| Summary: | Leaf morphology in <i>Populus</i> L. varies extensively among sections, species and clones under strong genetic control. <i>P. nigra</i> L. (section <i>Aigeiros</i>), with large and triangular leaves, is a commercial forest tree of economic importance for fast growth and high yield in Europe. <i>P</i>. <i>simonii</i> Carr. (section <i>Tacamahaca</i>) with small land rhomboid ovate leaves performs cold and dry resistance/tolerance in the semi-arid region of Northern China. Leaf morphological traits could be used as early indicators to improve the efficiency of selection. In order to investigate the genetic variation pattern of leaf morphology traits, estimate breeding values (combining ability), as well as evaluate crossing combinations of parents, 1872 intersectional progenies from eight families (<i>P. simonii</i> × <i>P. nigra</i>) and their parents were planted with cuttings for the clonal replicate field trial in Northern China. Four leaf size traits (area, perimeter, length, width) and roundness were measured with leaf samples from the 1-year-old clonal plantation. Significant differences regarding leaf traits were found between and among three female clones of <i>P. simonii</i> from Inner Mongolia, China and six male clones of <i>P. nigra</i> from Casale Monferrato, Italy. The genetic variation coefficient, heritability and genetic variance component of most traits in male parents were greater than these of female parents. Heritability estimates of male and female parents were above 0.56 and 0.17, respectively. Plentiful leaf variations with normal and continuous distributions exited in the hybrid progenies among and within families with the genetic variation coefficient and heritability above 28.49 and 0.24, respectively. Heritability estimates showed that leaf area was the most heritable trait, followed by leaf width. The breeding value ranking of parents allowed us to select the parental clones for new crosses and extend the mating design. Two male parental clones (N430 and N429) had greater breeding values (general combining ability, GCA) of leaf size traits than other clones. The special combining ability (SCA) of the crossing combination between <i>P. simonii</i> cl. ZL-3 and <i>P. nigra</i> cl. N430 was greater than that of others. Eight putatively superior genotypes, most combined with the female parental clone ZL-3, can be selected for future testing under near-commercial conditions. Significant genetic and phenotypic correlations were found between five leaf morphology traits with the coefficients above 0.9, except for leaf roundness. The results showed that leaf morphology traits were under strong genetic control and the parental clones with high GCA and SCA effects could be utilized in heterosis breeding, which will provide a starting point for devising a new selection strategy of parents and progenies.
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