Thesis (M.Sc. (Horticulture)) --University of Limpopo, 2018 / ‘Hass’ avocado fruit changes skin colour from green to purple and then black during ripening. However, markets importing South African avocado fruit have been complaining about the ‘Hass’ skin colour not changing to purple/black during ripening. Thus, the study aimed to investigate the effect of tree girdling, harvest time and ripening temperature on ‘Hass’ avocado fruit skin colour development during ripening. The mature ‘Hass’ avocado fruit were harvested from girdled and non-girdled trees during early (April), mid- (May) and late (June) harvest times. Upon arrival, in the laboratory fruit were cold stored at 5.5°C for 28 days. After storage, fruit were ripened at 25, 21 and 16°C for 8, 6 and 4 days, respectively. After withdrawal from clod storage fruit were evaluated for skin colour development, ripening and physiological disorders (chilling injury). Fruit from girdled trees showed high maturity (low moisture content) when compared with fruit from non-girdled trees during early and mid-harvest. With respect to skin colour development, the results indicate that skin eye colour development of fruit from girdled and non-girdled trees minimally increased from emerald green (1) to olive green (3) across all harvest times, ripening temperature and ripening duration. However, late season fruit from non-girdled trees improved to purple (4) when ripened at 21°C when compared with fruit from girdled trees. In terms of objective colour, lightness, hue angle and chroma decreased for fruit from girdled and non-girdled trees, across all harvest times, ripening temperature and ripening duration. Lightness and hue angle of fruit from girdled trees were slightly reduced when compared with fruit from non-girdled trees, throughout all harvest times, ripening temperature and duration. Early and mid-season fruit harvested from girdled trees showed rapid decrease of chroma when compared with fruit from non-girdled trees, throughout ripening temperature and x duration. In terms of softening, fruit from girdled trees showed higher firmness loss and ripening percentage within 6 (16°C) and 4 (21 and 25°C) days when compared with fruit from non-girdled trees during early and mid-harvest, whereas, late harvest fruit from girdled trees reached higher ripening percentage and firmness loss within 4 days throughout ripening temperatures. With respect to cold damage, late harvested fruit from girdled trees showed higher external chilling injury when compared with non-girdled trees, throughout ripening temperature. In general, girdling treatment improved fruit maturity, ripening rate and firmness loss. However, the incidence of variable skin colouring of ‘Hass’ avocado fruit during ripening was also prevalent in early harvested fruit from girdled tree, irrespective of ripening temperature. Keywords: girdling, harvest time, physiological disorder, ripening temperature, variable colouring

Avocados are an important horticultural crop in South Africa, especially in the provinces of KwaZulu-Natal, Mpumalanga and Limpopo. The distance to traditional export markets, phytosanitary restrictions to lucrative markets such as China, the USA and Japan and increased competition in the European market have challenged the South African avocado industry. The industry has responded with improved logistics and shipping, a co-ordinated market access program and a global system to co-ordinate exports of avocados to the European market. To remain competitive on the global market, further improvements and innovations are required to improve the efficiency of postharvest operations. These improvements and innovations should be guided by a greater understanding of postharvest physiology. Avocados are a relatively new export crop, so there is still much to be learnt about avocado postharvest physiology and the optimisation of postharvest management. In this regard, reduced temperature storage (1°C) and modified humidity packaging (MHP) were investigated for their effect on fruit physiology and quality, the effect of a water- and ABA-infusion on ripening was examined and the effect of a cold chain break on fruit physiology and quality determined; near-infrared spectroscopy was also examined for its potential for its use in the avocado industry. As an initial study, the relationships between individual sugars, protein and oil were studied to understand the changes in avocado fruit during ripening. It was found that mannoheptulose and perseitol were the predominant sugars at harvest, but declined to very low levels during the first 10 days postharvest. The concentrations of glucose and