Within-crown variation in dormant long-shoot apices of mature trees of Pinus resinosa

Abstract

Dormant long-shoot buds were harvested from two 45-year-old trees of Pinus resinosa. The shoot apices were examined for variation in shape, size, and histochemical staining in relation to their position in the crown. On nonshaded branches from the exposed south side of the trees there were no significant differences in size and shape of shoot apices associated with height in the crown. However, on the north side of the tree there was a marked increase in height and height–diameter ratio, and a decrease in diameter of shoot apices from branches below the point where the canopy closed and branches were subject to continual shading. Within the shaded portion of the crown, there were no significant differences in the size and shape of the apices studied. Megasporangiate strobili primordia were found in buds with terminal shoot apices having a low height–diameter ratio (<0.400), whereas microsporangiate strobili primordia were found in buds with terminal shoot apices having a relatively high height–diameter ratio (>0.400). Staining intensity for total carbohydrates, total proteins, and RNA decreased with decreasing height in the crown. There were no great differences in stain intensity between shoot apices from sun and shade branches from the same whorl. Therefore, the potential of a branch appears to be related to the size of the shoot apex which is correlated with its exposure within the crown.