Cone Production and Cone Structure in the Clones from Mutational Witches’ Brooms of Siberian Stone Pine

Abstract

Mutational witches' broom (WB) is a fragment of tree crown with abnormal morphogenesis, including slow growth, abundant branching, reduced apical dominance and often abundant seed production. Mutational WBs are the main source of ornamental dwarf grafting conifer cultivars. Siberian stone pine, like other conifers, is capable of forming mutational WBs. In order to reveal how the mutation affects the development of WB cones an analysis of the cone yield and cone size and structure in 20 WB clones was made, and they were compared with a mean normal Siberian stone pine cone with length 9 cm, width 6 cm, 57 seeds per cone and the proportion of the medial fertile zone 39.1-60.3% of the total number of scales. WB clones had a different number of cones. Thirteen-year old clones had 13 cones in average. All cone traits varied greatly among the WB clones. Three clones had shortest cones, their length was less than 3 cm. Two of the clones also had the smallest diameter, 2.3 and 2.2 cm, respectively. Four clones had relatively large cones, longer than 4 cm, and diameter 3.4 cm. The fertile zone in the cones of WB clones was from 27.8% to 55.6% from the total number of scales, i.e. only slightly smaller than that in normal cones. The cones contained 40-60 ovules, which gave rise to less than 20 seeds in 20% of the clones, and more than 40 seeds per cone in 15% of the clones. which is a third less than that in normal Siberian stone pine trees. The number of aborted seeds after pollination in cones was almost not high for all clones, however, for some, the proportion of aborted seeds was up to 30% of the total number of seeds per cone. Thus, the cones in WB clones were 2-3 times smaller, and they were generally inferior to normal Siberian stone pine cones, but the yield in WB clones was quite high. At the same time, the WB clones were highly variable in yield and cone quality. Only individual clones with good yield and relatively large cones can be promising for breeding as nut-bearing cultivars. Due to the negative WB crown density on the number and size of their cones. in most cases, the presence and abundance of cones can only serve as an additional feature that enhances the ornamental value of dwarf graft cultivars of Siberian stone pine. At the same time, all WB clones were fertile and could be used for cross-breeding and further selection work.