Genetic diversity of the ease of peeling in mandarins

The ease of peeling is an important fruit quality trait for fresh-eating citrus, particularly mandarins. In the practice of hand peeling of citrus fruits, both the peel-pulp adherence and peel resistance to penetration are relevant to the ease of peeling. In this study, we first elaborated an easy apparatus that facilitated precise quantification of peel-pulp adherence and peel resistance to penetration. Through measuring mandarins, oranges, pummelos and lemons that each included 10 varieties, the ascending order of the peel-pulp adherence was mandarin < orange < lemon < pummelo, and that of the peel resistance to penetration was mandarin < orange < pummelo < lemon. Eighty mandarin varieties from National Citrus Germplasm Repository, China exhibited great diversities of peel-pulp adherence and peel resistance to penetration. The hierarchical clustering analysis based on these two properties classified these mandarins into two major clades, representing 72 easy-peeling and 8 hard-peeling mandarins, and the easy-peeling mandarins were further divided into two subclades, namely, 42 very easy-peeling and 30 moderately easy-peeling subclades. The orange, grapefruit or Hassaku ancestral origin may contribute to the property of hard peeling in some moderately easy-peeling and hard-peeling mandarins. Some representative easy-peeling mandarins had loose albedo with great aerial spaces. The correlation analysis demonstrated that peel thickness was relevant to peel firmness but not the peel resistance to penetration, and peel mass per unit area but not peel density was relevant to peel firmness. Moreover, the peel-pulp adherence and peel resistance to penetration were positively correlated, implying a common mechanism controlling the two properties of the ease of peeling.