Cone structure and seed development in grafted witches' broom and normal crown clones from the same trees of Pinus sibirica

Mutational witches' broom represents a fragment of the tree crown with slower shoot growth, abnormally dense branching, decreased apical dominance and often with abundant cone-bearing compared with a normal crown. Mutational witches' broom is a major source for the majority of ornamental dwarf conifer cultivars with abundant branching. Pinus sibirica is also capable of forming mutational witches' brooms. The species has valuable edible seeds, and therefore, dwarf cultivars with abundant cone-bearing could be used to establish cone crop plantations. To reveal how the mutation causing witches' broom affects the quality of cones and seeds, we carried out a comparative analysis of cone size, cone structure and seed quality in six grafted clones with witches' broom and normal clones from the same trees, determining seed losses at the different developmental stages. Cones from mutant clones had 1.5-2.9 times smaller size than normal clones. Cones from 50% of the normal clones were more elongated in shape. In mutant clones, the numbers of fertile scales and ovules were 1.4 times lower than in normal clones. Seeds had a smaller size in all mutant clones, and seed quality and seed efficiency were lower in two-thirds of the clones compared with normal clones. Seed losses were observed at different developmental stages, but there were significantly more losses before pollination and during embryo development in mutants. Therefore, the majority of mutants had small cones and therefore a rather ornamental value because of reduced seed efficiency and small seeds. At the same time, individual mutant clones had a seed efficiency and the number of filled seeds close to those of normal clones. They could therefore be promising as cone-bearing cultivars for the commercial production of pine kernel.