Elongation of Scots pine seedlings under blue light depletion

Abstract The elongation response of Scots pine (Pinus sylvestris L.) seedlings to the removal of blue light (400–500 nm) was studied in field experiments in northern Finland. The seedlings were grown in orange or transparent plexiglass chambers or in ambient control plots. The orange plexiglass removed the blue wavelengths from sunlight, while the others served as controls. The experiment was conducted at sub-arctic (69°N) and mid-boreal (64°N) latitudes with three- and two-year-old seedlings originating from 67°N latitude. The response to blue light depletion was also investigated at the 69°N latitude in the following plant subjects: one-year-old Scots pine seedlings of northern (67°N) and southern (62°N) provenances, deciduous Betula pubescens ssp. czerepanovii and Betula pubescens f. rubra seedlings and herbaceous Epilobium angustifolium and Glechoma hederacea plants. Additionally, diurnal change in light quality at the 69°N latitude during the summer was measured. The elongation of Scots pine seedlings was increased by the removal of blue wavelengths. The increase was more pronounced at the 69°N latitude, while at the 64°N latitude the response was smaller or absent. This is due to increased amount of scattered growth-inhibiting blue light during the nights at the high latitude. The removal of blue light increased stem elongation in northern origin Scots pine seedlings much more compared to the southern origin seedlings, which suggests that the northern provenance is more sensitive to blue light. Irrespective of that, southern origins also suffer from reduced elongation in the north as they migrate according to climatic change scenarios. However, it is obvious that they grow longer than local origins in the north. Morphological variables and photosynthetic pigments confirm that the increased elongation of Scots pine seedlings under blue light depletion is not a result of etiolation or it is only a marginal factor. Also, it was neither dependent on temperature nor photosynthesis and growth resources. Instead, the increased elongation is probably a photomorphogenic regulation response of metabolism. In addition, shade intolerant Scots pine, Betula seedlings and herbaceous Epilobium angustifolium responded stronger to blue light removal compared to the more shade-tolerant herbaceous Glechoma hederacea