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 ectomycorrhizas of seedlings of Norway spruce [Picea abies (L.) Karst.] were studied in a growth room experiment. The seedlings were grown in forest humus mixed with quartz sand for 16 weeks. The B treatment was applied in the nutrient solution. Stem height, dry weight, number of root tips, mycorrhizas as well as B and N concentrations in the seedlings were monitored in sequential harvests. By the last harvest, in week 16, needle B concentrations were 6.6 mg kg(-1) at the lowest B level and 17.5 mg kg(-1) and 26.5 mg kg(-1) at the two 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 reduced the number of root tips and mycorrhizas as well as mycorrhizal percentage and root dry weight, which indicates the importance of B for root growth in Norway spruce seedlings. The seedlings crown with adequate internal B had 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. Therefore, the critical needle B concentrations should perhaps be re-examined.