Seasonal variations of auxin and gibberellin A4 levels in cambial-region tissues of three conifers (Pinus elliottii, Chamaecyparis obtusa, and Cryptomeria japonica) with inherently different wood densities

We previously reported the species-specific annual ring formation characteristics of three conifers (slash pine (Pinus elliottii), hinoki (Japanese cypress, Chamaecyparis obtusa) and sugi (Japanese cedar, Cryptomeria japonica)) grown in the same stand over 2 years. We found that the species-specific annual ring formation characteristics affected the inherent difference in wood density among these conifers (slash pine > hinoki > sugi). Plant hormones in cambial-region tissues were believed to affect annual ring formation. However, seasonal variation of the amounts of plant hormones in cambial-region tissues had only been examined in a few tree species. In this study, as the first step to elucidating the role of plant hormones in annual ring formation in conifers, we report the seasonal variations of the auxin (indole acetic acid, IAA) and gibberellin A4 (GA4) levels in cambial-region tissues and their effects on annual ring formation in three conifers (slash pine, hinoki, and sugi) with inherently different wood densities. Sugi (small wood density) had significantly higher levels of IAA and formed more tracheids in the early season than in the late season, although slash pine (large wood density) had higher levels of IAA and formed significantly more tracheids in the late season than in the early season. Hinoki (intermediate wood density) had constant IAA levels and formed a constant number of tracheids throughout the season. There were significant positive correlations between the levels of IAA in cambial-region tissues and the number of tracheids formed during late season in the two conifer species. A close relationship was observed between the seasonal ratio of the IAA levels (late/early season) and wood density. No consistent trend in the change in the level of IAA during the transition from earlywood to latewood formation was recognized among the three conifers. The IAA levels in slash pines were significantly higher than those in sugi and hinoki. The GA4 levels had no significant effect on number of tracheids formed in the three conifers. These results suggest that the species-specific seasonal variation patterns of the IAA levels might lead to the inherent differences in wood density among these three conifers through species-specific characteristics in the formation of annual rings.