Pattern of secondary thickening in a Quercus cerris root system.

We investigated changes in the pattern of secondary thickening along first-order lateral roots of Quercus cerris L. (turkey oak), a dominant species in the mixed forests of the Mediterranean basin. The eccentricity of radial growth was analyzed relative to the gravimetric center of the root, a measure that is unaffected by year-to-year variation in growth ring width. In cross sections collected at the taproot-lateral root junction, radial growth was greatest on the upper side of the root. However, at a distance of 10 cm from the taproot-lateral root junction, radial growth was greatest on the lower side of the root. With increasing distance from the taproot-lateral root junction, the direction of the vector of radial growth eccentricity changed in an anti-clockwise direction as viewed from the distal face of the root cross section. These findings applied to all lateral roots of the studied root system, irrespective of root size, depth of origin and axial growth direction. Ring analysis indicated that most first-order lateral roots grew to a length of 1 m in the first year.