Slow-growing pollen-tube of colchicine-induced 2n pollen responsible for low triploid production rate in Populus

To reveal the mechanism of low triploid production, meiotic observation, induction of 2n pollen by colchicine, hybridization of colchicine-induced 2n pollen and pollen germination were conducted in Populus tomentosa clone 5088. Meiosis of the PMCs was initiated 24 h after it was cultured in the greenhouse. Results showed that the meiosis developed in a consecutive, asynchronous process. Analysis of the occurrence rate of colchicine-induced 2n pollen using the GLM-univariate revealed significant differences among the dominant meiotic stages (F = 45.822, P = 0.000). Significant differences also occurred among various colchicine injection times (F = 10.150, P = 0.004). The most effective stage for colchicine-induced 2n pollen occurred between the zygotene to diakinesis substages. Sixty-eight triploids were confirmed with both flow cytometric analysis and chromosome number-counting among the 3346 offspring. The highest triploid induction rate was 13.2%—significantly lower than the expected triploid production rate of 47.0%—indicating that the colchicine-induced 2n pollen was weak during fertilization. We found positive correlation between the efficiency of triploid production and the frequency of colchicine-induced 2n pollen (r = 0.961, P = 0.001) which suggests that the triploid production rate increased as the occurrence rate of colchicine-induced 2n pollen increased. Analysis using the GLM-univariate indicated that both pollen types (F = 87.50, P < 0.001) and durations of pollen germination (F = 7.61, P = 0.002) did have significant effects on the lengths of pollen-tubes. Haploid pollen-tubes were significantly longer than colchicine-induced 2n pollen-tubes. This suggests that haploid pollen has an advantage during fertilization.