fructose increased, while sucrose declined slightly during ripening. The concentration of protein increased sigmoidally during ripening, reflecting the increase in the ripening enzymes, particularly cellulase and polygalacturonase. The oil content fluctuated slightly during ripening. It is suggested that mannoheptulose and perseitol are important carbon and energy sources during ripening. Glucose concentration was also found to increase earlier in fast ripening fruit compared to slow ripening fruit, which is related to increased cellulase activity and may be related to the ABA functioning. Thereafter, storage and ripening trials in two consecutive seasons showed that 1°C storage and the use of MHP for 28 days reduced mass loss, water loss, ethylene production, respiration, softening and heptose consumption, without appreciably affecting fungal rots, physiological disorders or external chilling injury, compared to fruit stored at 5.5°C and regular atmosphere respectively. Also, the storage of fruit in MHP delayed the rise in the activity of cellulase during ripening, compared to fruit not stored in MHP, but there was no significant difference in the peak activity of cellulase, polygalacturonase or pectin methylesterase. In a separate experiment, fruit ripening was significantly affected by the infusion of ABA in an aqueous solution. Water slightly reduced the variation in ripening while ABA reduced the time to ripening and the variation; it is suggested that water stress and ABA are intrinsically involved in the ripening processes and may act as a ripening trigger. The water concentration in fruit was measured non-destructively using reflectance NIR; this model was used to determine the maturity of fruit and the loss of water during cold storage. In the cold chain break experiment, it was found that although fruit recovered after a cold chain break, in terms of ethylene production and respiration, there was a loss in quality because of severe shrivelling as a result of increased water loss. Fruit that were stored at 1°C were generally of a better quality at ripeness, if the cold chain was broken, compared to fruit stored at 5.5°C. In a follow-up experiment, it was found that significant changes occurred in avocado physiology over a 6h period. The respiration rate of fruit significantly increased after 4h at room temperature and mannoheptulose declined by 32% in control fruit and by 16% in ethephon-treated fruit after 6h. This demonstrates the potential for quality loss in a short amount of time. Furthermore, a model of avocado ripening is proposed, outlining the role of water, ABA, ethylene, respiration, ripening enzymes and individual sugars. This study has contributed to the understanding of avocado postharvest physiology and should aid in better management of avocados for improved fruit quality and consumer satisfaction. / Thesis (Ph.D.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.

The South African avocado industry is export-orientated with forty percent of total production sold overseas. The avocado fruit is a highly perishable product with a relatively high rate of respiration which results in the quick deterioration of fruit quality. Good phytosanitary procedures are a necessity in ensuring good product quality. Due to the threat of pests and diseases becoming resistant to the conventional chemicals currently used to control them, there has been a great need to diversify from their usage. Silicon (Si), being the second most abundant element (28%) in the earth’s crust after oxygen, is a major constituent of many soils and has been associated with disease resistance in plants for a long time. It has been used in a number of crop species to provide resistance against pathogenic agents. In some horticultural crops Si has been found to offer protection against fungal infections by strengthening cell walls, thus making it more difficult for the fungi to penetrate and colonize the plant. The aim of this research was to investigate the effects of postharvest silicon application on the quality of ‘Hass’ avocado fruit. The specific objectives included investigating the effect of silicon on the ripening pattern as well as the metabolic physiology of the avocado fruit. Avocado fruit were obtained from two locations in the KZN Midlands (Everdon Estate in Howick and Cooling Estate in Wartburg). Fruit were treated with different forms of Si (potassium silicate (KSil), calcium silicate (CaSil), sodium silicate (NaSil) and Nontox-silica® (NTS)) at concentrations ranging from 160 ppm to 2940 ppm. After dipping for 30 minutes in the silicon treatments, the fruit were stored at -0.5°C, 1°C, 5°C