Genetic variation of leaf traits related to productivity in a Populus deltoides x Populus nigra family

In breeding and selection, two of the main goals of hybridization are to combine favourable traits from different species and to obtain high hybrid vigor (or heterosis). The objectives of out- study were (1) to determine which leaf traits are most closely linked to growth in a cross between Populus deltoides Bartr. ex Marsh. and Populus nigra L. and (2) to estimate the relevance of this cross for selection of highly productive genotypes. To achieve these objectives, 26 poplar F, hybrids and their parents were studied during their second growing season in central France. Tree growth (i.e., growth rates of stem height, circumference, and volume) was monitored during 1 month, and leaf traits (i.e., increases in number of leaves, maximum individual leaf area, specific leaf area, petiole length, and dry mass, leaf carbon and nitrogen contents, and internode length) were estimated at the end of the 1-month period. Growth traits were tightly correlated to most of the leaf traits. More precisely, it appeared that stem volume growth rate can be decomposed into two single leaf characteristics: maximum individual leaf area and leaf increment rate. All traits showed moderate values of broad-sense heritability. Heterosis as well as coefficients of genetic variation were also modest.