| Summary: | Thesis (MscAgric (Horticulture))--University of Stellenbosch, 2009. === ENGLISH ABSTRACT: For many apple (Malus domestica Borkh.) and pear (Pyrus communis L.)
cultivars, attractive colour is essential to their profitability on export markets. This
study focuses on problems related to poor green colour of ‘Granny Smith’ apples
and insufficient red colour of bi-coloured pear cultivars.
‘Granny Smith’ apples often suffer from poor green colour. Green colour of fruit
from various orchards was already found to differ midway through fruit
development, with these differences being carried through to harvest. In a trial
where nitrogen (N) fertilisers were applied using different forms at different times,
there was no improvement in green colour. In another trial, artificial shading was
applied to fruit only during their early development. Fruit that were shaded during
this time were less green at harvest than unshaded fruit. Additional N
applications may only improve colour where a deficiency exists. However, green
colour may be improved by increasing light distribution early during fruit
development.
Bi-coloured pears attain their maximum red colour midway through their
development, and this desired red colour is mostly lost prior to harvest. Red
colour can also increase transiently with the passing of cold fronts. Anthocyanins,
responsible for this red colour, may have a photoprotective function which would
explain this pigmentation pattern, as photosystems are particularly sensitive to
light damage at low temperatures. As ‘Rosemarie’ fruit bent over from a vertical
to hanging position during development, peel photoinhibition was reduced as
anthocyanins were synthesised. ‘Forelle’ peel was found to be very sensitive to
high light levels at low temperatures. Substantial anthocyanin development took
place in ‘Cripps’ Pink’ apples when weather conditions were cold, but clear
following a cold front. A photoprotective role seems to explain daily changes in
anthocyanins in response to temperature, but not the seasonal progression of
colour development. For many apple (Malus domestica Borkh.) and pear (Pyrus communis L.)
cultivars, attractive colour is essential to their profitability on export markets. This
study focuses on problems related to poor green colour of ‘Granny Smith’ apples
and insufficient red colour of bi-coloured pear cultivars.
‘Granny Smith’ apples often suffer from poor green colour. Green colour of fruit
from various orchards was already found to differ midway through fruit
development, with these differences being carried through to harvest. In a trial
where nitrogen (N) fertilisers were applied using different forms at different times,
there was no improvement in green colour. In another trial, artificial shading was
applied to fruit only during their early development. Fruit that were shaded during
this time were less green at harvest than unshaded fruit. Additional N
applications may only improve colour where a deficiency exists. However, green
colour may be improved by increasing light distribution early during fruit
development.
Bi-coloured pears attain their maximum red colour midway through their
development, and this desired red colour is mostly lost prior to harvest. Red
colour can also increase transiently with the passing of cold fronts. Anthocyanins,
responsible for this red colour, may have a photoprotective function which would
explain this pigmentation pattern, as photosystems are particularly sensitive to
light damage at low temperatures. As ‘Rosemarie’ fruit bent over from a vertical
to hanging position during development, peel photoinhibition was reduced as
anthocyanins were synthesised. ‘Forelle’ peel was found to be very sensitive to
high light levels at low temperatures. Substantial anthocyanin development took
place in ‘Cripps’ Pink’ apples when weather conditions were cold, but clear
following a cold front. A photoprotective role seems to explain daily changes in
anthocyanins in response to temperature, but not the seasonal progression of
colour development. Dwarfing rootstocks are known to improve red colour of bi-coloured pears due to
improved light distribution. ‘Forelle’ fruit from six rootstocks of varying vigour were
harvested from exposed positions only, so as to establish the effect of rootstock
on red colour development independent of the effect of rootstock on canopy light
distribution. Fruit from trees on quince (Cydonia oblonga Mill.) rootstocks were
found to have redder fruit than those from vigorous BP pear rootstocks. This may
be due to higher chlorophyll and carotenoid concentrations present in the peel of
fruit from BP rootstocks, whose leaf and peel N were also high. The use of
quince rootstocks is recommended where red colour development of bi-coloured
pears is a problem.
An early season bi-coloured cultivar with good red colour is required. Breeding
trials to find such a cultivar are resource intensive. To streamline the process, a
method to preselect immature seedlings for their future fruit colour is required.
