Developmental changes in Sitka spruce as indices of physiological age

S ummary Adventitious rooting ability is considered a good indicator of juvenility in trees and has often been used to assess explants subjected to treatments to alter developmental stage. However, rooting assessments are not always practical and the callusing of needles in vitro was investigated as an alternative bioassay of physiological age. Shoots were sampled from the uppermost whorl of Sitka spruce trees aged 1, 3, 4, 6, 11, 16, 21, 35 and 39 years and from the lowest whorls of 3‐ and 4‐year‐old trees. Needles were sampled for measurement of length, width, length/width ratio, projected and total surface areas, dry weight and specific leaf area. For explants of uppermost whorl origin, there were decreases in the rooting ability of cuttings and the callusing response of needles with increasing age of tree. Associated with the loss in callusing ability was an increase in the production of a black exudate. The changes with age for rooiing, callusing and exudation could each be adequately described by the Gompertz growth function which showed their rates of change to be similar. The observed changes are closely associated with changes in needle area, weight and specific leaf area. Euclidean distances (using the mean values of selected characteristics for each age class) were computed as a measure of physiological age. The shoots and needles of basal shoots are more juvenile in character than those taken from the uppermost whorls of the same trees.