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Keywords:

  • bud set;
  • latitude;
  • light-emitting diode;
  • light quality;
  • photoperiod;
  • phytochrome

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Seedlings of trees with a free growth pattern cease growth when night-lengths become shorter than a critical value, and this critical night-length (CNL) decreases with increasing latitude of origin. In northern populations, the light quality also appears to play an important role and a clinal variation in requirement for far-red (FR) light has been documented. In this study we dissected the light quality requirements for maintaining growth in different latitudinal populations of Norway spruce (Picea abies (L.) H. Karst.) using light emitting diodes for red (R), FR and blue (B) light, as 12 h day extension to provide 24 h photoperiod. At equal spectral photon flux, FR light was more effective than R light in maintaining growth, and the requirement of both R and FR increased with northern latitude of origin. One-to-one mixtures of R and FR light were more effective in maintaining growth than either FR or R light alone, indicating a possible interaction between R and FR light maintaining growth. Using the blue light as day extension could not prevent growth cessation in any of the populations, but delayed the bud set slightly in all populations. Our results suggest that phytochrome(s) are the primary photoreceptors in high irradiance responses maintaining growth in Norway spruce seedlings.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

It is vital for trees growing in temperate and boreal regions to stop growth in autumn to prepare for the freezing temperatures of the winter season. Growth cessation in autumn is a prerequisite for the development of terminal buds and full cold hardiness (Weiser 1970; Junttila & Kaurin 1990; Olsen et al. 1997). First year seedlings of most tree species have a free pattern of growth, and respond to night-lengths longer than a critical period, by ceasing apical growth and forming terminal bud (Nitsch 1957). The critical night-length (CNL) increases clinally with decreasing latitude (or altitude) of origin of the population, and is considered to be an adaptation to the length of the growing season (Håbjørg 1972b). The photoperiod has generally been considered the primary factor controlling shoot elongation in these plants, but day extension treatments of different light qualities have indicated another light monitoring system than night timekeeping. Experiments with northern populations of downy birch (Betula pubescens Ehrh.), bay willow (Salix pentandra L.) and Norway spruce (Picea abies (L.) H. Karst.) have documented a requirement for far-red (FR) light in the light period to maintain growth, and this demand increases clinally with increasing northern latitude of origin (Håbjørg 1972a; Junttila & Kaurin 1985; Clapham et al. 1998a). This has been suggested to be an adaptation to arrest growth in long photoperiods (close to or 24 h) at high northern latitudes (Nilsen 1985). These two light monitoring systems have been referred to as night timekeeping and day timekeeping, and appear to co-exist in woody species with a wide geographic distribution including regions above the arctic circle (Clapham et al. 1998b). The relative importance of either timekeeping system varies along a latitudinal cline of origin, with prevalence for night timekeeping at southern latitudes and day timekeeping at far northern latitudes.

Plants have four known classes of light sensing pigments regulating photomorphogenesis: phytochromes detecting light in the red (R)/FR region, cryptochromes and phototropins detecting wavelengths in the blue part of the spectrum, and UVB-receptors (Quail 2002). Phytochromes are the photoreceptors known to be directly involved in the photoperiodism of plants, whereas cryptochromes have been shown to play some regulatory roles in photoperiodism and to be necessary for phytochrome function (Devlin & Kay 2000). Night and day timekeeping in trees share many characteristic features with photoperiodism of flowering in herbaceous plants. Herbaceous short-day plants (SDP) rely on night timekeeping for flowering, and short night-breaks of low-fluence R light inhibit the response. These night-breaks are reversible if they are quickly followed by a short FR exposure. Seedlings of Populus have been shown to be responsive to short night-breaks of red (R) light maintaining growth, while a subsequent short FR treatment reverses this effect (Howe et al. 1996). Low fluence R/FR reversibility is a feature of light stable phytochromes, such as phyB, and a PHYB has been mapped close to a quantitative trait loci for photoperiodism in Populus (Frewen et al. 2000). Herbaceous long-day plants (LDP) rely on day timekeeping for flowering, and several LDP are induced to flower by prolonged FR- or R : FR-exposure in a high irradiance response (HIR). Mixtures of R : FR are often required or most effective in these HIRs, and the stimulus is also most effective when detected at dusk (Thomas & Vince-Prue 1997). Seedlings of northern trees do not respond to short night-break treatments, and prolonged FR-exposure is more effective in certain phases of the light period, predominately at dusk (Vince-Prue 1984). The FR-HIR response of flowering is mediated by the light-labile phyA in the quantitative LDP Arabidopsis thaliana L. (Shinomura, Uchida & Furuya 2000). Altering the level of phyA in trees has been shown to modify the response to photoperiod. Increased phyA levels in transgenic Populus rendered the plants insensitive to short photoperiods (SD), and decreased phyA levels enhanced the response to SD (Olsen et al. 1997; Eriksson 2000). PhyA is thus important for the photoperiodism in angiosperm trees, although the nature of its role is not known. Gymnosperm phytochromes are not well characterized, but have been shown to contain a light labile phytochrome similar to phyA (Burgin et al. 1999).