Fruit colour from bearing seedlings was compared with colour of their immature
leaves. Trees with red leaves were likely to produce fruit that were too red for the
breeders’ requirements. Trees with green or blushed leaves were capable of
producing blushed fruit. It would be feasible to cull red-leaved seedlings with
minimal risk of losing potential bi-coloured cultivars. === AFRIKAANSE OPSOMMING: Verskeie appel (Malus domestica Borkh.) en peer (Pyrus communis L.) kultivars
se winsgewendheid word bepaal deur hul aantreklike kleur. In hierdie studie word
die swak groen kleur van ‘Granny Smith’ appels asook rooi kleurontwikkeling van
blospere ondersoek.
Die groen kleur van ‘Granny Smith’ appels is dikwels onvoldoende. Verskille in
groen kleur tussen boorde was reeds gedurende vroeë vrugontwikkeling
aanwesig, en hierdie verskille het voortgeduur tot met oes. Groen kleur kon nie
deur verskillende bronne en tye van stikstofbemesting verbeter word nie.
Stikstofbemesting verbeter groen kleur moontlik net in boorde met ‘n
stikstoftekort. Vrugte wat gedurende hul vroeë ontwikkeling oorskadu is, se groen
kleur was swakker by oes in vergelyking met vrugte wat nie oorskadu is nie.
Groen kleur kan moontlik verbeter word deur ligverspreiding tydens vroeë
vrugontwikkeling deur middel van snoei aksies te verhoog.
Blospeerkultivars bereik hul maksimum rooi kleur halfpad deur hul ontwikkeling,
maar is geneig om hul rooi kleur grootliks voor oes te verloor. Rooi kleur mag
egter kortstondig toeneem in reaksie op die lae temperature gepaardgaande met
koue fronte. Antosianiene, wat verantwoordelik is vir die rooi kleur, het moontlik
‘n beskermende funksie teen hoë ligvlakke, en hierdie funksie mag moontlik die
bogenoemde patroon van rooikleurontwikkeling verklaar. Die natuurlike buiging
van ‘Rosemarie’ pere van hul aanvanklike regop oriëntasie tot hul karakteristieke
hangende posisie, is gekenmerk deur ‘n afname in fotoinhibisie van die skil en ‘n
gelyklopende sintese van antosianien. ‘Forelle’ skil was uiters sensitief vir hoë
ligvlakke in kombinasie met lae temperature (16 ºC). ‘Cripps’ Pink’ appels het ‘n
vinnig toename in rooi kleur getoon met die koue, maar helder, weerstoestande
wat gevolg het op ‘n kouefront. Dit is welbekend dat dwergende onderstamme die rooi kleur van blospere
verbeter deur ligverspreiding in die boom te verhoog. Ten einde die effek van
onderstam op rooi kleurontwikkeling onafhanklik van die effek van onderstam op
ligverspreiding te ondersoek, is ‘Forelle’ pere wat blootgestel was aan vol son
geoes van bome geënt op ses onderstamme met verskillende groeikrag.
Kweperonderstamme (Cydonia oblonga Mill.) het rooi kleur verbeter in
vergelyking met die groeikragtige BP peeronderstamme. ‘n Moontlike rede vir die
verbetering is die laer chlorofiel- en karotenoïedkonsentrasies in die skil van
vrugte op kweperonderstamme. Bome op peeronderstamme het ook hoër blaaren
skil stikstofvlakke gehad. Kweperonderstamme word aanbeveel in gevalle
waar rooi kleurontwikkeling van blospere ‘n probleem mag wees.
Die RSA vrugtebedryf benodig ‘n vroeë blospeerkultivar met goeie rooi
kleurontwikkeling. Die teling van so ‘n kultivar is hulpbronintensief en baie duur.
Ten einde die teelproses meer effektief te maak, word ‘n metode benodig om
saailinge al voor uitplanting in die boord te selekteer na gelang van hul
toekomstige vrugkleur. Die vrugkleur van oesryp pere van draende saailinge is
vergelyk met die kleur van hul onvolwasse blare. Bome met rooi blare is geneig
om vrugte te dra wat té rooi is om te kwalifiseer as blospere. Die meerderheid
blospere is afkomstig van bome met blos of groen onvolwasse blare. Dit is
prakties haalbaar om rooiblaarsaailinge uit te dun, met net ‘n klein, aanvaarbare
risiko om ‘n moontlike blospeerkultivar in die proses te verloor. === medg2010-1 === Imported from http://etd.sun.ac.za April 2010.
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