Investigation of micropropagation potential of some avocado (Persea americana Mill.) rootstocks under in vitro conditions

The use of clonal rootstocks has begun to spread in commercial avocado cultivation worldwide, particularly in countries such as the United States, South Africa, and Australia. Among the reasons for the increasing use of clonal rootstocks are the ability to obtain genetically uniform individuals, their resistance to biotic and abiotic stress conditions, high yield potential, and the production of uniform fruits. The use of clonal rootstocks in Türkiye is still limited to the research stage. However, considering the continuous expansion of production areas and the adverse impacts of climate change, the use of clonal rootstocks in Türkiye is expected to become inevitable in the near future due to their resistance to biotic and abiotic stresses. Nevertheless, there are still challenges worldwide regarding the micropropagation of avocado. Therefore, in this thesis study the micropropagation potential of the 'Duke 7' and 'Toro Canyon' clonal avocado rootstocks, which are widely used around the world and have high commercial value, was investigated. In the initiation stage of the study, four different basal nutrient media (WPM, MS, B5, and DKW) were evaluated to determine the most suitable basal nutrient media for micropropagation. During the shoot proliferation stage, the effects of different types of cytokinins (BAP and mT) and concentrations (1 and 2 mg/L) were investigated on the basal nutrient media that yielded the most successful results for each rootstock. In the rooting stage, different types of auxin (IBA and NAA) and concentrations (0, 0.5, 1, 2, and 3 mg/L) were evaluated to induce root formation in microcuttings. In the acclimatization stage, the adaptation performance of in vitro plantlets to ex vitro conditions was evaluated. In the basal nutrient media test, data related to morphological parameters (viability rate, shoot length, number of leaves, shoot development status, browning rate, callus rate, and vitrification rate) were evaluated in the first, second, and third subculture stages; while measurements related to physiological parameters (leaf area, leaf fresh and dry weight, and SPAD value) were only performed in the third subculture stage. While investigating the effects of cytokinin types and concentrations on shoot development and proliferation, number of shoots was also taken into account in addition to the parameters considered in the basal nutrient media experiment. During the rooting stage, data on rooting rate, number of roots, root length, root development status, and callus development status were evaluated six weeks after transfer to the rooting medium. During the acclimatization stage, the acclimatization rate of in vitro plantlets to external conditions was determined 2 and 6 weeks after the plantlets were removed from in vitro conditions and transplanted into Jiffy-7 tablets in each rootstock. The research findings revealed that, when considering all morphological and physiological parameters together in the basal nutrient media experiments, the highest shoot growth was recorded in the 'Duke 7' clonal rootstock on WPM medium and the 'Toro Canyon' rootstock on MS medium. In the shoot development and proliferation, BAP was identified as the most effective cytokinin for both clonal rootstocks. While 1 mg/L BAP was found to be optimal for promoting shoot growth and development, 2 mg/L was determined to be the most suitable concentration for enhancing shoot proliferation. The most successful outcomes in maintaining morphological stability and minimizing physiological disorders such as browning, callus formation, and hyperhydricity were achieved with 1 mg/L mT treatment in both clonal rootstocks. Moreover, mT was identified as the most effective cytokinin across all physiological parameters, with the optimal concentration determined as 1 mg/L for the 'Duke 7' clonal rootstock and 2 mg/L for the 'Toro Canyon' clonal rootstock. In the rooting experiment, IBA was determined to be the most effective auxin type for both clonal rootstocks. The optimal concentration was found to be 1 mg/L for the 'Duke 7' clonal rootstock and 2 mg/L for the 'Toro Canyon' clonal rootstock. In the final stage of acclimatization, the highest acclimatization rates were 87.50% for the 'Duke 7' clonal rootstock and 81.25% for the 'Toro Canyon' clonal rootstock after 2 weeks. These rates decreased to 43.75% for the 'Duke 7' clonal rootstock and 31.75% for the 'Toro Canyon' clonal rootstock at 6 weeks after transplantation of in vitro plantlets into Jiffy-7 tablets. Our research identified that to increase success in the micropropagation of clonal rootstocks and obtain healthy plants suitable for commercial production, the culture medium, growth regulators, and their concentrations must be individually optimized for each genotype. Furthermore, to enhance adaptation to ex vitro conditions, the acclimatization process should be supported by gradual transitions and conducted under appropriate greenhouse conditions, which is crucial for improving survival rates. Accordingly, future studies are recommended to focus on developing genotype-specific optimization and acclimatization approaches. As a result, both clonal rootstocks were successfully propagated using the micropropagation method, but six weeks after transfer to the external environment, the survival rate was found to be lower in the 'Toro Canyon' clonal rootstock than in the 'Duke 7'.