New branch roots produced by vascular cambium derivatives in woody parental roots of Populus nigra L.

In literature, it has been suggested that new branch roots are produced in a woody parental along its axis not only in the proximal zone where a primary anatomical structure is present but even where old branch roots are present, and a thick production of wood internally characterizes the parental root anatomy. This paper confirms this hypothesis by showing, for the first time, the presence of a new branch root primordium forming within the secondary phloem tissues of a woody parental root. The new branch root primordia found in this work are designated secondary branch roots (SBRs) and seem to develop from derivatives of the vascular cambium that abandon the formation of conducting elements to become the mother cells of these new root primordia. We find that traces belonging to SBRs present some anatomical difference compared with those belonging to branch roots deriving from primary tissues designated, according to literature, primary branch roots (PBRs). This difference in traces could help to distinguish between the origin of the SBRs and PBRs present along a woody parental axis, and this could be helpful in understanding how the root system of a woody plant is developed. The possibility of branching, wherever needed, along the root axes, independently of their internal anatomical organization, evokes a new scenario in which woody plants, in analogy with herbaceous plants, continuously modify their root system in an attempt to adapt and to better exploit their rooting environment.