatellite markers, shown to be homologous to the LG now known to be LG 9 ( Kenis and Keulemans, 2004). These results contribute towards a better understanding regarding the genetic control of IVB in aplle and will also be used to elucidate the genetic basis of other dormancy related traits such as time of initial reproductive budbreak and number of vegetative and reproductive budbreak.

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.

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.