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The avocado mesocarp contains up to 60-70% oil by dry weight where triacylglycerol (TAG) is the major constituent. This neutral lipid, TAG is utilized by plants for the carbon and energy source when stores in seed tissue. There is significant human nutritional demand for vegetable oil, but its use in the production of renewable biomaterials and fuels has intensified the need to increase oil production. In plants, the final and committed step in TAG biosynthesis is catalyzed by diacylglycerol acyltransferases (DGAT) and/or a phospholipid: diacylglycerol acyltransferases (PDAT). However, the regulation of TAG biosynthesis is not well-studied in nonseed tissues such as mesocarp of avocado. Based on the transcriptome data of Persea americana it is hypothesized that both DGAT and PDAT are likely to catalyze the conversion of diacylglycerol to TAG. In this study, putative DGAT1 and PDAT1 were identified and comprehensive in silico analyses were conducted to determine the respective start codons, full-length coding sequences, transmembrane domains, predicted protein structures and phylogenetic relationships with other known DGAT1s and PDAT1s. These data reveal that the putative DGAT1 and PDAT1 of a basal angiosperm species retain features that are conserved not only among angiosperms but also other eukaryotes. For transient expression, DGAT1 and PDAT1 were transformed into N. benthamiana leaves by agrobacterium-mediated transformation. Lipid droplet was visualized by Nile Red staining and lipid content and compositions were analyzed by TLC and GC-MS. It was found that avocado DGAT1 and PDAT1 increase lipid content significantly when expressed in tobacco leaves. These results suggest that avocado DGAT1 and PDAT1 are functional and synthesize TAG when expressed in planta.

Thesis (M. Sc. (Horticulture)) -- University of Limpopo, 2019 / The South African Avocado Industry is continuously expanding with ‘Hass’ cultivar occupying the largest land and therefore, the most exported cultivar. Expansion of the industry implies an increased demand for irrigation water. However, South Africa is a water scarce country characterised by low and erratic rainfall, where fruit production relies heavily on irrigation. The agricultural sector is under an obligation to reduce water use due to the increasing demand for water by other economic sectors. Therefore, it must find efficient water use techniques to save water and ensure water availability for other economic sectors. Partial root-zone drying (PRD) is an irrigation technique which may lead to 50% reduction in water use, half of the root system is kept wet while the other half is left dry to a predetermined level of soil water depletion, the dry and the moist sides are alternated at regular intervals. During storage and transportation of fresh produce, low temperature is a post-harvest tool used to maintain quality, especially when fruit are destined for long distant export markets. This study investigated the effect of PRD and low temperature storage on post-harvest quality parameters of ‘Hass’ avocado fruit. The experiment was laid as a factorial arranged in randomised complete block design (RCBD). The treatment factors for chilling injury, electrolyte leakage and vascular browning were 2 x irrigation regimes (Full Irrigation (FI) vs. PRD) and 2 x storage temperatures (2.0 and 5.5°C). However, treatment factors for fruit weight loss, respiration rate, fruit firmness, ripening percentage and fruit skin colour were 2 x irrigation regimes (Full Irrigation (FI) vs. PRD), 2 x storage temperatures (2.0 and 5.5°C) and 4 ripening days (0, 2, 4 and 6 days). Mature ‘Hass’ avocado fruit were harvested from PRD and fully irrigated trees and thereafter fruit of each treatment were stored at 2.0 and 5.5°C for 28 days. Each treatment consisted of 6 replicates, with an exception of electrolyte leakage, whereby the experiment was replicated 4 times. After 28 days’ storage at 2.0 and 5.5°C, fruit were ripened at 21°C. The effect of PRD and low temperature storage was determined by evaluating the following physico-chemical fruit parameters during ripening: external chilling injury, electrolyte leakage, fruit weight loss, respiration rate, firmness, fruit skin colour and vascular browning. During ripening, ‘Hass’ avocado fruit stored at 2.0°C showed significantly higher incidences of external chilling injury symptoms compared with xiii 5.5°C, irrespective of irrigation treatment. Furthermore, an interaction between irrigation treatment and low storage temperature had a significant (P<0.05) effect on cell membrane electrolyte leakage. All evaluated fruit showed similar weight loss, irrespective of irrigation and storage treatment. Irrigation, storage temperature and ripening days did not have effect on respiration during ripening. Fruit reached the respiratory climacteric peak on the same day (day 2). Furthermore, there was no significant interaction effect (P˃0.05) on fruit firmness. However, PRD treated fruit showed slightly low firmness when compared with control fruit. ‘Hass’ avocado fruit harvested from PRD and fully irrigated trees and stored at 5.5°C ripened quicker compared to fruit stored at 2.0°C. There was no significant interaction effect (P˃0.05) on fruit skin lightness (L), chroma (C), hue angle (h°) and fruit eye colour due to irrigation and cold storage treatment. In general, fruit showed skin colour change from emerald green to approximately 75% coloured. Furthermore, irrigation and storage temperature did not have effect on vascular browning, however, there incidence was high on fruit stored at 2.0°C when compared with 5.5°C. The results of this study indicated that ‘Hass’ avocado fruit stored at 2.0°C was negatively affected by low temperature storage and this cold storage temperature is not recommended. PRD reduced water use during irrigation, however, its effect on post-harvest quality of ‘Hass’ avocado fruit subjected to 5.5°C must be further investigated before recommended for export markets. / Agricultural Research Council-Tropical and Subtropical Crops (ARC-TSC) and Agricultural Sector Education Training Authority (AgriSeta)

