Growth responses of 15 Salix genotypes to temporary water stress are different from the responses to permanent water shortage

Hybrids of willow (Salix spp.) are today grown as biomass crops for energy in several cool-temperate regions including Sweden. These trees have great growth potential also under warmer climates, but may suffer from drought and heat. Research on tree growth under drought has often focused on the responses to permanent water stress. Less attention has been paid to the effects of temporary water stress on tree growth, although periods of strong water shortage are recurrent in many regions of the world. We performed a greenhouse experiment to assess the impact of five treatments including permanent (PWS; 30 days of water shortage) and temporary (TWS; 4-12 days of water shortage followed by normal watering) water stress on growth and leaf traits of 15 Salix genotypes. Plant biomass and height increment decreased across treatments in the order control (well-watered) > TWS > PWS, but there was also a significant genotype x treatment interaction. The pattern in growth traits was not directly reflected by the leaf traits, for which there was no significant genotype x treatment interaction (except for leaf angle). The significant genotype x treatment interactions for growth traits indicate a potential for breeding. Permanent water shortage does not necessarily represent an appropriate environment for the selection of genotypes for environments characterised by temporary water stress.