Methods for generation of triploid aspen and poplar

Triploid trees often show higher growth potential, longer fibers and improved pulp properties in comparison with their diploid counterparts and were also found to have a greater ecological adaptability. Different methods of sexual polyploidisation were established and applied to breed triploid trees. As prerequisite for the crossings, parent clones with high performance were selected. In total, 48 triploid clones could be generated. The majority, including 13 intersectional hybrids, resulted from crossings using enriched diploid pollen. Spontaneously occurring diploid (2n) pollen, which can reach up to 5% of total pollen under natural conditions, was enriched using micro sieves. Additionally, in some clones, unreduced 2n pollen could be induced by heat treatment of microspore mother cells during meiosis. Some triploid plants were generated by heat treatment of female flowers after pollination to induce the formation of 2n eggs or were selected from conventional crossings based on their size and morphological characteristics. Out of the 29 triploid clones, tested under outdoor conditions to select the best-growing triploid clones, 14 clones showed a significantly increased shoot height in comparison to the fast-growing triploid clone"Astria". Another approach to produce triploids is to cross diploid with tetraploid trees. By combining polyploidisation methods with in vitro cultivation techniques, stable tetraploid clones from ten tested parent-clones were produced using colchicine and oryzaline for chromosome doubling. After flower formation, these plants can be used as valuable breeding material for mass production of highly productive triploids. All in all, results show that the methods applied here are suitable to generate triploid plants, with some of them showing excellent growth.