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.
Stem taper functions for white birch (Betula platyphylla) and costata birch (Betula costata) in the Xiaoxing’an Mountains, northeast China

 White birch (Betula platyphylla Sukaczev) and costata birch (Betula costata Trautv.) are valuable hardwood tree species growing in northeast China. Several taper studies have analysed birch species in the countries harbouring the boreal forests. However, this study presents an initial attempt to develop stem taper models using the fixed- and mixed-effects modelling for white birch and costata birch in Xiaoxing’an Mountains, northeast China. Ten commonly used taper models were evaluated by using 228 destructively sampled trees of both tree species comprising of 4582 diameter and height measurements. The performance of these models was tested in predicting diameter at any height, total volume and merchantable volume (10 and 20 cm top diameters). We incorporated a second-order continuous-time error structure to adjust the inherent autocorrelation in the data. The segmented model of Clark best predicted the diameter and total or merchantable volume when the upper stem diameter at 5.3 m was available. When diameter measurements at 5.3 m were not available, the models of Kozak and Max and Burkhart were superior to other models for white birch and costata birch, respectively. After model comparison, the best model of Clark was refitted as the NLME model.
Heterosis in Intervarietal Crosses in Maize and Its Implication in Breeding Procedures 1
TN RECENT years there has been an increased interest in -•• heterosis resulting from population crosses of various species (3, 16, 19, 20). There is need for more information relative to the characterization and magnitude of hybrid vigor to provide a basis for more efficient breeding procedures in both plants and animals. Hybrid vigor resulting from Ft crosses of corn varieties was first reported by Beal (2). This led to rather extensive study of variety crosses by plant breeders in an attempt to increase grain yields during the early 1900's. Interest in variety crosses as a practical measure gave way to interest in hybridization of inbred lines derived from varieties after about 1920. Richey (11) presented a summary of 244 reported comparisons between the parental varieties and their F1 crosses as a sample of the results obtained up to that time. His summary showed that 82.4% of the crosses exceeded the parental average, with 55.7% exceeding the higher yielding parent. The lack of consistency of results where different substrains of any given variety were used together with the spectacular results from crosses of selected inbred lines resulted in a loss of interest in varietal crosses of corn at that time. Recently a renewed interest in the type of gene action responsible for heterosis and the expected progress from selection within and among populations has resulted in further study of varietal crosses in corn. Investigations of the relative magnitude of additive genetic and dominance variance in corn varieties (15) led to a study of crosses among southern prolific varieties by this group of workers. The results from a set of diallel crosses among six southern varieties (16) showed the F1 to average 19-9% more than the midparent values and 11.5% more than the high parent. The range in superiority over midparent values was 4.6 to 46.2%. If previous selection had resulted in these varieties being at equilibrium between mutation and selection and if gene frequencies affecting yield were similar from one population to another, the above results would be considered surprising. The fact that corn varieties differ in their underlying genetic constitution is evident from these and earlier data. Many varieties have been preserved by plant breeders since the advent of hybrid corn and have been regrown on a rather small sample basis when necessary to maintain via-