The involvement of other photoreceptors than phytochromes in growth cessation and bud set has not been investigated in detail for trees. Blue light detected by cryptochromes has been shown to be involved in the photoperiodic control of flowering in Arabidopsis, setting the endogenous clock to the photoperiod and being necessary for phyA function (Devlin & Kay 2000). A study of the effects of different light sources as day extension for a northern population of bay willow has suggested that blue light together with R : FR light may promote bud set (Junttila & Kaurin 1985). In addition to this, B light has been reported to influence phytochrome control of elongation growth in seedlings of Scots pine (Pinus sylvestris L.) (Fernbach & Mohr 1990).

Despite the reported responses of trees to light qualities, uncertainties exist in respect to the actual active wavelengths involved. Common to the previous light quality experiments on trees has been the use of broad spectrum light sources rich in R, FR or B light, covering large ranges of wavelengths outside the activity maxima of phytochromes or cryptochromes (Håbjørg 1972a; Junttila & Kaurin 1985; Clapham et al. 1998a). The purpose of this study was to analyse the effects of distinct light qualities on growth in latitudinal populations of the woody gymnosperm, Norway spruce. Day extensions were obtained with light-emitting diodes (LED) with discrete wavelength-ranges peaking at R (660 nm), FR (730 nm) and B (460 nm) light, corresponding to the wavelengths for maximum activation of the phytochrome Pr form, the Pfr form, and cryptochrome, respectively (Borthwick et al. 1952; Ahmad et al. 2002).

MATERIALS AND METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Seedlings were grown from seeds of three different latitudinal provenances of Norway spruce (Picea abies (L.) H. Karst), northern 66°N (Rana, provenance P1, seed lot number 4145), intermediate 64°N (Snåsa, provenance L1, seed lot 5611) and southern 59°N (Halden, provenance CØ1, seed lot 98063, all from Skogfrøverket, Hamar, Norway). The seedlings were germinated in moist fertilized peat containing 20% perlite, and grown for 6 weeks prior to the LED treatments in climatically regulated dark chambers under 24 h photoperiod of 40 W m−2 PAR (400–750 nm, Phillips TLD 58 W/840 fluorescent tubes; Eindhoven, The Netherlands and incandescent Osram 75 W; Munich, Germany). During the experiments with LED treatments all seedlings got 12 h PAR (30–35 W m−2, Phillips TLD 15 W/840) and photoperiod was extended to 24 h using LED light (Quantum Devices, Barneveld, WI, USA) either during the 12 h day extension period only or continuously (24 h). The spectral photon distributions of the light sources are shown in Fig. 1. Light treatments with LED light were given to the seedlings for 4–6 weeks. The LEDs were mounted within enclosed trolley systems, with reflective covers to maximize irradiation within the system and keeping external light out. The enclosed trolley systems were kept during the experimental periods in climate-controlled dark chambers. The temperatures were constant at 18 °C (± 0.5 °C) and humidity was adjusted to give 0.5 kPa water vapour pressure deficit. The seedlings were watered daily with a complete nutrient solution (Junttila 1976).

image

Figure 1. Wavelength ranges of monochromatic light emitting diodes (LED) of far-red (FR), red (R) and blue light (B), and fluorescent light (Phillips TL-D 840).

