QUANTITATIVE GENETICS OF GROWTH AND DEVELOPMENT IN POPULUS .1. A 3-GENERATION COMPARISON OF TREE ARCHITECTURE DURING THE FIRST 2 YEARS OF GROWTH

One approach to gain an insight into the genetics of tree architecture is to make use of morphologically divergent parents and study their s egregating progeny in the F-2 and backcross (B-1) generations. This ap proach was chosen in the present study in which material of a three-ge neration pedigree growing side by side in a replicated plantation, was analyzed. The pedigree included Populus trichocarpa (T) and P. deltoi des (D) parents, their F-1 and F-2 hybrids and their B-1 hybrids to th e D parent. The trees were grown in the environment of the T parent an d measured for the first 2 years of growth. Nine quantitative traits w ere studied at the stem, branch and leaf levels of tree architecture, in which the original parents differed. Strong F-1 hybrid vigor relati ve to the better parent (T) was expressed in growth and its components . Most quantitative traits in the F-2 and B-1 hybrids were intermediat e between the T and D parents but displayed a wide range of variation due to segregation. The results from the analysis of variance indicate d that all morphometric traits were significantly different among F-2 and B-1 clones, but the B-1 hybrids were more sensitive to replicates than the F-2. Broad-sense heritabilities (H-2) based on clonal means r anged from moderately high to high (0.50-0.90) for the traits studied, with H-2 values varying over age. The H-2 estimates reflected greater environmental ''noise'' in the B-1 than in the F-2, presumably due to the greater proportion of maladaptive D alleles in those hybrids. In both families, sylleptic branch number and length, and leaf size on th e terminal, showed strong genetic correlations with stem growth. The l arge divergence between the two original parents in the traits studied , combined with the high chromosome number in Populus (2n=38), makes t his pedigree well suited for the estimation of the number of quantitat ive trait loci (QTLs) underlying quantitative variation by Wright's bi ometric method (1968). Variation in several traits was found to be und er the control of surprisingly few major QTLs: 3-4 in 2nd-year height and diameter growth, a single QTL in stem diameter/height ratio.