Seasonal Variations in Endogenous Indole-3-Acetic Acid and Abscisic Acid in the Cambial Region of Pinus densiflora Sieb. et Zucc. Stems in Relation to Earlywood-Latewood Transition and Cessation of Tracheid Production

Seasonal changes in the total amount (measured as ng cm -2 ) of endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) in the cambial region of Pinus densiflora Sieb, et Zucc. trees with crowns of different sizes were determined at different stem heights. The total amount of IAA varied seasonally in all trees and at all stem positions, being maximal in early summer (May or July). In a tree with a large crown, the total amount of IAA remained high after peaking and declined in autumn, whereas in a tree with small crown it decreased rapidly after peaking, in particular in the lower stem. The transition from earlywood to latewood occurred concurrently with the decrease in the total amount of IAA after it had peaked, suggesting the involvement of IAA in the control of latewood formation. Cessation of the production of tracheids also paralleled the decline in the total amount of IAA early in the growing season in the lower stem of the tree with a small crown, but not in the tree with a large crown. Thus, other factors in addition to declining IAA seem to be involved in halting the production of the tracheids. The total amount of ABA was lower than that of IAA in all trees and at all stem heights, and changes were not correlated with specific changes during the annual cycle of cambial activity and dormancy.