Needle and bud scale development in Picea abies

Variation in lateral organ morphology identifies the timing of annual transitions between bud scale and needle identity in conifer buds.

Vegetative tissue is generated from the shoot apical meristem (SAM) at branch tips. Temperate and boreal conifers have determinate growth, in which a year’s complement of needles all begin to form in the summer of Year 1 (Y1), overwinter in a protective bud, then emerge and mature in spring and summer of Year 2 (Y2). Buds are protected by bud scales (bss), which are initiated in spring Y1 prior to needle initiation. Through the annual cycle of needle and bud formation, the SAM alternately produces needle or bs lateral organs. Detailed classification of vegetative lateral organ morphologies in Picea abies (Norway spruce) showed that while the majority of lateral organs are clearly either of needle or bs type, a number of organs in any given bud are intermediate, having both needle and bs characteristics. These transitional organs form the basal outer protective layers of the bud. This work documents the variety of bs morphologies involved in forming the overall bud. Needle and bs counts show a clear distinction in initiation times for each type. bs initiation coincides with spring bud elongation, while needle initiation lags this by 2–3 months. This suggests distinct, temporally separated signals for bs and needle tissue differentiation. The transitional forms suggest some primordia form at times when they are susceptible to both needle and bs differentiation signals. Measurements of shoot apex dimensions show that, in addition to an increasing number of needle primordia on the shoot apex over the summer, the needle primordia themselves undergo an expansion in the fall prior to dormancy.