Maturation of the tracheary elements in the roots of Pinus banksiana and Eucalyptus grandis

Tracheary elements of the xylem are responsible for the longitudinal (axial ) transport of water and ions that have moved radially across the root. The se vessel members and (or) tracheids mature some distance behind the root t ip, and it is generally believed that this distance is directly related to root growth rate. To test this idea, the distances behind the root tip at w hich tracheary elements of pouch-grown Pinus banksiana Lamb. and Eucalyptus grandis W. Hill ex Maiden mature were examined. From each species, three r oot tip types (white, brown, and ectomycorrhizal short lateral) were assess ed. Unlike previous studies of this topic, two methods of testing tracheary element maturity were employed concurrently. The first was anatomical and involved visualizing the deposition of lignin in the walls of the tracheids or vessel members. The second was functional and consisted of determining the capability of the tracheary elements to conduct a fluorescent, tracer d ye. The distance behind the root tip at which the conductive xylem cells ma ture varied from 0.16 to 1.6 mm and was highly dependent on species and roo t type. No significant correlation was found between growth rate and proxim ity of tracheary element maturation to the tip for white roots. The presenc e of lignin in the tracheary element wall was not a reliable indicator of t he cell's functional maturity.