Improving production and quality of Gac (Momordica cochinchinensis Spreng.) fruit

Research Doctorate - Doctor of Philosophy (PhD)The focus of the work undertaken in this study on the Gac plant was to develop production and postharvest practices that would increase yield and maximize fruit quality. The plant Gac is dioecious. The fruit has high nutritional value, and is used in traditional medicine and in processed foods. However, the agronomic and postharvest practices for this crop are not well developed. Specifically, improved ways of germinating seeds and propagating seedlings, methods of assisted pollination, managing fruit size, and controlling fruit quality are required. The methods used for other crops in agronomy and postharvest to improve practices were expected to provide suitable models for Gac. Greenhouse and hydroponic methods were used to grow experimental crops and postharvest techniques were also used to analyse the quality of fruit in this study. In Chapter 2, the effect of temperature and seed age on seed germination, the effect of auxin concentration on the rooting and survival rate of cuttings and the effects of three rootstock ages and two grafting methods on the survival of grafts were investigated. Seed germinated well at 25-35 °C, IBA (indole-butyric acid) concentrations of 3-5gL-1 were suitable for propagating softwood cuttings and top-wedge and slice type grafts were successful with 4 and 8 weeks old rootstocks. These methods could be widely used to propagate Gac with the purposes of increasing the number of female plants. Ways to prolong the viability of seed in storage is a recommend area for further research. Chapter 3 included developing an in vitro method to evaluate pollen viability. The effect of pollen storage on pollen viability and on Gac fruit quality was also investigated. Following this, the effect of temperature on pollen germinability was investigated. The medium including 1% agar, 0.01% H3BO3, 0.01% KNO3 15% sucrose, 250mg MgSO4.7H2O and 700mg Ca(NO3)2.4H2O was found to be suitable for Gac pollen germination at 35 °C. Although pollen germination declined with storage time, hand pollination with stored pollen (for up to four weeks at 4 °C and eight weeks at -20 °C) showed a high fruit set (>73%) and no differences in fruit quality (lycopene and β-carotene concentrations) compared with fresh pollen. Better storage regimes will require an understanding of the desiccation sensitivity of Gac pollen. In Chapter 4, the effect of fruit load and fruit-set order on fruit weight and quality of Gac was evaluated. The resource allocation among leaves and fruits was also explored. With increased fruit load and fruit-set order, declines in fruit weight and aril quality were found in fruit highlighting the important effect that fruit load can have on fruit quality. This study highlights that leaf area index (LAI) provided a non-destructive indicator of canopy area, having a positive relationship with leaf dry weight (r2 = 0.56), and it may be suitable for use in future studies requiring canopy area estimates. Chapter 5 evaluated the impact of postharvest storage on some physiochemical characteristics of mature Gac fruit in Vietnam and Australia. Gac fruit harvested prior to full maturity continues to ripen, increasing nutritional quality, in terms of oil, lycopene and β-carotene concentrations in aril. Fruit firmness, skin colour and the TSS (total soluble solids) of aril were identified as potential indicators of the lycopene and β-carotene concentrations. The postharvest research on Gac showed that it may be possible to use a simple measurement of TSS (total soluble solids) in aril juice, or whole-fruit firmness to indicate fruit quality during ripening. In Chapter 6, Gac fruit harvested at five maturity stages were described in terms of their physicochemical characteristics. Fruit maturity stages M4 and M5 showed the highest quality characteristics in terms of aril oil, lycopene and β-carotene concentrations. The respiration rates and ethylene produced from fruits during storage suggested a climacteric nature which needs further investigation. This will assist in determining the appropriate storage conditions for Gac fruit. In conclusion, methods of propagation, pollination, canopy management and postharvest practices have been improved as a result of this study and provide information that can be used as a base for further developments in the commercialization and conservation of this species