Indole-3-acetic acid concentration in the leading shoot and living stem bark of Scots pine: seasonal variation and effects of pruning

Seasonal changes in the concentration of indole-3-acetic acid (IAA) in the leading shoot and in the live tissue external to the xylem (living bark) of 2-3 m tall Scots pine (Pinus sylvestris L.) trees were observed over a period of three years. Both the leading shoot and the living bark showed a consistent pattern of seasonal variation in IAA content, with maxima of about 150 and 350 ng g(-1) (fresh weight), respectively. The IAA concentration in both tissues increased in late June at the time of shoot growth termination, decreased from late August, and by October reached a low value, which was maintained throughout the winter. Excision of current year shoots on the upper branches decreased the annual stem diameter increment without affecting, the concentration of IAA in the living bark. However, excising the leading shoot, or girdling the mainstem beneath the leading shoot, caused a substantial reduction in the IAA content of the living stern bark.