Abstract:

To clarify the relationship between the ease of peeling of mandarins and cell wall polysaccharides,which are thought to support the plant cell integration,adherence and mechanical strength,the ease of peeling,peel morphology,cell wall polysaccharide contents,the activities of the enzymes related to cell wall polysaccharide degradation and the expression level of some related genes were analyzed in four mandarin varieties,Citrus reticulata Blanco‘Mukaiyama’,C. clementina hort. ex. Tanaka,C. reticulata Blanco‘Fremont’and C. reticulata Blanco‘Yaoxianggan’. During the fruit ripening,peel adherence and firmness,which reflect the ease of peeling,remarkably decreased,suggesting that the peeling is easier as the fruit mature. In different varieties,the ease of peeling also showed great diversity. Fremont and Yaoxianggan had higher peel adherence than Mukaiyama and Clementine,and Yaoxianggan had the highest peel firmness in the four varieties,whereas Mukaiyama and Clementine had the similarly lowest peel firmness,indicating that Fremont and Yaoxianggan were hard peeling and Mukaiyama and Clementine were easy peeling. The correlation analysis indicated the significant positive correlation between the peel adherence and firmness,and that the ease of peeling is relevant to the low level of cell wall polysaccharides and high activities of their degradation-related enzymes. The contents of cell wall polysaccharides in all varieties dramatically dropped during the fruit maturation,and coincidently,the activities of some enzymes related to cell wall polysaccharide degradation and the expression level of the encoded genes also ascended. At the same time the expression of some genes encoding xylanase inhibitors and pectin methylesterase inhibitors was decreased. In these four varieties,the peels in the easy-peeling Mukaiyama and Clementine had lower content of total cell wall biomass as well as some polysaccharide fractions including hemicellulose,cellulose and protopectin,which were relevant to the higher activities of xylanase,cellulose and pectin degradation-related enzymes. Nevertheless,the pivot gene controlling the distinct ease of peeling in different varieties remains obscure.

Key words: mandarin, ease of peeling, peel firmness, peel-pulp adherence, cell wall polysaccharides, enzyme activity, gene expression