QUANTITATIVE GENETICS OF YIELD BREEDING FOR POPULUS SHORT-ROTATION CULTURE .2. GENETIC DETERMINATION AND EXPECTED SELECTION RESPONSE OF TREE GEOMETRY

Understanding the genetic determination of tree geometry and its assoc iations with productivity are important research goals of short rotati on intensive culture of fast-growing Populus species. A morphological development model of a 4-year Populus deltoides Bartr. X Populus simon ii Carr. and P deltoides X Populus nigra L. F1 hybrid poplar was descr ibed at four levels of organization: (i) leaf, (ii) branch, (iii) main stem, and (iv) whole tree. Quantitative genetic analyses were done to explore the amounts and patterns of genetic variation and responses t o artificial selection for 58 morphometric traits at the four levels. Leaf traits were compared across current terminal, sylleptic and prole ptic branches, and pooled type; branch traits were compared across syl leptic and proleptic branches and pooled type. The alternate locations of sylleptics with proleptics on the main stem was considered to be a n adaptive response of poplars to environmental conditions in the midd le temperate zones. The components of tree geometry showed abundant ge netic variation among clones and high broad-sense heritability levels, both with apparent divergence across traits, trait origins (for leaf and branch traits), and hybrid populations. Desired trait such as main stem height and diameter at breast height had decreased genetic bases and, thus, decreased selection responses compared with other morphome tric traits. Despite weaker genetic control over leaf and branch trait s on sylleptics than on current terminal and proleptics, high genetic variabilities would be expected to generate substantial genetic gain f or some traits such as leaf number, size, and orientation on sylleptic branches. Under moderate selective intensity (p = 15%), P. deltoides X P. simonii tended to display more open branching systems with great photosynthetic area and light interception, while tree geometry of P. deltoides X P. nigra showed a better balance between maximum photosynt hetic organ production and minimum energy investment.