Fine roots of Cryptomeria japonica were separated into two functional groups: primary roots that serve as the principal agent for water and nutrient absorption and secondary roots that have transport capacity and protect the plant from environmental stress. Individual roots can also be categorized by three characteristics: diameter, branching order, and number of protoxylem groups. We investigated the relationships of these two functional groups with the three categories and evaluated which category was a better index for distinguishing primary from secondary roots by using the Pianka overlap index. Primary and secondary roots showed no exact correspondence to any of the three categories and had overlap in each category. Therefore neither was a useful indicator to distinguish primary from secondary roots. However, in the case of Cryptomeria japonica, we can roughly distinguish primary from secondary roots on the basis of whether root diameter is less than or greater than 0.6 mm.
2015 Japanese Society for Root Research