Growth response of branches to variation in the intra- and interbranch supply of photosynthate

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Autonomy of mid-crown branches in supply and utilization of carbohydrates is tested using a series of defoliation and shade treatments on young loblolly pine trees (Pinus taeda L.). The response of branch extension to variation in the intra- and interbranch supply of carbohydrate was inferred in young loblolly pine trees (Pinus taeda L.). A series of defoliation and shading treatments were designed and applied on selected branches, such that the extension of their terminal buds would depend on carbohydrate stored in the target branch, and carbohydrate imported from the stem or neighboring branches. The potential carbohydrate import from neighboring branches was also reduced by defoliation or shading of branches immediately above and immediately below the target branch. All combinations of branch treatments were analyzed to suppose the net effect of carbohydrate supply and source on branch extension. Both defoliation and shading of target branches significantly reduced growth in length and diameter of target branches as compared to the control. If imported carbohydrate compensated measurably for the loss of photosynthetic potential in the target branch, then extension of treated branches would have been longer than recorded when the neighboring branches were not treated and perhaps shorter when the neighboring branches were defoliated or shaded compared to the control. Neither was the case, however, because treatments on the neighboring branches had no significant effect on extension of the target branch. The results of this study provide additional support to the theory that branches within the crown are autonomous. These results, however, do not always suggest that branches do not import carbohydrates from the stem. A companion study found significant reductions in stem diameter of the internodes adjacent to defoliated or shaded branches.