Unlike most other fruits that accumulate oil in seed tissues, avocado stores large amounts of oil in the form of triacylglycerol (TAG) in its mesocarp tissue. The regulation of TAG biosynthetic pathways in such nonseed tissues is poorly understood. RNA-Seq was used to identify the transcriptional networks underlying TAG biosynthesis in developing mesocarp of avocado. Deep transcriptional profiling studies revealed that several transcripts were differentially represented between the early and late developmental stages of mesocarp. The temporal expression pattern of transcripts associated with fatty acid biosynthesis in plastid coincided with increasing oil content. Furthermore, except for the transcripts that likely encode enzymes involved in the terminal step in TAG synthesis, others involved in TAG assembly in the endoplasmic reticulum were poorly expressed. This pattern of higher expression for genes involved in fatty acid synthesis but not TAG assembly was similar to previously observed pattern in other seed and nonseed tissues. Comparative analysis of transcript levels for about 1500 transcription factors, across different species and oil-storing tissues, also revealed that WRINKLED1 may be highly conserved across species but transcriptional regulation of oil biosynthesis, includes distinct tissue-specific features. In conclusion, while the biochemical pathways for oil biosynthesis in nonseed tissue are highly similar to that of seed tissues, certain distinctive modes of regulation of fatty acid biosynthesis and TAG assembly likely occur in oil-rich nonseed tissues.

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)

Thesis (M. Sc. (Agriculture, Horticulture)) --University of Limpopo, 2016 / ‘Reed’ avocado is a late season cultivar introduced to South Africa from California. The cultivar has shown good adaptation and produces quality fruit with export potential. Its pre-harvest adaptation and production aspects have been researched and documented. However, the effect of harvest time, post-harvest storage and ripening temperature has not been comprehensively studied on this newly introduced ‘Reed’ avocado cultivar. Therefore, the aim of this work was to investigate the effect of different harvest time, post-harvest storage and ripening temperature on the quality of late season ‘Reed’ avocado fruit. Matured ‘Reed’ avocado fruit were harvested based on moisture content indexing in December (2015) and January (2016). The experiment was carried out in a factorial, arranged in a completely randomised design (CRD) with three replicates. Treatment factors were: 2 x harvest time (mid-and late), 2 x post-harvest storage (2.0 and 5.5°C), 3 x ripening temperature (16, 21 and 25°C) and 5 x ripening day (0, 2, 4, 6 and 8). Fruit were stored at 2.0 and 5.5°C for 28 days, thereafter, ripened at 16, 21 and 25°C until fully ripe. During ripening, fruit were evaluated for weight loss, skin colour, firmness, respiration rate, physiological and pathological disorders. Mid-harvest fruit had higher moisture content when compared with late harvest fruit. However, harvest time, post-harvest storage, ripening temperature and ripening time (days) significantly influenced fruit weight loss, firmness, respiration rate, ripening percentage of ‘Reed’ avocado fruit during ripening. Moreover, fruit firmness decreased faster at higher temperatures (25 and 21°C) with fruit ripening within 4 and 6 days, respectively. In addition, ripening at a lower temperature (16°C) was slower with fruit fully ripened within 8 days after withdrawal from cold storage at both harvest times. ‘Reed’ avocado fruit respiration rate followed a climacteric pattern, however, significantly higher rate at higher temperature (25°C) when compared with lower temperature (16°C) after withdrawal from cold storage during both harvest times. Interestingly, mid-harvest fruit showed high electrolyte damage after withdrawal from 2.0°C when compared with late harvest fruit at the same temperature. Furthermore, mid-harvest fruit stored at 2.0ºC and ripened at 21°C showed higher chilling injury when compared with fruit ripened at 16 and 25°C. High electrolyte leakage positively correlated (R2 = 0.242) with high chilling damage for xiv mid-harvest fruit stored 5.5°C. Treatment factors had a significant effect (P < 0.05) on lightness (L *) and hue angle (h ) but no significant effect (P > 0.05) on chroma (C *) and eye colour of ‘Reed’ avocado fruit during ripening, irrespective of harvest time. Overall results showed a visual change in ‘Reed’ avocado skin colour, with eye colour changing from green to bright yellow. Furthermore, late harvest fruit showed high post-harvest pathological diseases after removal from high temperature (5.5°C) when compared with mid-harvest fruit stored at low storage temperature (2.0°C). Ripening at a higher temperature (21 and 25°C) resulted in higher incidence of stem-end rot and body rot when compared with lower temperature (16°C) for both harvest times. Late harvest fruit showed a higher incidence of vascular browning at higher ripening temperatures (21 and 25°C) when compared with lower temperature (16°C) after withdrawal from cold storage. Moreover, overall results showed that harvest time, post-harvest storage and the ripening temperature had a profound influence on the quality of ‘Reed’ avocado fruit. In conclusion, ‘Reed’ avocado fruit can be harvested during mid- or late season and stored at recommended low temperature (2.0ºC); and thereafter, ripened at either 16 or 21ºC. In addition, future studies should focus on identifying pre-harvest practices that promote higher post-harvest fruit quality for ‘Reed’ avocado fruit under South African production environment. Keywords: ‘Reed’ avocado fruit; firmness; electrolyte leakage; respiration rate; stem-end rot; body rot; vascular browning