Effect of defoliation on tracheid production and the level of indole‐3‐acetic acid in Abies balsamea shoots

Abstract

To simulate feeding by the spruce budworm (Choristoneura fumiferana Clem.), the apical current‐year shoots on 1‐year‐old branches in the uppermost whorl of 6‐year‐old balsam fir [Abies balsamea (L.) Mill.] trees were either removed completely by debudding before the start of the growing season or defoliated 0, 50, 90 or 100% shortly after budbreak. Debudded branches were treated at the apical end with 0, 0.1 or 1.0 mg of indole‐3‐acetic acid (IAA) (g lanolin)−1. Ninety % of the 1‐year‐old needles were also removed from some of the experimental branches. After ca 4 weeks of growth, the radial width of new xylem and the level of IAA were determined in the 1‐year‐old internode. The IAA content was measured by radioimmunoassay.The removal or defoliation of current‐year shoots inhibited tracheid production and decreased the IAA level. Exogenous IAA stimulated tracheid production and increased the IAA level in debudded branches. Current‐year shoot defoliation also inhibited current‐year shoot elongation. The inhibitory effect of current‐year needle removal on all parameters generally increased with increasing intensity of defoliation. The removal of 1‐year‐old needles did not affect the IAA level or current‐year shoot elongation, nor did it influence tracheid production in branches with current‐year shoots. However, removal of 1‐year‐old needles inhibited tracheid production in debudded branches supplied with exogenous IAA. The results indicate that (1) IAA is involved in the control of tracheid production in the 1‐year‐old internode, (2) IAA is supplied primarily by current‐year shoots, and (3) defoliation by the spruce budworm inhibits tracheid production partly by decreasing the supply of IAA.