Abstract
Young seedlings of the conifer Norway spruce exhibit short day (SD)-induced cessation of apical growth and bud set. Although different, constant temperatures under SD are known to modulate timing of bud set and depth of dormancy with development of deeper dormancy under higher compared to lower temperature, systematic studies of effects of alternating day (DT) and night temperatures (NT) are limited. To shed light on this, seedlings of different provenances of Norway spruce were exposed to a wide range of DT-NT combinations during bud development, followed by transfer to forcing conditions of long days (LD) and 18°C, directly or after different periods of chilling. Although no specific effect of alternating DT/NT was found, the results demonstrate that the effects of DT under SD on bud set and subsequent bud break are significantly modified by NT in a complex way. The effects on bud break persisted after chilling. Since time to bud set correlated with the daily mean temperature under SD at DTs of 18 and 21°C, but not a DT of 15°C, time to bud set apparently also depend on the specific DT, implying that the effect of NT depends on the actual DT. Although higher temperature under SD generally results in later bud break after transfer to forcing conditions, the fastest bud flush was observed at intermediate NTs. This might be due to a bud break-hastening chilling effect of intermediate compared to higher temperatures, and delayed bud development to a stage where bud burst can occur, under lower temperatures. Also, time to bud burst in un-chilled seedlings decreased with increasing SD-duration, suggesting that bud development must reach a certain stage before the processes leading to bud burst are initiated. The present results also indicate that low temperature during bud development had a larger effect on the most southern compared to the most northern provenance studied. Decreasing time to bud burst was observed with increasing northern latitude of origin in un-chilled as well as chilled plants. In conclusion, being a highly temperature-dependent process, bud development is strongly delayed by low temperature, and the effects of DT is significantly modified by NT in a complex manner.
Highlights
To survive the winter, temperate and boreal zone trees must cease their growth, develop dormant winter buds and acquire cold hardiness in time before the onset of the harsh conditions
By employing a substantial number of DT-night temperatures (NT) combinations resulting in eight different daily mean temperatures (DMT), our results clearly demonstrate that bud set is a temperature-dependent process, and that, the DMT is an important determinant of the time to bud formation, the actual temperature combination is important
The present results demonstrate that the effect of DT on time to bud set is significantly modified by NT in a complex manner, alternating DT and NT in itself does not affect time to bud set
Summary
Temperate and boreal zone trees must cease their growth, develop dormant winter buds and acquire cold hardiness in time before the onset of the harsh conditions. In addition to the significance of the light climate, in dormancy induction, temperature is well known to play important roles in several aspects of growth-dormancy transitions and development of cold hardiness, and has been shown to modify the sensitivity to photoperiod and influence the duration of bud formation (Olsen, 2010; Tanino et al, 2010, 2014; Rohde et al, 2011; Cooke et al, 2012; Junttila and Hänninen, 2012; and references therein). Some woody species of the rose family, such as apple (Malus domestica) and pear (Pyrus communis) are insensitive to photoperiod, and instead show growth cessation and formation of dormant winter buds in response to relatively low temperature www.frontiersin.org
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