Cambial activity and intra-annual xylem formation in roots and stems of Abies balsamea and Picea mariana.

BACKGROUND AND AIMS Studies on xylogenesis focus essentially on the stem, whereas there is basically no information about the intra-annual growth of other parts of the tree. As roots strongly influence carbon allocation and tree development, knowledge of the dynamics of xylem production and maturation in roots at a short time scale is required for a better understanding of the phenomenon of tree growth. This study compared cambial activity and xylem formation in stem and roots in two conifers of the boreal forest in Canada. METHODS Wood microcores were collected weekly in stem and roots of ten Abies balsamea and ten Picea mariana during the 2004-2006 growing seasons. Cross-sections were cut using a rotary microtome, stained with cresyl violet acetate and observed under visible and polarized light. The number of cells in the cambial zone and in differentiation, plus the number of mature cells, was counted along the developing xylem. KEY RESULTS Xylem formation lasted from the end of May to the end of September, with no difference between stem and roots in 2004-2005. On the contrary, in 2006 a 1-week earlier beginning of cell differentiation was observed in the stem, with cell wall thickening and lignification in roots ending up to 22 d later than in the stem. Cell production in the stem was concentrated early in the season, in June, while most cell divisions in roots occurred 1 month later. CONCLUSIONS The intra-annual dynamics of growth observed in stem and roots could be related to the different amount of cells produced by the cambium and the patterns of air and soil temperature occurring in spring.