Studies on selection of natural triploids of Populus tomentosa.
Natural triploids of poplar, which exhibit large volume growth, have been found in a few species of Populus in many countries, but investigations on their indigenous occurrence in China were not done until the national poplar breeding programme was established in 1983. A selection and breeding programme for triploids of Populus tomentosa has being conducted in Beijing Forestry University for over 15 yr (1983-97). Research started with natural triploid selection from stands, observations on larger 2n pollen, and the artificial induction of triploids using colchicine treatment. This paper summarizes work done on 2n pollen and the selection of natural triploids. Larger pollen grains (35-45 µm) occur naturally as 3.9% of total P. tomentosa pollen. They have 38 chromosomes (1n = 2x = 38), and are, therefore, unreduced 2n in character; the smaller pollen has 19 chromosomes (1n = 1x = 19), and is normal haploid pollen. Natural triploids (2n = 3x = 57) of P. tomentosa occur in the (normal) distribution area of the species. So far 5 clones of natural triploids have been found in the northern, western, and southern parts of the P. tomentosa distribution area, and far separated from one other. The volume growth, fibre length, stem form and pest resistance of natural triploids of P. tomentosa are superior. The average genetic gain for volume growth of 5 natural triploids exceeded 56.4% over control values, with the best clone (B383) exceeding 154%. These natural triploids are very important for pulp and timber production, and tree breeding research. They are already grown on a large-scale and their use has spread up to the northern part of China.