ObjectiveTriploid poplars have been proved to be characterized with outstanding advantages in vegetative growth over diploids. However, it still has some difficulties to take full advantage of maternal effect in present P. tomentosa breeding work. By careful selection of female parents with good fertility, this study aims to produce hybrid triploids by megaspore chromosome doubling in P. tomentosa, which would provide technical support for triploid breeding in further research.

MethodThe P. tomontosa clones MC1 and MC2 were served as the female parents, and the hybrid clone YX1 of P. alba×P. glandulosa was selected to be the male parent. According to the correlation between megasporocytes and microsporocytes during meiosis, the meiotic stage of megaspore in female flowers could be determined by the meiotic stage of microspore in male flowers. When the microspore developed into early stage of single nucleus, late uninucleate stage and binucleate stage, the female catkins were treated for 4 hours with high temperatures of 38, 40 and 42 ℃ to induce chromosome doubling of megaspore, respectively.

ResultA total of 189 triploids were obtained in the two hybrid combinations. Among all the 189 triploids, 70 triploids were obtained from the cross combination MC1×YX1, and the average induction rate was 56.45%. 119 triploids were obtained from the combination MC2×YX1, and the mean induction rate was 47.04%. No triploids were achieved in the control groups. When the microspores developed into the uninucleate eccentric stage, the corresponding meiotic stage of macrospore mother cell was at the stages from pachytene to metaphase I, which could be the optimal treating period for inducing megaspore chromosome doubling by 38 and 40 ℃ high temperature treatment. The average seedling height and the ground diameter of triploid hybrids derived from two cross combinations were higher than that of the diploids, amid there were some differences between the two combinations, which suggested that some attentions should be paid to the parental selection in triploid breeding of P. tomentosa.

ConclusionOur findings reveal that induction of megaspore chromosome doubling by high temperature treatment is an effective way to produce triploid hybrids in P. tomentosa, and the progress of breeding technologies plays an important role in genetic improvement of P. tomentosa.