Growth dynamics and mycorrhizas of Norway spruce (Picea abies) seedlings in relation to boron supply

The effects of three boron levels on the growth dynamics and ectomycorrhiza s of seedlings of Norway spruce [Picea abies (L.) Karst.] were studied in a growth room experiment. The seedlings were grown in forest humus mixed wit h quartz sand for 16 weeks. The B treatment was applied in the nutrient sol ution. Stem height, dry weight, number of root tips, mycorrhizas as well as B and N concentrations in the seedlings were monitored in sequential harve sts. By the last harvest, in week 16, needle B concentrations were 6.6 mg k g(-1) at the lowest B level and 17.5 mg kg(-1) and 26.5 mg kg(-1) at the tw o higher levels. Boron slightly increased the stem hei-ht and the total dry weight, but did not affect N content of the seedlings. Low internal B redu ced the number of root tips and mycorrhizas as well as mycorrhizal percenta ge and root dry weight, which indicates the importance of B for root growth in Norway spruce seedlings. The seedlings crown with adequate internal B h ad more root tips than those receiving the two lower B treatments as early as week 9, when needle B concentrations at the lowest B supply were 16.0-17 .3 mg kg(-1), which has previously been considered a sufficient B level. Th erefore, the critical needle B concentrations should perhaps be re-examined .