Physiological and Metabolic Changes in ‘Xinyu Mandarin’ Following Natural Tetraploidization

The mandarin is an important fruit crop worldwide, and ‘Xinyu mandarin’—a local Citrus reticulata variety—is widely cultivated in Jiangxi Province, in China. Autopolyploidy has frequently been used for the improvement of crop varieties. In a previous study, we identified a tetraploid (4X) material of ‘Xinyu mandarin’ generated from its diploid (2X) mother seedling via a spontaneous mutation. However, the physiological and metabolic variations after polyploidy are not clear enough, which is not conducive to the early screening. In the present study, we analyzed the morphological, physiological, and metabolic differences between the tetraploid and diploid plants. The tetraploids had larger and thicker leaves, and the activities of key enzymes in photosynthesis, the total chlorophyll, phenolic, and proline contents, were all higher in tetraploids. In the non-targeted metabolomic profile, a total of 522 metabolites were identified, of which 61 were significantly different between diploids and tetraploids. The differential metabolites included similar proportions of primary and secondary metabolites, and most of these were up-regulated in tetraploids, especially stress-related metabolites such as phenolic acids, flavonoids, alkaloids, and so on, indicating that tetraploids may possess better stress tolerance ability and vigor. Therefore, the tetraploids of ‘Xinyu mandarin’ may serve as an excellent female parent for the improvement of citrus varieties.