Thesis (PhD(Agric) (Horticulture))--University of Stellenbosch, 2011. / Includes bibliography. / ENGLISH ABSTRACT: Mealiness, a soft, dry textural disorder of ‘Forelle’ pear (Pyrus communis L.), is a problem for the South
African fruit export industry. Soft, dry textural disorders seem to be related to changes in cell wall
breakdown. The aim of this work was, therefore, to investigate the occurrence of mealiness‐associated
changes in the cell wall and elucidate the mechanism by which mealiness occurs in ‘Forelle’ pear, as well as
to characterise cell wall changes occurring during normal ripening.
Mealy ‘Forelle’ tissues had significantly lower total galacturonic acids associated with the middle lamella
(water‐ and CDTA‐soluble fractions). The water‐soluble pectin of mealy tissues was depolymerised at an
earlier stage of ripening. The widespread disintegration of cell‐to‐cell adhesion in mealy cell walls only,
suggests that the middle lamella and the plasmodesmata are more broken down. In mealy ‘Forelle’ tissues
there was no indication of less broken down high molecular weight polyuronides in the CDTA fraction,
normally associated with these dry, soft textures. The pectins from mealy tissues were more broken down
and both mealy and non‐mealy tissue polyuronides depolymerised. Furthermore, there was a lack of light
toluidine staining in the larger air spaces, which would indicate such water‐insoluble pectins. These data
suggest that the formation of high molecular weight pectate gels is unlikely in mealy ‘Forelle’ pear. The
slight increase in the galactose content in mealy tissues in CDTA‐ and Na2CO3‐soluble fractions and slight
decrease in the 1 M KOH glycan fraction during later stages of ripening (6+11, 9+7, 9+11; weeks at ‐0.5°C
plus days at 15°C) may indicate that galactose loosely interlinked into the glycan fraction broke down
sooner for mealy tissues. This didn’t increase molecular size profiles in the CDTA fraction. Arabinose
content was slightly higher in the 4 M KOH fraction and slightly lower in mealy tissues of water‐ and
CDTA fractions. This did not influence the molecular weight of the glycans compared to those in the nonmealy
tissues. ‘Forelle’ data therefore seem to be more congruent with a decrease in intercellular adhesion
as the mechanism by which mealiness occurs, rather than the formation of high molecular weight pectins
taking up the cellular fluid.
‘Forelle’ pear water‐soluble pectin content increases with increased ripening. High amounts of watersoluble
pectin and low amounts of Na2CO3‐soluble pectin suggests that solubilisation of
rhamnogalacturonan‐I pectins must have taken place during early ripening (at a fruit firmness of > 4.7 kg
(7.9mm tip). Galactose and glucose in the pectin fraction dramatically decreased after fruit ripened to a
firmness of 4.5 kg, whereafter they remained unchanged. This was also the period in which fruit softened
the most and the biggest increase in pectin water‐solubility occurred. It is not known whether these events
are coincidental, or linked causally. Rhamnose and arabinose extractability increased in the water fraction
and xylose, fucose and mannose increased in glycan fractions with ripening. The biggest changes in
polyuronide solubilisation and depolymerisation occurred in water‐ and CDTA fractions between storage
and ripening durations of 3+7 (4.7 kg) and 6+4 (2.7 kg). / AFRIKAANSE OPSOMMING: Melerigheid, ʼn sagte droë tekstuur afwyking van ‘Forelle’ pere (Pyrus communis L.), is ʼn probleem vir die
Suid Afrikaanse vrugte uitvoerbedryf. Sagte, droë tekstuur afwykings blyk betrekking te hê op
selwandafbraak veranderinge. Die doel van die studie was dus om die melerigheid‐geassosieerde
veranderinge in die selwand te ondersoek, sowel as om vas te stel wat die meganisme betrokke is by
melerigheid ontwikkeling in ‘Forelle’ pere. Die selwand veranderinge gedurende normale rypwording is ook
gekarakteriseer.
Melerige ‘Forelle’ weefsel het betekenisvol laer totale galakturoonsuur wat geassosieer is met die
middellamella (water‐ en CDTA‐oplosbare fraksies). Die water‐oplosbare pektien van melerige weefsel was
op ʼn vroeër stadium van rypwording gedepolimeriseer. Die wydverspreide disintegrasie van sel‐tot‐sel
adhesie, slegs in melerige selwande, dui aan dat die middellamella en die plasmodesmata meer afgebreek
is. Daar is geen indikasie van hoë molekulêre massa poliuroniedes in die CDTA fraksie van melerige ‘Forelle’
weefsel, wat gewoonlik geassosieer word met droë, sagte teksture nie. Die pektiene van melerige weefsel
was meer afgebreek en melerige en nie‐melerige weefsel se poliurone was gedepolimeriseer. Daar was ook
geen ligte toluïdien verkleuring in die groter intersellulêre lugruimtes nie, wat ʼn aanduiding sou wees van
wateronoplosbare pektiene. Hierdie data dui dus aan dat die vorming van hoë molekulêre pektien jel in
melerige ‘Forelle’ pere onwaarskynlik is. Die klein toename in galaktose inhoud in die CDTA‐ en Na2CO3‐
oplosbare fraksies en ʼn klein afname in 1 M KOH glikaan fraksie tydens latere rypheidstadiums (6+11, 9+7,
9+11; weke by ‐0.5°C plus dae by 15°C), kan beteken dat los verweefde galaktose in die glikaan fraksie
vroeër afgebreek het in melerige weefsels. Die molekulêre grootte profiel is nie verander in die CDTA
fraksie nie. Arabinose inhoud was bietjie hoër in die 4 M KOH fraksie en bietjie laer in melerige weefsel van
die water‐ en CDTA fraksies. Die molekulêre massa van die glikane was klaarblyklik onbeïnvloed hierdeur.
‘Forelle’ data blyk dus meer saam te stem met die meganisme waar ʼn vermindering in intersellulêre
adhesie ʼn rol speel in melerigheid, eerder as die meganisme waar hoë molekulêre pektien selvloeistowwe
bind.
‘Forelle’ peer water‐oplosbare pektieninhoud neem toe met toenemende rypheid. Hoë vlakke wateroplosbare
pektien en lae vlakke Na2CO3‐oplosbare pektien stel voor dat die oplossing van
rhamnogalakturonan‐I pektiene gedurende vroeë rypwording moes plaasgevind het (by ʼn fermheid van
> 4.7 kg (7.9mm punt). Galaktose en glukose in die pektienfraksie het drasties verminder nadat vrugte tot ʼn
fermheid van 4.5 kg ryp geword het, waarna hul onveranderd gebly het. Dit was ook die periode waarin
vrugte die meeste sag geword het en die grootste toename in poliuronied wateroplosbaarheid gevind is.
Dit is nie bekend of die gebeure toevallig of oorsaaklik verbind is nie. Rhamnose en arabinose
ekstraheerbaarheid het vermeerder in die water fraksies, en xylose, fukose en mannose het vermeerder in
die glikaan fraksies gedurende rypwording. Die grootste verandering in oplosbaarheid en depolimerisasie
het plaasgevind in die water‐ en CDTA fraksies tussen opberging en rypwordingsperiodes van 3+7 (4.7 kg)
en 6+4 (2.7 kg).
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/6746 |
| Date | 03 1900 |
| Creators | Crouch, Elke Monika |
| Contributors | Huysamer, Marius, Holcroft, Deirdre, University of Stellenbosch. Faculty of AgriSciences. Dept. of Horticulture. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 159 p. : ill. |
| Rights | University of Stellenbosch |
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