Graduate Thesis Or Dissertation
 

The physiology of plagiotropic growth of rooted cuttings of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco]

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  • Experiments were conducted to determine if the plagiotropic growth habit of rooted cuttings of Douglas-fir might result from a system favoring the accumulation of indoleacetic acid in the adaxial side of the shoot in response to vertical placement. Actively growing excised branch terminals were subjected to gravitational disorientation to determine if they would respond to displacement from horizontal by curving epinastically. Horizontal shoots showed slight epinastic curvature within 24 hr but returned to near their original positions within 48 hr. Shoots placed vertically reached an equilibrium curvature of 50-60° after 72 hr. This response was significantly reduced by both decapitation and defoliation. Shoots placed horizontally with their abaxial sides up (inverted) showed up to 110° of epinastic (dorsoconvex) curvature after 72 hr. Treatment with 50 ul/1 ethylene had no effect on the curvature response of horizontal or horizontal inverted shoots. Shoots rotated on a horizontal clinostat showed curvatures intermediate between those of vertical and horizontal inverted positions. These results suggest that the stimulus for the epinastic response varies with the orientation of the shoot with respect to the gravity vector. The stems of cuttings of 3 clones examined 2 seasons after rooting exhibited dorsoconvex curvature averaging from 15-47° in the region comprising the original 12-cm cutting. Cross sections taken 1.5-2.0 cm from the basal and distal ends of the original cuttings were examined microscopically. The annual ring produced the year before rooting (ring 1) was symmetrical in both basal and distal sections with a small amount of compression wood on the abaxial side. The annual ring produced the season after rooting (ring 2) was asymmetrical in both basal and distal sections. In basal sections, the upper side of ring 2 was 2.5-3.5 times wider on the adaxial than on the abaxial side. Eighty -90% of the width of the adaxial side was compression wood. In the annual ring produced the second season after rooting (ring 3), compression wood formation had begun to shift back to the abaxial side. In distal portions of the original cuttings of 2 clones, where curvature had displaced the stems from vertical, the width of compression wood was twofold greater on the abaxial than on the adaxial side of ring 3. These results indicate that compression wood plays an important role in the development of the plagiotropic growth habit of rooted cuttings of Douglas-fir. Radioactivity from ¹⁴C-indole-3-acetic acid applied in droplets to decapitated but otherwise intact rooted cuttings moved down the stem as a pulse. After 24 hr, nearly twice as much activity was detected in extracts from the adaxial than from the abaxial sides of cutting stems. In a similar experiment with branches on the tree, significantly more activity was extracted from the adaxial than from the abaxial side of horizontal but not vertically restrained branches. There was no significant difference in transport of activity from ¹⁴C-IAA donor blocks between the adaxial and abaxial sides of segments excised from stems of rooted cuttings or branch terminals. The epinastic curvature of vertically placed, excised branch terminals and the asymmetric cambial activity and compression wood formation in stems of rooted cuttings are indirect evidence for a transverse gradient in auxin content in vertical shoots and rooted cuttings. The accumulation of exogenously applied ¹⁴C-IAA on the adaxial sides of rooted cuttings lends further support to this interpretation. However, based on branch transport studies, the accumulation of activity in the adaxial sides of rooted cuttings was apparently not simply a response to vertical placement.
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