Witches' broom and normal crown clones from the same trees of Pinus sibirica: a comparative morphological study

Mutational witches' broom clones show a growth redistribution compared with normal clones. The main factor affecting the variations in mutant clone morphology is the strength of the mutation. Mutational witches' broom is a fragment of the tree crown with abnormally dense branching and slower shoot growth, compared with those of a normal crown. Thousands of dwarf ornamental cultivars widely used in landscape design have been developed from mutational witches' broom. In this study, crown morphology was compared between grafted clones of witches' broom and normal clones from the same trees. The results quantify variations in crown structure between the mutants and normal clones. The sample materials were 10 pairs of grafted witches' broom and normal crown clones of Pinus sibirica. The mutant and normal clones were discrete sets. Many morphological traits were affected in the mutants. Compared with the normal-crown clones, the mutants showed male sterility, decreased apical dominance, reduced shoot and needle length, and increased branching and seed cone bearing. In terms of morphogenic changes induced by the mutation, the shoots of the witches' broom clones were bicyclic, generated seed cones that were much shorter than those of normal clones, and had acquired the ability to form lateral buds. The extent of interclonal variation was significantly greater among witches' broom clones than among normal clones. Compared with the morphological traits of normal clones, those of the mutants were shifted in the same direction but to different extents. Therefore, mutational witches' broom is the expression of a mutation that can be weak, medium, or strong. These results will be useful for unraveling the genetic basis of witches' broom in conifers and for breeding new dwarf cultivars.