Effect of plasma treatment on papaya seeds / Xi Dengke

Xi , Dengke (2023) Effect of plasma treatment on papaya seeds / Xi Dengke. PhD thesis, Universiti Malaya.

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Abstract

Seed germination is a prerequisite part of plant growth and crop production, being crucial for next-generation plant development. The germination of papaya seeds is erratic, slow, and asynchronous. High dormancy of papaya seeds is due to their hard shell-like testa with poor penetrability and inhibitors in vivo suppressing the metabolism. In this thesis, appropriate plasma treatments to overcome dormancy in papaya seeds were explored by cold plasma interaction and plasma-activated liquid. The objective was achieved by cold plasma irradiation combined with soaking treatment, giving an enhanced germination rate of 92%. Plasma-activated liquid was obtained by high-frequency discharge and microsecond-pulsed discharge on deionized water, giving the plasma-activated water (PAW) and microsecond-pulsed plasma-activated water (μPAW). PAW treatment has been found to enhance the germination rate by 20% and induce better growth performance in early seedling development. The plant height was increased significantly, and the average mass measured after 30 days of growth in greenhouse was 103% more than the control plants. The cold plasma treatment was performed in three systems: dielectric barrier discharge (DBD) system, low-pressure DBD system, and plasma jet. The plasma treatments in low-pressure DBD performed at 0.1 mbar and 1 mbar air were found detrimental to seed germination. The germination rate of the seeds treated by an air plasma jet without gas flow conducted at 16p-p kV decreased with the treatment time. Cold plasma treatment, conventional hot water (50 ˚C) treatment, and untreated seeds were studied together as reference and control groups. The results confirmed that plasma-assisted priming combining cold plasma treatment with soaking gave the optimum effects on papaya seeds, with higher germination rate and enhanced growth at the early developmental stage. Early seeding growth was observed to be most in the PAW treated group, 40 % advanced in dry mass compared to plasma-assisted priming methods. This makes PAW an excellent choice in field application for its adaptability, security, and green to environment. The seeds treated by μPAW were investigated by SEM showing the change in morphology by physical etching effect. The biochemical mechanism was elucidated from the evolution of the surface chemistry, functional groups, and ABA content. The evolution based on analysis indicated that the reactive species of μPAW have boosted the metabolism of protein, lipid, and starch in the seed coat and decreased ABA content. These changes allowed the papaya seed to rapidly break dormancy and germinate. The extracts from the papaya seedlings contained high SOD activity and low MDA concentration, indicating the reduced oxidative damage due to the increase in antioxidant enzyme activities. This ensured an enhancement of early seedling development.

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