LAUSR

Abstract Light quality affects the morphological development of a plant. The aim of the experiment was to examine the effect of far-red radiation (FR, 700–800 nm) on growth, morphology, and root growth capacity (RGC) in Scots pine seedlings originating from latitudes 61°N and 67°N, and on gas exchange and subsequent field growth in seedlings originating from 61°N. The seedlings were grown in a greenhouse under natural daylight, receiving either FR-rich (+FR) or FR-deficient (−FR) supplemental light for 20 h per day, provided using light-emitting diodes (LEDs). The seedlings were outplanted and grown in the field for two growing seasons. Growth under +FR hastened terminal bud formation in seedings originating from 61°N, and delayed it in seedlings from 67°N. At the end of the growth period under the LEDs, +FR seedlings were dominated by secondary needles and had a higher needle mass than −FR seedlings, whose foliage comprised mainly primary needles. −FR improved root growth capacity in the seedlings originating from 61°N, but not in seedlings originating from 67°N. In seedlings from 61°N, photosynthesis (P n ) at the needle area level was higher and stomatal conductance lower under −FR, resulting in higher water use efficiency than under +FR. Due to the greater amount of photosynthetic tissue under +FR, the whole-seedling-level P n was higher under +FR. Height and diameter growth were greater in −FR seedlings than in +FR seedlings during the second growing season. Growth habit and growth rate of Scots pine seedlings can be manipulated by controlling the FR content in the supplemental light, enabling production of seedlings targeted for outplanting on sites with specific characteristics.