Thesis (M.Sc.(Agricultural Management )) -- University of Limpopo, 2020 / Avocado fruit ‘Hass’ harvested during early-season and exposed to temperature at 5.5°C for 28 d are susceptible to chilling injury (CI); and therefore, develop poor skin colour during ripening. In ‘Hass’ avocado fruit, skin colour change during ripening is used by European market to indicate fruit ripeness and softness. Therefore, the aim of this study was to evaluate the use of hot water (HW) and methyl jasmonate (MJ) as postharvest treatment dips to mitigate CI; and thereby, enhance ‘Hass’ avocado fruit peel colour during ripening. Fruit were harvested randomly from 5 selected trees treated alike during early season (April 2018); and thereafter, transported to the laboratory. At the laboratory, experiments of this study were divided into 2: experiment (1) fruit were dipped into HW (38, 42 and 46°C for 30, 25 and 20 min, respectively); and experiment (2) fruit were dipped into MJ (10 and 100 µmol/L for 2 min) treatments. In both experiments after these treatments, fruit were allowed to dry for 60 minutes at ambient (±25°C) temperature and untreated fruit were used as control. Thereafter, fruit were stored at commercial shipping temperature (5.5°C) for up to 28 d. After removal from cold storage, fruit were ripened at ambient temperature (±25°C) and evaluated every after 2 d for weight loss, firmness loss, objective colour parameters (lightness-L*, chroma-C* and hue angle-h*), subjective colour (eye colour) and ripening percentage. However, chilling injury (CI) and electrolyte leakage (EL) were evaluated immediately after removal from cold storage. The results showed that HW significantly (P< 0.05) increased weight and firmness loss during ripening. Furthermore, HW reduced EL and external chilling injury (ECI) of ‘Hass’ avocado fruit during cold storage. In addition, the results showed that HW had significant effect (P< 0.05) on colour parameter L* and eye colour rating, but did not affect (P> 0.05) C* and h*. Avocado ‘Hass’ fruit subjected to HW at 42°C/25 and 46°C/20 min developed purple colour (eye colour rating 4.47 and 4.36, respectively) during ripening when compared with HW at 38°C/30 min and control fruit. Moreover, results showed that dipping fruit in 10 µmol/L had a significant effect (P< 0.05) on reducing weight loss during ripening. Methyl jasmonate (10 and 100 µmol/L) treatment reduced EL and alleviated external chilling injury (ECI) of ‘Hass’ fruit during cold storage. The results showed that MJ (10 and 100 µmol/L) treatments had significant effect (P< 0.05) on colour parameter L*, h* and eye colour rating, but did not affect (P> 0.05) C*. Furthermore, ‘Hass’ fruit treated with 10 and 100 µmol/L MJ reached the purple skin colour (eye rating 5.39 and 5.19, respectively) during ripening. Fruit dipped in MJ (10 µmol/L) had low weight loss when compared with fruit treated with MJ (100 µmol/L). In conclusion, the results of this study indicated that HW (42°C/25 minutes) and MJ (10 µmol/L) effectively alleviated external chilling injury; and therefore, improved ‘Hass’ skin colour development during ripening / Agricultural Research Council-Institute (Agriseta) and University of Limpopo

