Evaluation of biochemical and physiological traits in the leaves of Malase Saveh pomegranate mutants and its relationship with spring cold tolerance

Background and objectives: In biological studies, low dose gamma radiation has been used as a stimulus to high dose as an inhibitor. Gamma ray is a physical mutagen. Cold and frost are limiting factors for pomegranate cultivation in many parts of the world, including Iran. The purpose of this study was to investigate the effect of gamma radiation on the physiological and biochemical characteristics of pomegranate leaves after exposure to gamma radiation and its relationship with tolerance to spring cold stress.
Materials and methods: The latent buds on one-year-old branches of Meles Saveh pomegranate were subjected to gamma radiation with a dose of 36 Gy from the cobalt 60 source. After that, the irradiated shoots were kept in the vault for almost one year and these mutant plants were used for vegetative propagation and production of second generation vegetative plants (mV2). In the winter of the following year, mV2 mutant clones were transferred to the main garden in Saveh. After a four-year growth period, all trees entered the flowering stage. The plants in the fifth growing season (mV5) were selected based on their resistance to winter cold and diseases visually (based on the percentage of dry shoots and browning of branches). 18 genotypes were selected from mV5 generation. One-year-old branches were harvested from the middle of each tree in April 1401 and kept on ice to be analyzed in the laboratory for physiological and biochemical traits such as frost tolerance, malondialdehyde, proline, soluble carbohydrates, total phenol, Antioxidant capacity and photosynthetic pigments should be used.
Findings: The results of variance analysis of the data showed that there was a significant difference in the freezing tolerance of pomegranate mutant clones. Among the mutant clones, the highest and lowest freezing tolerance was observed in mutants 6 and 3, respectively. The highest amount of proline was observed in mutant 4, while the lowest amount was assigned to mutants 9, 2, 7, 10 and control. Among the mutant clones, mutants 1, 4, 13 and 18 had the highest leaf soluble carbohydrate content, while the lowest was found in mutant 9. The highest content of total phenol was observed in mutants 1, 4, 6, 13, 16, 17 and 18 and the lowest in control. The highest antioxidant capacity was related to mutants 4, 6, 13 and 18, while the lowest was found in control and mutants 2, 3 and 9. The results showed that the lowest and highest amount of malondialdehyde was related to mutants 13 and 2, respectively. Comparing the average data of photosynthetic pigments showed that the highest amount of chlorophyll a and carotenoid was observed in mutant 18 and the highest amount of chlorophyll b and total chlorophyll was observed in mutants 4, 6 and 18. There was a negative and significant correlation between LT50 and proline, soluble carbohydrates, total phenol, antioxidant capacity and photosynthetic leaf pigments, while a significant positive correlation was observed between LT50 and malondialdehyde. In general, gamma irradiation increased membrane lipid peroxidation and electrolyte leakage and decreased membrane stability. Although some pomegranate mutants were able to protect cell membranes against oxidative damage caused by gamma irradiation by increasing the concentration of osmolytes (such as carbohydrates and proline) and phenolic compounds, and thus increased cold tolerance.
Conclusion: Irradiation of plant buds is an effective method to improve plant varieties. Pomegranate mutant clones 6, 4, 13 and 18 had significantly more proline, soluble carbohydrates, total phenol, antioxidant capacity and photosynthetic pigments and less malondialdehyde and as a result more cold-tolerance than the control. Mutant clones can be used in future breeding programs to produce cold-tolerant cultivars for cultivation in cold regions.