Changes in metabolite profiles in Norway spruce shoot tips during short-day induced winter bud development and long-day induced bud flush

The seasonal change in photoperiod is a primary environmental signal influencing tree growth. Long days (LD) sustain growth, whereas short days (SD) induce winter bud formation. In this respect, metabolomic responses have been studied to a limited extent only in conifers. Here we identified changes in metabolite profile in the conifer Norway spruce after transition to SD and following re-transfer to LDs inducing bud flush. After 1 week in SD initial changes in metabolite profile was visible but for the majority of compounds magnitudes of changes were small. However, the ascorbate content was strongly reduced and there were often temporary increases in several energy metabolism-related compounds, secondary metabolites, nucleosides, amino acids and lipids. After 8 weeks in SD substantial changes were observed; proper winter buds had high pools of ABA, antioxidants, flavonoids, terpenoids, phenylpropanoids, sugars, amino acids and lipids related to stress tolerance and hardening, and low levels of nucleosides and metabolites in energy metabolism. One week after re-transfer to LD the metabolome was generally relatively similar to under long-term SD, except e.g. increased urate and strongly decreased ABA and oxidized glutathione. Two weeks later, bud flush had occurred, and the metabolite profile resembled the situation before transfer to SD. This study thus revealed comprehensive modulation of the metabolome in Norway spruce in response to a day length shift, indicating substantially increased stress resistance under SD-induced bud set, and reversal upon bud flush in LD.