Jouni Partanen, Risto Häkkinen, Anneli Viherä-Aarnio, Niina Stenvall, Heikki Hänninen. (2026). Short-day treatment in late summer reduces the chilling requirement in Norway spruce seedlings. Silva Fennica vol. 60 no. 2 article id 25053. https://doi.org/10.14214/sf.25053

Keywords: bud burst percentage; days to bud burst; rest; endodormancy release; forcing, quiescence

Highlights: Short-day treatment reduces the chilling requirement and the time to bud burst in Norway spruce seedlings; Forcing photoperiod does not affect the time to bud burst of short-day treated seedlings; In natural outdoor conditions, the short-day treatment advances the bud burst slightly.

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Artificial shortening of photoperiod with short-day (SD) treatment in late summer is a common measure in forest nurseries to advance the growth cessation and increase the frost hardiness of seedlings in the autumn. We conducted an experimental study of the effect of SD treatment (12 h) in the preceding summer and the photoperiod (16 h, 8 h) prevailing in the regrowth test under forcing conditions. The effect of these factors on the chilling requirement in second-year Norway spruce Picea abies (L.) Karst. seedlings was studied. Half of the seedlings were first subjected to a three-week SD treatment between 11 July and 1 August 2005, whereas the other half were used as the control group. The seedlings were first exposed to chilling in natural conditions. Subsequently, samples of the seedlings were transferred at intervals between September and May to a regrowth test in growth-promoting forcing conditions in a greenhouse. Additionally, we observed the bud burst of the seedlings in natural conditions during the next spring. Our main result was that SD treatment reduced the chilling requirement in the seedlings. Long photoperiod (16 h) in the forcing conditions also reduced the chilling requirement. The buds of the SD-treated seedlings burst earlier than those of the control seedlings and generally at almost the same time in both photoperiods. This suggests that the photoperiod prevailing in the forcing conditions has no additional delaying or advancing effect on the bud burst of SD-treated seedlings. In natural outdoor conditions, the SD treatment in the preceding summer advanced the springtime bud burst slightly.

Partanen, Natural Resources Institute Finland (Luke), Production Systems, Juntintie 154, FI-77600 Suonenjoki, Finland https://orcid.org/0009-0007-1782-4283 E-mail: ext.jouni.partanen@luke.fi
Häkkinen, Natural Resources Institute Finland (Luke), Production Systems, P.O. Box 2, FI-00791 Helsinki, Finland https://orcid.org/0009-0007-1687-1045 E-mail: risto.hakkinen@yahoo.com
Viherä-Aarnio, Natural Resources Institute Finland (Luke), Production Systems, P.O. Box 2, FI-00791 Helsinki, Finland https://orcid.org/0000-0002-1777-637X E-mail: annelivihe@outlook.com
Stenvall, Natural Resources Institute Finland (Luke), Production Systems, P.O. Box 2, FI-00791 Helsinki, Finland https://orcid.org/0000-0002-9447-8712 E-mail: niina.stenvall@luke.fi
Hänninen, State Key Laboratory for Development and Utilization of Forest Food Resources, Zhejiang A&F University, 666 Wusu Street, Hangzhou 311300, PR China https://orcid.org/0000-0003-3555-2297 E-mail: hhannin@zafu.edu.cn

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