Bordered-pit development in Larix laricina (Du Roi) K. Koch

Three studies were conducted to determine firstly, the developmental pattern of bordered-pit formation during cambial reactivation in Larix laricina (Du Roi) K. Koch; secondly, to examine if bordered-pit production could be regulated by an auxin (1-naphthalene acetic acid, NAA) during tracheid differentiation in vitro; and lastly, to study variations in bordered-pit development in the callus produced in vitro.;Experimental trees originated from the University of New Brunswick Forest, Fredericton, N.B., Canada. For the first study, trees were harvested early in May and stems were systematically sectioned for microscopy. The second and third studies required harvests during cambial dormancy in the winter months. Eight-year-old stem segments were removed from mature trees, and stem chips (2 cm axial x 2 cm tangential x 0.5 cm radial) from these were grown on a defined sterile culture medium.;The developmental pattern of cambial reactivation and wood formation in Larix laricina was found to be quite similar to that of other softwood species. Reactivation occurred first in the mid-stem region, next in the more apical areas, and last in the basal areas. Branches lagged behind the main stem but showed a similar pattern of reactivation. This study suggests that bordered-pit development begins early in the cell division phase.;In vitro, wood formation and bordered-pit production occurred at several NAA concentrations. Radial as well as tangential cell-wall bordered pits were seen with variations in size, number and position between treatments.;Callus proliferating from responsive culture chips produced thick walled, bordered-pitted tracheid-like cells in an organized pattern, whereas non-responsive chip callus contained thin-walled, non-pitted parenchyma cells growing in a random pattern.;This research has provided further evidence that earlywood formation in vitro using merchantable stem sections of Larix laricina is possible. This procedure, using chip or callus cultures, has the potential of becoming a reasonably rapid system of testing auxin and nutrient effects on wood formation.