QTL mapping of biomass and nitrogen economy traits in willows (Salix spp.) grown under contrasting water and nutrient conditions

In order to efficiently grow Salix for biomass production in warmer climates, varieties with a desirable response to drought and nutrient-limiting conditions are needed. The main objective of this study was to investigate the genetic background of biomass production and nitrogen (N) economy in contrasting conditions of water and nutrient availability and to identify candidate genes with a putative function in the expression of the different traits. Quantitative trait loci (QTLs) analysis was conducted using data from 198 individuals of a back-cross population between S. viminalis and S. schwerinii grown in a greenhouse under three combinations of N and water supply. In total, 60 QTLs were identified for biomass and N economy traits in the different treatments. Most of the QTLs mapped to linkage groups II, III, VI, X, XIII and B. At linkage groups III, VI and X, QTLs for both N economy and biomass traits co-located. The phenotypic variation explained by each QTL varied from 7.7 to 41.9 % of the total variance. QTLs for N economy traits explained most of the variation. Gene ontology (GO) analyses, performed on QTL intervals for each trait and projected to the Populus trichocarpa genome, revealed that genomic intervals connected to 19 traits were enriched for at least one GO term. Candidate genes were selected among genes linked to the enriched GO terms. These results represent a first necessary step for additional mapping and functional studies and encourage the development of marker-assisted breeding of Salix varieties adapted to drier climates.