An investigation into the genetic variation of chilling requirement in apple (Malus x domestica Borkh.) progenies

[摘要] ENGLISH ABSTRACT: Various experiments were undertaken over a period of five years to investigate the feasibility ofinitiating a large-scale programme of controlled apple breeding and selection for theimprovement of climatic adaptation, using budbreak number (NB) as a practical criterion ofselection. NB is preferred to time of budbreak as sole criterion on the grounds that early budbreakis associated with low NB under local conditions. Variation within and between adult andjuvenile seedling families was investigated and the genetic control of the traits involved wasassessed, as well as direct and correlated responses to selection. In initial experiments differentrating criteria for NB as measure of chilling requirement were tested in association withvegetative and reproductive budbreak time and flowering duration, viz, a classification indexbased on number and distribution of budbreak (pDS grade), an index where shoot length withincreased budbreak was included in the index calculation (pDS index) and bud break numberexpressed as number per 100 cm of shoot length (NB index). Variance analysis (ANOVA andVariance component analyses) detected significant variation within seedling families forbudbreak time and NB, but estimates of genetic components of variance between families weregenerally low. High genetic variance among seedlings within families is most likely due to thehigh level of heterozygosity in the parental cultivars as is characteristic of vegetativelypropagated crops. Intra-class correlation coefficients for clones within and between familiesindicate moderate genetic determination for NB with broad sense heritabilities around 30 percent.Realized heritabilities calculated from response to two-way truncation selection were between 40and 60 percent. For budbreak time (reproductive and vegetative), the broad sense heritabilityaveraged around 75 and 69 percent, respectively, indicating a high degree of geneticdetermination. Significant response to selection for NB of one-year-old shoots of young seedlingsand from seedlings grown into adult trees showed that pre-selection for increased budbreaksuccessfully identified seedlings genetically inclined to more and better distribution of budbreakwithin a set time of 21 days after initial budbreak. Correlated responses indicated additionaladvantages of practical and horticultural value, viz, uniformity and position of bud break, and thenumber and length of side shoots. In general, it is concluded from responses to two-way selectionthat utilizable genetic variance in NB is present within seedling families and thus that selectionmay successfully be applied as an early screening method for increased budbreak in adult trees. The NB index of intact one-year-old shoots under prevailing sub-optimal winter conditions istherefore proposed as criterion of selection for improvement of climatic adaptation, andcombined selection utilizing genetic variation between and within crosses as the selectionmethod.

[发布机构] Stellenbosch University