Evaluation of leaf traits for indirect selection of high yielding poplar hybrids

Two Populus families growing at two sites in Europe (i.e., northern Italy versus central France) were used to investigate: (1) the relationships between various leaf structural and growth traits and biomass production, (2) the dependence of these relationships on environmental conditions and genetic background (i.e., Populus deltoides × Populus nigra family versus P. deltoides × Populus trichocarpa family), and (3) the subsequent relevance of the use of these morphophysiological traits as indirect indicators of productivity. Tree growth and leaf characteristics, as well as the links between them, were intensively studied for 3 weeks. The P. deltoides × P. trichocarpa family was more productive than the P. deltoides × P. nigra family at both sites. The two families inherited complementary leaf characteristics from their respective male parents, i.e., large leaves from P. trichocarpa and fast leaf production from P. nigra. The traits were clearly dependent on site conditions, trees being much bigger in Italy than in France. Moreover, the G × E interaction caused a significant change in the genotypic ranking between sites in terms of productivity for the P. deltoides × P. trichocarpa family, which might represent a limit for the selection of productive hybrids showing a large environmental spectrum. Three categories of leaf traits could be considered: (i) traits linked to whole tree growth irrespective of site and family (e.g., leaf area, petiole dimensions), (ii) traits for which the relationships with tree growth were dependent on site and/or family (e.g., specific leaf area, nitrogen content, leaf number increment), and (iii) traits showing no link with tree growth irrespective of site and family (e.g., chlorophyll and carbon contents).