Integration of copper subcellular distribution and chemical forms to understand copper toxicity in apple trees

• The translocation of Cu from apple tree root to aerial parts was limited.
• Most accumulated Cu in apple tree was stored in fibrous roots.
• The largest proportion of Cu in fibrous roots was bound to cell wall.
• The accumulated Cu in fibrous root was mainly in phosphate and oxalate forms.
• Ralls can be considered as a Cu-tolerant species of apple trees.

Little information is known about the tolerance mechanisms of apple trees growing in soils with excess Cu. This study investigated not only the accumulation and translocation of Cu in the organs of two apple tree cultivars (Fuji and Ralls), but also the detoxification mechanisms of their roots at the cell level. Most accumulated Cu was within apple tree root, especially in the fibrous root, for both cultivars. Compared with Fuji, Ralls had significantly higher Cu concentrations in roots, especially for fibrous roots, while lower concentrations in the aboveground parts. Combining the results from subcellular distribution and chemical forms in the fibrous roots of two apple tree cultivars, more Cu amounts were sequestrated in cell wall and vacuole for Ralls and Cu accumulated in the fibrous root of Fuji had higher in vivo mobility. The results of this study implied that different apple tree cultivars differ in the accumulation, transportation and tolerance to toxic levels of soil Cu, and the chemical forms and subcellular partitioning of Cu in the fibrous roots can elucidate some of the variation of apple trees in Cu sensitivity.