Mutational witches’ broom is a part of the tree crown with abnormally dense branching and
slow shoot growth, which putatively originates from a cell in a bud apical meristem and can be visually
differentiated from the normal crown. The witches’ broom forms a large branching system which competes
sufficiently with other parts of the tree crown. However, the competitive relationship and the interaction
between the mutant and normal crown parts have not yet been studied. We investigated the patterns and
dynamics of the competitive relationship between witches’ broom and normal branching systems of the
same tree of five Pinaceae species by tree ring analysis. Three saw-cuts were made in each tree: on the lateral
branch with the witches’ broom, on the proximal part of the axial branch in front of the branch with the
witches’ broom, on the distal part of the axial branch right after the branch with the witches’ broom. An-
nual radial growth was measured to the nearest 0.01 mm, and annual ring areas from three saw-cuts were
analysed to compare the growth dynamics before and after witches’ broom appearance. The growth of the
lateral branch with the witches’ broom sharply increased in the year of witches’ broom appearance by 2–10
times. The growth of the distal part of the axial branch sharply decreased in the same year, which could
be explained only by the witches’ broom appearance. All branches with witches’ brooms gradually became
thicker than the distal part of the branch, 3–150 times at the peak of growth, and occasionally outgrew even
the proximal part of the branch. Thus, witches’ broom is drastically morphologically and physiologically
different from the rest of the crown. All witches’ brooms had their own vertically oriented axis of symmetry
and represented autonomous branching systems that were not subordinate to any other branching systems.
They break the normal donor-acceptor relationship in a tree and eagerly consume resources without being
fully involved in their production. The mutation clearly affects the hormonal regulation of growth and morphogenesis and turns a witches’ broom into a new metabolic sink.
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