Apical control is the inhibition of a lateral branch growth by shoots above it (distal shoots). If the distal shoots are cut off to remove apical control, the lateral branch can grow larger and may bend upwards. Apical control starts when new lateral buds grow after passing through a period of dormancy. Buds initially break and produce leaves, then apical control is exerted and the lower (proximal) laterals stop growing. Apical control also inhibits growth of large, old branches. Gravimorphism and restricted water and nutrient transport can inhibit branch growth, but they are not primary mechanisms of apical control. Apical control may reduce branch photosynthesis. Under apical control allocation of branch-produced assimilate to the stem is relatively high, so low assimilates in the branch may limit branch growth even though hormone levels are adequate for growth. Hormones appear to be involved in apical control, but it is not known how. One role of hormones may be to maintain the strength of the stem sink for branch-produced assimilate. Upward bending of a woody branch after release from apical control requires both new wood production and production of wood cells that can generate an upward bending moment. Apical control inhibits radial growth of branches and, in some species, may regulate the production of wood with an upward bending moment.

No abstract

No abstract

Natural stands and a 3-year-old plantation of red alder (Alnusrubra Bong.) trees were used to study the incidence of leaning stems, the level of growth stresses and tension wood formation, and the ability of the stems to right themselves to vertical. Overall, 10% of the 512 trees in 10 natural stands leaned >22°. The largest diameter trees on the steepest slopes leaned most. Most (61%) of the trees curved upward, showing a righting response. For samples without tension wood, growth stress levels on the upper side of leaning stems, but not on the lateral or lower sides, were positively correlated with lean angles above 6°. These leaning stems had a significant righting response without tension wood. Tension wood formation was variable at leans from 9° to 26° both within and among trees, but was correlated with eccentric growth rings. We measured stem recovery in the year-old stem of 3-year-old trees bent to angles of 0–37.5°. During the 5-month experiment all stems righted to near vertical. Tension wood formed on the upper side in stems bent >6°, but reversed to the lower side before reaching vertical in 22 of 30 trees.

We investigated effects of stem phloem girdles on apical control of branch angle, stem and branch growth and stem air content in six conifer species. A stem girdle 2 cm above a branch caused the branch to bend upward in all six species. Upward bending was associated with increased formation and action of compression wood (CW) in the lower portion of the branch. Compression wood also formed in the main stem below the branch, suggesting increased auxin production in the branch. A stem girdle 2 cm below a branch (the branch remained directly connected to the apex and distal branches) released the branch from apical control in Tsuga canadensis (L.) Carr., Pinus contorta Dougl. ex Loud. and Pseudotsuga menziesii (Mirb.) Franco. The branch bent up, but there was no CW formation in the stem. In Pinus rigida Mill., the branch exhibited increased cambial activity but did not bend up. A stem girdle > 20 cm below a branch did not release the branch from apical control in any of the species. These results support the hypothesis that branches compete with the subjacent stem for branch-produced photosynthate and that when the branch lacks this competitive sink it is released from apical control. A stem girdle 2 cm below a branch did not cause release of apical control in either Juniperus virginiana L. or Picea abies (L.) Karst. In these species, decreased shoot elongation and cambial activity above the girdle probably prevented release. A stem girdle 2 cm below a branch increased air content in the stem below the girdle in four of five species, whereas the other girdle treatments had no significant effect on stem air content. Although growth was inhibited above the girdle in the two species with the largest increase in air content, growth was not inhibited in the other species. High air content in stem segments isolated from distal auxin and carbohydrate sources is consistent with the hypothesis that a carbohydrate supply is required to refill embolized cells.

White spruce trees (Picea glauca (Moench) Voss) producing annually the same number of tracheids had a much shorter season for cambial activity in Alaska (65° N) than in New England (43° N). We counted the number of potential dividing cells in the cambial zone (NCZ) and estimated the rate of cell division by determining the percentage of cambial zone cells in mitosis (MI) for trees of different vigor (annual tracheid production) from each region during the early summer period of relatively constant mitotic activity. Within each region, NCZ was dependent on tree vigor and MI was independent of tree vigor. Rate of tracheid production was higher in Alaskan trees because of their higher rate of cell division (higher MI).

Terminal shoot growth and bud formation in striped maple (Acerpensylvanicum) were followed both in vigorous and suppressed forest trees and in suppressed trees grown in the greenhouse and garden under 10 light intensities from 6 to 82% of solar radiation in the open. All buds contained (1) a pair of preformed early leaves that grew rapidly and (2) a pair of rudimentary primordia that became either bud scales in suppressed trees, after abortion of the blade, or a second pair of leaves in vigorous or released trees. The fate of the rudimentary primordia could be regulated by light intensity. They formed bud scales at 6% light or leaves at about 18% light. Maximum height growth and leaf pair formation occurred at 30–60% solar radiation in the open, although the first internode was longest under the lowest light. When leaf production stopped, bud formation was similar in vigorous and suppressed trees. The inner and outer bud scales and the pair of preformed early leaves were produced at 2- to 4-week intervals. The rudimentary primordia were not formed until after an interval of 8–12 weeks, in late summer near the end of bud formation.

No abstract