Progeny test of tetraploid Betula platyphylla and preliminary selection of hybrid parents
Estimating genetic parameters of parental lines through progeny testing and choosing good hybrid parents are important for genetically improving seed orchard trees. In this study, 24 tetraploid progeny seedlings were used as experimental materials, which came from test cross design: six tetraploid Betula platyphylla lines (Q33, Q13, Q103, Q19, Q83 and Q14) as female parents and four individual B. platyphylla diploid lines (F3, F4, F9 and F11) as male parents were crossed. Variance analysis of height, diameter, height-to-diameter ratio, and internodal distance showed that the differences between hybrid combinations reached highly significant levels. Using multi-objective decisionmaking, we performed a comprehensive assessment of the various hybrid combinations. Using a selection rate of 20 % of the standard, five hybrids were selected; their genetic gains in average height, diameter, height-to-diameter ratio, and internodal distance were 20.95, 6.07, 13.07 and 8.96 %, respectively. We also analyzed the combining ability and genetic parameter effect values of parents and hybrid combinations. The combined analysis revealed that Q13, Q103, Q33 and Q83 were superior females; F3, F4, and F9 were superior males; and F3 × Q13, F4 × Q83 and F9 × Q33 were superior hybrid combinations. The heights and diameters of these progenies were 22.49 and 11.48 % greater than average, respectively.
A comparison of the effect of genetic improvement, seed source and seedling seed orchard variables on progeny growth in Eucalyptus nitens in South Africa
Eucalyptus nitens is an important forestry species grown for pulp and paper production in the temperate, summer-rainfall regions of South Africa. A tree improvement programme has been ongoing at the Institute for Commercial Forestry Research for two decades, but genetic improvement in the species has been slow due to delayed and infrequent flowering and seed production. Three trials were established, firstly, to quantify the gains that have been made in the first generation of improvement in the breeding programme and, secondly, to establish whether a number of seed source and orchard variables influence the performance of the progeny. These variables were the amount of flowering trees in the seed orchard, year of seed collection, seed orchard origin and composition of seed orchard bulks. Diameter at breast height and tree heights were measured in the trials at between 87 and 97 months after establishment, and timber volumes and survival were calculated. Improved seed orchard bulks performed significantly better (p < 0.01) than unimproved controls in the field trials. Genetic gains ranging from 23.2 to 164.8 m3 ha−1 were observed over the unimproved commercial seed. There were significant differences (p < 0.01) in progeny growth between the levels of seed orchard flowering, with higher levels of flowering (≥40 %) producing substantially greater progeny growth than lower flowering levels (≤20 %). The seed orchard had no effect on progeny growth in this trial series. This suggests that seed collected from any of the four seed orchards tested will produce trees with significant improvement in growth.
Effect of winter chilling and paclobutrazol on floral bud production in Eucalyptus nitens
Flower bud production of Eucalyptus nitens is comparatively abundant at specific high altitude sites in the South African summer rainfall region. This suggests that cumulative cold may be implicated in the floral induction process. Therefore, three chill models were used to investigate whether winter temperature data can be related to E. nitens flower bud production at sites differing in chill accumulation. Amount of accumulated winter chill, in conjunction with paclobutrazol treatment, was able to explain between 66 and 72% of the variation in E. nitens flower bud production. Although the potential chilling requirement of E. nitens can be calculated with similar accuracy by each of the chill models tested, the Dynamic Model performed best under the particular range of experimental conditions. At low to moderate levels of winter chill (43 to 81 Chilling Portions (CPs) of the Dynamic Model), paclobutrazol application hastened first flowering and increased the percentage reproductive trees by between 5–27% in the case of seedlings, and between 2–62% in the case of grafts. At high levels of winter chill (> 87 CPs), paclobutrazol had a negligible effect on the percentage reproductive trees in the case of either seedlings or grafts. At four and five years after planting, very high levels of accumulated winter chill (96 CPs) stimulated a high percentage of seedlings (25–50%) and grafts (55–64%) to produce flower buds. Considerable variation in precocity and chilling requirement (for floral induction) is evident within the South African E. nitens breeding population. A better accuracy in relating E. nitens floral bud production to amount of prior winter chilling could be achieved by further experimentation involving fewer genotypes and a wider range of chilling conditions.