Ph.D. (Agriculture)) -- University of Limpopo, 2021 / The rind physiological disorders incidence such as rind pitting is a challenge to the
citrus industry as it affects appearance; and ultimately, acceptability and purchase in
both local and international markets. Although the internal quality is not directly
affected by rind pitting, fruit damaged by this disorder are rejected in the fresh fruit
market. The susceptibility to this disorder varies among citrus fruit cultivars. Other
factors impacting rind physiological disorders include; pre-harvest environmental
conditions and postharvest storage conditions. However, the main cause of this
disorder is still unknown. In South Africa, ‘Benny’ valencias are the most prone
orange cultivars to rind pitting disorder within the sweet-orange-type. Therefore, the
aim of this study was to investigate production site and postharvest treatments’ effect
on physico-chemical, biochemical properties, antioxidants, rind soluble sugars and
gene expression in relation to rind pitting development of ‘Benny’ valencia citrus fruit.
During 2016 and 2017 seasons, the study was conducted, whereby, ‘Benny’ valencia
citrus fruit were harvested from Tzaneen, Groblersdal and Musina in South Africa.
After harvesting, the fruits were transported to the Agricultural Research Council-
Tropical and Subtropical Crops (ARC-TSC) postharvest laboratory in Nelspruit for
sorting, grading treatment, cold storage and post-storage quality evaluation. After
sorting and grading, fruits were subjected to the following treatments: T1 = no wax
plus dehydration, T2 = wax plus dehydration and T3 = wax plus no dehydration.
Dehydrated treatments were applied for 3 days at relative humidity of ±45%,
thereafter, fruit were stored at -0.6 and 4.5°C for 28 days plus 7 days shelf-life. After
removal from cold storage plus 7 days shelf-life, fruit were analysed for rind pitting
incidence (RPI), weight loss percentage (WL), firmness, total electrolyte leakage
(TEL), total soluble solids (TSS), titratable acidity (TA) and TSS: TA ratio.
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Afterwards, fruit were peeled to remove flavedo, thereafter; the flavedo peels were
freeze-dried, milled and stored at -21°C for further physiological analysis. Freeze
dried flavedo peel was analysed for total flavonoids, total phenolics, vitamin C,
soluble sugars (glucose, fructose and sucrose), antioxidant assays (FRAP, DPPH,
ABST and ORAC) and genes.
The results showed that rind pitting incidence was high on fruit subjected to wax plus
no dehydration across all storage temperatures and production sites. Furthermore,
results showed that fruit harvested from Musina exposed to T1 had higher incidence
of rind pitting than those from Groblersdal and Tzaneen, irrespective of storage
temperature. Meanwhile, fruit harvested from Musina had the highest TEL when
compared with Groblersdal and Tzaneen irrespective of treatments and storage
temperatures. A significantly higher (P<0.05) WL was observed in Musina fruit
harvested from Tzaneen exposed to T3 at both storage temperatures. Moreover,
increased TSS was observed after storage across all production site and postharvest
treatments. The study showed that production site and postharvest treatments had a
significant influence on rind pitting and total electrolyte leakage. Additionally, fruit
treated with no wax + dehydration was found to be more susceptible to rind pitting.
However, fruit sourced from Tzaneen had significantly (P<0.0001) high TPC and
TFC, irrespective of postharvest treatments therefore, low rind pitting incidence.
While rind vitamin C was higher in fruit from Groblersdal when compared with
Tzaneen and Musina. However, low RPI was also observed in fruit sourced from
Groblersdal. Fruit from Musina subjected to wax plus dehydration had higher RSA
and low RPI at both temperatures when compared with fruit sourced from
Groblersdal and Tzaneen. Therefore, wax plus dehydration resulted in low rind
pitting with an increased accumulation of rind biochemical concentrations,
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irrespective of cold storage temperatures. These results suggested that there is a
link between rind pitting and rind biochemical concentrations in the ‘Benny’ valencia
citrus fruit. Moreover, fruit from Musina subjected to wax plus dehydration had higher
antioxidant measured by DPPH and low RPI at both low storage temperatures when
compared with fruit harvested from Groblersdal and Tzaneen. Furthermore, fruit
harvested from Musina and treated with no wax plus dehydration, thereafter, stored
at -0.6°C had low RPI with high antioxidant activity measured by FRAP than
Tzaneen and Groblersdal regions in both seasons.
With respect to sugars, fructose was not significantly (P<0.05) affected by production
sites, postharvest treatments and cold storage temperature, hence, the low pitting
incidence. The highest glucose was observed in fruit harvested from Groblersdal,
irrespective of treatments and cold storage temperatures when compared with those
from Tzaneen and Musina, low RPI was also observed in fruit harvested from
Groblersdal. However, fruit harvested from Groblersdal treated with wax plus
dehydration and stored at 4.5°C had higher sucrose and low RPI when compared
with Tzaneen and Musina. Moreover, this study suggested that soluble sugars in
‘Benny’ valencia flavedo during cold storage is involved in rind pitting tolerance
mediated by wax plus dehydration treatment.
Three homologic genes: CsCP gene; CsNAC-domain protein gene; CsCP-F gene;
were chosen to examine the relationship between their expression and citrus rind
pitting through quantitative RT-PCR analysis in pitting and no-pitting fruits. Results
showed that the expression of CsCP, CsNAC and CsCP-F genes were all higher in
the pitting rind fruit harvested from Tzaneen and low in fruit with low pitting.
Groblersdal and Musina fruit had low expression of genes and low rind pitting was
observed. Therefore, findings suggested that CsCP, CsNAC and CsCP-F genes may
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be linked to non-chilling rind pitting and could serve as targets for future
investigation.
Generally, the overall results obtained in this study provided an understanding into
the previous unknown complexities of citrus non-chilling rind pitting. Moreover, the
study revealed that the studied factors had an influence on non-chilling rind pitting
and physico-chemical properties of ‘Benny’ valencia citrus fruit. In addition,
postharvest treatments resulted in low non-chilling rind pitting with an increased
accumulation of rind biochemical concentrations. The fruit with high antioxidant
capacity were found to be tolerant to rind pitting, whereas, fruit with low antioxidant
capacity were found to be susceptible to rind pitting. Furthermore, soluble sugars are
believed to be involved in the defence mechanisms against non-chilling rind pitting in
the fruit. Gene expression changes also provided clues about the possible
mechanisms involved in non-chilling rind pitting development. / National Research Fund, Agricultural Research Council Tropical and Subtropical Fruit Crops and Citrus Academy
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ul/oai:ulspace.ul.ac.za:10386/3697 |
| Date | January 2021 |
| Creators | Mothapo, Matholo Joyce |
| Contributors | Mathaba, N., Mafeo, T. P. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | xxviii, 164 leaves |
| Relation |
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