or at room temperature (25°C). Energy dispersive x-ray (EDAX) analysis was then conducted on the exocarp and mesocarp tissues to determine the extent of silicon infiltration within each treatment. Firmness measurements, ethylene evolution and CO2 production were recorded as fruit approached ripening. The CO2 production of fruit that were stored at room temperature was analysed daily until they had fully ripened, while fruit from cold storage were removed weekly to measure respiration. Mesocarp tissue from each fruit was extracted using a cork borer and subsequently freeze-dried and stored for physiological analysis. The freeze-dried mesocarp tissue was then finely ground and later analysed for sugar content, total anti-oxidant capacity (TAOC), total phenolic (TP) content and phenylalanine ammonia lyase (PAL) activity using their respective assays. Statistical analyses were carried out using GenStat® version 11 ANOVA. Treatment and storage temperature means were separated using least significant differences (LSD) at 5% (P = 0.05). The experimental design in this study was a split-plot design with the main effect being storage temperature and the sub-effect being treatments. Each replication was represented by a single fruit. EDAX analysis revealed that Si passed through the exocarp into the mesocarp tissue in fruit treated with high concentrations of silicon, i.e., KSil 2940 ppm. Significant differences (P < 0.001) were observed in temperature means with regards to firmness. Fruit treated with KSil and NTS only and stored at 5°C were firmer than fruit stored at other temperatures. Fruits treated with Si in the form of KSil 2940 produced the least amount of CO2, while non-treated fruits (Air) had the highest respiration rate. Fruit stored at room temperature (25°C) produced significantly higher amounts of CO2 and peaked much earlier than fruit stored at other temperatures. Ethylene results showed that there were differences (P < 0.05) between temperature means with the highest net ethylene being produced by fruit stored at 25°C. There were also significant differences amongst treatment means (P < 0.001), with fruits treated with KSil 2940 ppm producing the least ethylene. There were significant differences (P < 0.001) in temperature means with regards to the total phenolic concentration with fruits stored at 1°C having the highest TP concentration (26.4 mg L-1 gallic acid). Fruit treated with KSil 2940 ppm had the highest total phenolic concentration whilst the control fruit (Air and Water) had the lowest. There were also differences (P < 0.05) in storage temperature means with respect to the total antioxidant capacity. Fruit stored at -0.5°C had the highest TAOC (52.53 μmol FeSO4.7H2O g-1 DW). There were no significant differences in TAOC (P > 0.05) with regards to treatment means although fruit treated with KSil 2940 ppm and stored at -0.5°C showed the highest TAOC of 57.58 μmol FeSO4.7H2O g-1 DW. With regards to the concentration of major sugars in avocado, mannoheptulose and perseitol (mg g-1), no significant differences (P > 0.05) were observed in temperature means. However, fruit stored at -0.5°C had the highest concentration of these C7 sugars compared with fruit stored at other temperatures. There were significant differences in treatment means (P < 0.001) showing that fruit treated with KSil 2940 ppm had the highest concentration of both mannoheptulose (18.92 mg g-1) and perseitol (15.93 mg g-1) in the mesocarp tissue. Biochemical analyses showed differences (P < 0.05) in storage temperature means with respect to PAL enzymatic activity. Fruit stored at 5°C had the highest PAL activity (18.61 mmol cinnamic acid g-1 DW h-1) in the mesocarp tissue compared with fruit stored at other temperatures. There were significant differences in treatment means (P < 0.001) with regard to PAL activity. Fruit treated with KSil 2940 ppm had the highest PAL activity (23.34 mmol cinnamic acid g-1 DW h-1). This research has demonstrated the beneficial effects, particularly applications of 2940 ppm Si in the form of KSil. This treatment successfully suppressed the respiration rate of avocado fruit. Biochemical analyses of total antioxidants, total phenolics and PAL activity in the mesocarp tissue have shown the usefulness of Si in improving the fruit’s metabolic processes. The C7 sugars (D-mannoheptulose and perseitol) also seem to be more prevalent in avocado fruit treated with Si (particularly KSil 2940 ppm) than in non-treated fruit. This suggests that an application of Si to avocado fruit can aid in the retention of vital antioxidants (C7 sugars). / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.