Optimizing nitrogen economy under drought: increased leaf nitrogen is an acclimation to water stress in willow (Salix spp.)

Background and Aims The major objective was to identify plant traits functionally important for optimization of shoot growth and nitrogen (N) economy under drought. Although increased leaf N content (area basis) has been observed in dry environments and theory predicts increased leaf N to be an acclimation to drought, experimental evidence for the prediction is rare. Methods A pedigree of 200 full-sibling hybrid willows was pot-grown in a glasshouse in three replicate blocks and exposed to two water regimes for 3 weeks. Drought conditions were simulated as repeated periods of water shortage. The total leaf mass and area, leaf area efficiency (shoot growth per unit leaf area, E-A), area-based leaf N content (N-A), total leaf N pool (N-L) and leaf N efficiency (shoot growth per unit leaf N, E-N) were assessed. Key Results In the water-stress treatment, shoot biomass growth was N limited in the genotypes with low N-L, but increasingly limited by other factors in the genotypes with greatest N-L. The N-A was increased by drought, and drought-induced shift in N-A varied between genotypes (significant G x E). Judged from the E-A-N-A relationship, optimal N-A was 16% higher in the water-stress compared with the well-watered treatment. Biomass allocation to leaves and shoots varied between treatments, but the treatment response of the leaf : shoot ratio was similar across all genotypes. Conclusions It is concluded that N-uptake efficiency and leaf N efficiency are important traits to improve growth under drought. Increased leaf N content (area basis) is an acclimation to optimize N economy under drought. The leaf N content is an interesting trait for breeding of willow bioenergy crops in a climate change future. In contrast, leaf biomass allocation is a less interesting breeding target to improve yield under drought.