Levels of endogenous indole‐3‐acetic acid in the vascular cambium region of Abies balsamea trees during the activity ‐ rest ‐ quiescence transition

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Abstract
Indole‐3‐acetic acid (IAA) levels in the cambial region were measured by radioimmunoassay during the annual cambial activity ‐ rest ‐ quiescence transition at the crown top (current‐year shoots) and at the middle and base of the stem of two 21‐year‐old Abies balsamea (L.) Mill. trees about 7.5 m in height. The IAA level declined throughout the experimental period (July 24 ‐ December 3) at all positions in both trees. The earlywood ‐ latewood transition, which occurred throughout each tree about the end of July, was associated in time with the largest decrease in the absolute amount of IAA. Tracheid production ceased towards the end of September, the cessation occurring earlier at the top of the tree than at the base. The cessation of tracheid production was only poorly correlated in time and space with the decline in IAA level, and it was not prevented by exogenous IAA. The results suggest that IAA level is involved in the control of tracheid radial enlargement, but not in the regulation of the cessation of tracheid production.