Thesis (Ph.D. (Plant Production)) -- University of Limpopo, 2022 / In 'Hass' avocado fruit, pre-harvest and postharvest factors affecting exocarp colour
change during ripening are vital to maintain the industry’s credibility, competitiveness
and profitability. Currently, the South African ‘Hass’ avocado fruit exocarp colour
development is affected by pre- and postharvest factors, ultimately, fruit does not
develop the required purple colour during ripening. These pre- and postharvest factors
must be understood in order to implement strategies that avoid downgrading of South
African 'Hass' avocado fruit by lucrative markets due to insufficient purple colour
development during ripening. In 'Hass' avocado fruit, exocarp colour development is
associated with an increase in anthocyanin synthesis and accumulation during
ripening. However, limited information is available regarding factors regulating
anthocyanin synthesis and accumulation in 'Hass' avocado fruit during ripening.
Therefore, the overall aims of this study were to investigate pre-harvest practices and
postharvest treatment that increase exocarp anthocyanin synthesis during ripening. In
addition, determine whether exocarp glucose and other antioxidants contribute to
'Hass' avocado fruit exocarp colour development during ripening.
In chapter 3, the study looked at how crop load adjustment affects ‘Hass’ avocado fruit
exocarp colour development during ripening at three different harvest maturities. The
crop load adjustment treatments were applied as: high (100%), moderate (50%) and
low (25%) at three harvest times (early, mid- and late). After harvest, fruit were stored
at 5.5°C for 28 days, thereafter, ripened at 25°C. The experimental design was carried
out as 3 x 3 factorial, arranged in a completely randomized design (CRD) with three
replications. The results showed that total anthocyanin and cyanidin 3-O-glucoside concentrations of ‘Hass’ avocados increased following crop load adjustment from
normal (100%) to moderate (50%) and low (25%) loads, resulting in improved exocarp
colour development during ripening. Furthermore, we discovered that fruit harvested
from moderate (50%) and low (25%) crop loads accumulated higher exocarp sugars
(D-mannoheptulose and perseitol) at three harvest maturities when compared with
high crop load (100%). Moreover, total phenolic concentration of fruit harvested from
moderate (50%) and low (25%) crop loads was higher than that obtained from high
load fruits, irrespective of harvest maturities.
In chapter 4, the study examined the interaction between branch girdling and harvest
maturation on the development of 'Hass' avocado fruit exocarp colour during ripening.
The experimental design was carried out as 2 x 2 factorial, arranged in a completely
randomized design (CRD). The results showed that fruit harvested from girdled trees
had poor exocarp colour development as compared to fruit harvested from control
trees, regardless of harvest time. Fruit harvested from girdled and ungirdled avocado
trees did not show significant differences in visual exocarp colour during early and
mid-maturity. Apart from crop load adjustment and girdling as pre-harvest methods to
manipulate postharvest exocarp colour, glucose was also infused through the pedicel.
Studies on the effect of glucose infusion through the pedicel on the exocarp colour of
the ‘Hass’ avocado fruit during ripening were presented in chapter 5. The study
included five treatments; control fruit with pedicel and infused with distilled water and
glucose concentrations (0.05, 0.13 and 0.28 mM). The distilled water, glucose infused
and control fruit were stored at 5.5°C for up to 28 days. After cold storage, fruit were
kept at ambient temperature 25°C for ripening. The experiment was conducted as a
completely randomized design (CRD) with three replications per treatment. The results
showed that glucose infusion through the pedicel markedly increased anthocyanin and
cyanidin 3-O-glucoside concentration during ripening. Interestingly, glucose
concentrations (0.05 and 0.13 mM) resulted in purple colour development after 8 days
at 25°C when compared with control, distilled water and highest concentration (0.28
mM).
In chapter 6, the relationship between 'Hass' avocado fruit size, exocarp colour and
related pigments with antioxidants capacity and sugar concentration during ripening
were investigated. The fruit were categorized by their weight; small (< 200 g) and large
(> 201 g). Their diameter and length were also measured using a vernier calliper. Fruit
ware stored at 5.5°C for 28 days, then ripened at 25°C. The experimental design was
carried out as a completely randomised design (CRD), using 25 fruit replications per
category. The results showed that small-sized fruit developed the desirable purple to
black exocarp colour when compared with large-sized fruit. Additionally, the results
showed that small-sized fruit had higher antioxidant capacity as measured by 2,2
diphenyl 1 picrylhydrazyl (DPPH), ascorbic acid and flavonoid content during ripening
when compared with large-sized fruit. Furthermore, it was found that small-sized fruit
accumulated higher exocarp and seeds (D-mannoheptulose, perseitol, sucrose and
glucose) sugar concentration. We demonstrated and concluded that exocarp colour,
pigments, antioxidants and sugar concentration are closely related to size in 'Hass'
avocado fruit. Knowledge from this thesis contributes toward the understanding of pre and postharvest factors that may influence colour development of ‘Hass’ avocado fruit
during ripening. This study contributes towards bridging the gap in the literature on the
biochemical changes associated with colour development of ‘Hass’ avocado fruit
during ripening / Agricultural Research Council-Tropical
and Subtropical Crops (ARC-TSC) and National Research Foundation (NRF)
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ul/oai:ulspace.ul.ac.za:10386/4242 |
| Date | January 2022 |
| Creators | Shikwambana, Kingsly |
| Contributors | Mafeo, T. P., Mathaba, N. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | xxiv, leaves 202 |
| Relation |
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