Definition of fine roots on the basis of the root anatomy, diameter, and branch orders of one-year old Fraxinus mandshurica seedlings

Abstract

Fine roots are important in root absorption of nutrient and water, and in root turnover. Accurate definition of fine roots is a prerequisite to improved estimation of the physiological and ecological functions of forest ecosystems. Root development and physiological functions are reflections of root anatomical structure. In this study, the anatomical structures of different root orders were analyzed by examining paraffin sections of one-year old Fraxinus mandshurica seedlings. One-year-old F. mandshurica seedlings had over five root orders. The root anatomical structures of all orders showed more differences. First and second order roots consisted of four sections: the epidermis, cortex, pericycle, and vascular bundles. Fourth and fifth order roots were mainly composed of the skin and peripheral vascular bundles (including the xylem and phloem). Third order roots had root epidermal and cortical structures, but the quantity and integrity of the cortical cells were inferior to those of the first and second order roots, and superior to those of the fourth and fifth order roots. All the first and second order roots and some third order roots with discontinuous cork layer (< 0.4 mm in diameter), but not the fourth and fifth order roots, were the fine roots of one-year old F. mandshurica seedlings. Although they had similar diameters, different portions of root systems had different anatomical structures and therefore, vary in capacity to absorb water and nutrients. Fine roots were accurately defined by root diameter, branch orders, and anatomical structural features of one-year old F. mandshurica seedlings.