The study’s aim was to determine how consumers relate to specific factors that affect the way in which fruit is marketed in supermarkets. This might have an influence on effective commercialization of new intellectual property in the global avocado industry.

The study’s aim was to determine how consumers relate to specific factors that affect the way in which fruit is marketed in supermarkets. This might have an influence on effective commercialization of new intellectual property in the global avocado industry.

The avocado industry needs improved cultivars with better agronomic traits such as increased resistance to pests and diseases. These could be developed with biotechnological approaches to breeding such as the use of genetic engineering. a prerequisite to the use of this technique if the development of an efficient in vitro regeneration system. The objective of this study therefore, was to develop a tissue culture protocol suitable for the propagation and transformation of avocado. To do this, the effects of the preconditioning of mother plants, different explant sources, growth regulator pretreatments of explants and different compositions of the growth medium were examined. The results of these confirmed that avocado is typical of woody plants, as avocado tissues shows a decrease in morphogenetic capacity as they age, and the most juvenile explant, the embryonic shoot axis, showed the highest potential for shoot regeneration. Regrowth of explants after removal of the first flush of shoots resulted in the production of shoot bases in vitro. Shoot bases were most efficiently produced when embryonic axes were dissected transversely to the axis of growth. This study has resulted in the production of an efficient system for the production of multiple shoots from embryonic shoot tissues of avocado through the induction of shoot bases. This tissue culture system could be extended to the use of somatic tissues as a source of explants. If shoot bases can be established from such tissues, this will provide a means by which avocado can be clonally propagated and provides a system by which genetic transformation and other techniques of biotechnology can be applied to the production of new and superior cultivars. / Doctor of Philosophy (PhD)

This study focuses on the soil- and water-borne plant pathogen Phytophthora cinnamomi Rands and the phenomenon of P. cinnamomi suppressive soil. In particular, this thesis reports on the outcome of field surveys and glasshouse assays undertaken to locate P. cinnamomi suppressive soils and to confirm the involvement of biological processes in suppression. The potential role of cellulase and laminarinase in suppression was investigated and a molecular technique known as length heterogeneity PCR (LH-PCR) was used to analyse the structure and diversity of bacterial and fungal communities in avocado orchard soils that were suppressive and conducive to P. cinnamomi. Four avocado orchards with P. cinnamomi suppressive soils were identified and soils were ã-irradiated to destroy their suppressive capacity, thus confirming biological suppression. Suppression was also partially transferred to ã-irradiated and conducive soils by mixing with 10% suppressive avocado soils. Cellulase and laminarinase activities measured in avocado orchard soils inoculated with P. cinnamomi were not associated with disease severity in lupin seedlings during glasshouse assays involving the same soil samples. Minor shifts in bacterial and fungal community structure were observed in response to mixing conducive and irradiated soils with suppressive soils. This was associated with decreased disease severity in avocado seedlings in these treatments. The shift in bacterial community structure was partially determined by the appearance and increased abundance of several bacterial 16S rDNA sequences, which were unique to the suppressive soils, in the mixed soil treatments. It is suggested that the bacteria and fungi from which these sequences originated may be involved in suppression and further work should be undertaken to determine their identity and confirm their potential role in the development and maintenance of P. cinnamomi suppressive soils. / Doctor of Philosophy (PhD)