Interactive Effects of Light Quality during Day Extension and Temperature on Bud Set, Bud Burst and PaFTL2, PaCOL1-2 and PaSOC1 Expression in Norway Spruce (Picea abies (L.) Karst.)

Light and temperature are crucial factors for the annual growth rhythm of tree seedlings of the boreal and temperate zone. Dormant, vegetative winter buds are formed under short days (SD) and altered light quality. In the conifer Norway spruce, expression of FTL2 increases and PaCOL1-2 and PaSOC1 decrease under light regimes, inducing bud set. Although temperature is known to modulate the timing of bud set, information about combined effects of light climate and temperature on bud phenology and gene expression is limited. We studied the interactive effects of temperature (18, 22/24 degrees C) and day extension with blue (B), red (R) or far-red (FR) light or different R:FR ratios compared to SD on growth-dormancy cycling and expression of FTL2, PaCOL1-2 and PaSOC1 in Norway spruce seedlings. Day-extension with B light and all treatments involving FR light sustained shoot elongation, with increased growth at higher temperature. The R light treatment resulted in delayed/prevented bud set compared to SD, with more delay/prevented bud set at 24 degrees C than 18 degrees C. This was associated with lower PaFTL2-transcript levels at 24 degrees C and more rapid subsequent bud burst. For the growth-sustaining treatments (long days, FR and B light), the PaFTL2-transcript levels were generally lower and those of PaCO1-2 and PaSOC1 higher compared with SD and R light. In conclusion, our results demonstrate more reduced/prevented bud set and faster bud burst with increased temperature under day extension with R light, indicating less deep dormancy than at lower temperature. Also, sustained shoot elongation under the B light treatment (27 mu mol m(-2) s(-1)) in contrast to the lower B light-irradiances tested previously (<= 13 mu mol m(-2) s(-1)), demonstrates an irradiance-dependent effect of day extension with B light.

Automated summary

Light and temperature play critical roles in the annual growth rhythm of tree seedlings. This study found that increasing temperature under extended red light delayed/prevented bud set and resulted in faster bud burst, indicating less deep dormancy compared to lower temperatures. Additionally, sustained shoot elongation under blue light treatment demonstrated an irradiance-dependent effect of day extension with blue light.