Abstract
The role of phytohormones in tracheid formation remains unclear in conifers. In this study, to obtain information on the role of auxin (IAA) and gibberellin A4 (GA4), we examined seasonal variation in the amount of phytohormones in cambial-region tissues and tracheid formation of sugi cultivar planted in a Nelder plot with different tree densities. We demonstrated that the amount of IAA was positively correlated with the number of tracheids formed in early and mid-season, but not in late season, and had no relation to tracheid differentiation. Crown length and height at the crown base had a positive and negative effect, respectively, on IAA amounts in early and mid-season, but not in late season. Height-to-diameter ratio was negatively correlated with IAA amounts in early and mid-season, but not in late season. Sugi trees with wider spacing continued tracheid formation in late season with smaller amounts of IAA, although the trees with narrower spacing ceased tracheid formation with larger amounts of IAA. Cambial growth cessation in late season might be controlled not by IAA amount, but by short-day-induced insensitivity to IAA. GA4 had no relation with the indexes of growth traits or tracheid formation.
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