Thesis (PhD)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: Various experiments were undertaken over a period of five years to investigate the feasibility of
initiating a large-scale programme of controlled apple breeding and selection for the
improvement of climatic adaptation, using budbreak number (NB) as a practical criterion of
selection. NB is preferred to time of budbreak as sole criterion on the grounds that early budbreak
is associated with low NB under local conditions. Variation within and between adult and
juvenile seedling families was investigated and the genetic control of the traits involved was
assessed, as well as direct and correlated responses to selection. In initial experiments different
rating criteria for NB as measure of chilling requirement were tested in association with
vegetative and reproductive budbreak time and flowering duration, viz, a classification index
based on number and distribution of budbreak (pDS grade), an index where shoot length with
increased budbreak was included in the index calculation (pDS index) and bud break number
expressed as number per 100 cm of shoot length (NB index). Variance analysis (ANOVA and
Variance component analyses) detected significant variation within seedling families for
budbreak time and NB, but estimates of genetic components of variance between families were
generally low. High genetic variance among seedlings within families is most likely due to the
high level of heterozygosity in the parental cultivars as is characteristic of vegetatively
propagated crops. Intra-class correlation coefficients for clones within and between families
indicate moderate genetic determination for NB with broad sense heritabilities around 30 percent.
Realized heritabilities calculated from response to two-way truncation selection were between 40
and 60 percent. For budbreak time (reproductive and vegetative), the broad sense heritability
averaged around 75 and 69 percent, respectively, indicating a high degree of genetic
determination. Significant response to selection for NB of one-year-old shoots of young seedlings
and from seedlings grown into adult trees showed that pre-selection for increased budbreak
successfully identified seedlings genetically inclined to more and better distribution of budbreak
within a set time of 21 days after initial budbreak. Correlated responses indicated additional
advantages of practical and horticultural value, viz, uniformity and position of bud break, and the
number and length of side shoots. In general, it is concluded from responses to two-way selection
that utilizable genetic variance in NB is present within seedling families and thus that selection
may 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 is
therefore proposed as criterion of selection for improvement of climatic adaptation, and
combined selection utilizing genetic variation between and within crosses as the selection
method. / AFRIKAANSE OPSOMMING: Verskeie proewe is oor 'n periode van vyf jaar uitgevoer om die toepaslikheid van 'n
grootskaalse appelteel- en seleksieprogram vir die verbetering van klimaatsaanpasbaarheid te
ondersoek met 'aantal knopbreke' (NB) as praktiese seleksiekriterium. NB word verkies bo tyd
van knopbreek op grond daarvan dat vroeë knopbreek onder plaaslike toestande met lae NB
gepaard gaan. Variasie binne en tussen volwasse en jong saailingfamilies en die genetiese beheer
van die betrokke eienskappe is ondersoek, asook direkte en gekoreleerde seleksieresponsie. In die
aanvangs-eksperimente is verskillende kriteria vir die kwantifisering van aantal knopbreke
getoets as potensiële maatstawwe van die inherente kouebehoefte in appelsaailinge. Die tyd van
vegetatiewe en reproduktiewe knopbreek en blomperiode is ook getoets. Die volgende indekse is
gebruik: 'n klassifikasie-indeks om die aantal en verspreiding van knopbreke te beskryf (pDS
graad), 'n indeks waar die lootlengte, met verhoogde aantal knopbreke, ingesluit is in die
berekening van die indekswaarde (PDS indeks), en knopbreke uitgedruk as die aantal per 100 cm
lootlengte (NB indeks). Variansie analise (ANOVA en variansie komponent analise) het
betekenisvolle variasie binne saailingfamilies aangetoon vir tyd van, en aantal knopbreke.
Ramings van genetiese komponente van variansie tussen families was relatief klein. Hoë
genetiese variansie tussen saailinge binne families is waarskynlik te wyte aan die hoë vlak van
heterosigositeit in die ouergenotipes, wat kenmerkend is van gewasse wat vegetatief voortgeplant
word. Intraklas korrelasie koëffisiënte vir klone tussen en binne families het gedui op 'n
middelmatige oorerflikheid in die breë sin (ongeveer 30 persent) vir aantal knopbreke. Verhaalde
oorerflikhede wat bereken is vanaf responsie op twee-rigting atknottingsseleksie was tussen 40
en 60 persent. Vir tyd van knopbreek (vegetatief en reproduktief) was die breësin oorerflikhede
ongeveer 75 en 69 persent, onderskeidelik, wat aanduidend is van 'n hoë graad van genetiese
bepaling. Betekenisvolle responsie op seleksie vir NB van jong saailinge en saailingbome wat
volwassenheid bereik het toon dat pre-seleksie vir knopbreke saailinge kan identifiseer wat
geneties meer knopbreke en 'n beter verspreiding van knoppe binne 'n periode van 21 dae na die
eerste knopbreek lewer. Gekorreleerde responsie op seleksie toon 'n addisionele voordeel van
praktiese en tuinboukundige belang, naamlik, meer en langer sylote. In opsomming kan dit gestel
word dat responsie op twee-rigting seleksie bruikbare genetiese variasie vir NB binne
saailingfamilies ontgin het en dat seleksie vir verhoogde aantal knopbreke suksesvol toegepas kan word. Die NB indeks op een-jaar-oue hout word dus voorgestel as seleksiekriterium vir
verbetering van klimaatsaanpasbaarheid onder plaaslike sub-optimale wintertoestande, en
gekombineerde seleksie "combined selection" wat genetiese variasie binne en tussen kruisings
benut as seleksiemetode.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52677 |
| Date | 03 1900 |
| Creators | Labuschagne, Iwan Frederick |
| Contributors | Louw, J. H., Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 161 pages : illustrations |
| Rights | Stellenbosch University |
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