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Five seedlings were grown per pot, with up to eight pots per provenance and LED treatment. Pots from different provenances were randomized within LED treatments. The result from each pot was used as the experimental unit for percentage bud set, average growth. The growth was measured every second week and bud set was registered weekly, and number of flushing buds was also registered. At the end of the experiment the number of axillary branches with or without bud set was registered. All the data were analysed using the GLM procedure in SAS (Release 8.02 (1999–2001), SAS Institute Inc. Cary, NC, USA), regarding light treatments and provenances as fixed effects and residuals as the random experimental error.

RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Northern populations require higher irradiance of FR and R light than southern populations to prevent bud set

To examine the light energy requirements for maintaining growth, different latitudinal populations were grown in a 24 h photoperiod, with a 12 h day extension of LED R or FR light of different irradiances. The southern population (59°N) and the intermediate population (64°N) were able to maintain growth in 1.7 W m−2 of FR, whereas the northern population (66°N) showed 43% bud set after 4 weeks at this irradiance (Table 1a). In all populations, R light was less effective at maintaining growth than FR at 1.7 W m−2 (Table 1b). The southern population was most sensitive to R light and had incomplete bud set in irradiance as low as 0.1 W m−2 for R light at night (Table 1b).

Table 1.  Average percent bud set in latitudinal populations of Norway spruce after 4 weeks in a 24 h photoperiod of different irradiances of monochromatic far-red (FR) or red (R) light extension at night
LED/populationIrradiance (W m−2)
00.10.21.7
  1. The main light period consisted of 12 h PAR fluorescent light with a 12 h day extension with monochromatic light. n = 4 pots with five seedlings per pot.

(a) FR-light
  Halden (59°N)100100100  0
  Snåsa (64°N)100100100  0
  Rana (66.5°N)100100100 43
(b) R-light
  Halden (59°N)100 60 37 53
  Snåsa (64°N)100100100 85
  Rana (66.5°N)100100100100

Effects of FR and R light in maintaining growth

To further dissect the roles of FR and R in maintaining growth in Norway spruce, seedlings were grown in a 24 h photoperiod with monochromatic R, FR or a 1 : 1 mixture of R : FR. These treatments were given both as day extension and during the 12 h PAR light period. There was a clear clinal response to R light in the three populations, ranging from complete bud set in the northern population to 77% bud set in the intermediate and 38% bud set in the southern population after 6 weeks (Fig. 2). The seedlings of all populations grown in FR light had a low percentage of bud set after a treatment for 6 weeks: 7, 1 and 3% in the northern, intermediate and southern population, respectively (Fig. 2). When FR was given in combination with R light in equal irradiance, there was no bud set in any of the latitudinal populations (Fig. 2), indicating a possible additive effect of R and FR light. To eliminate the possibility that a higher irradiance sum of R and FR was responsible for maintaining growth, this was further investigated using lower irradiance of LED light extensions and adjusting the total irradiance of combined R : FR to the same level as in the R and FR treatments. Percentage bud set was higher in this experiment (Fig. 3) than in the previous (Fig. 2), and a clinal response to R light was observed with 93, 42 and 5% bud set after 3 weeks in the northern, intermediate and southern populations, respectively (Fig. 3). A clinal response was also apparent in FR, with 20% bud set in the northern population after 4 weeks, 2% bud set in the intermediate population after 5 weeks, and no bud set in the southern population during 6 weeks treatment (Fig. 3). In contrast to the treatments with either R or FR as day extensions, combined R : FR (1 : 1) did not produce bud set in any of the populations during the experiment.

image

Figure 2. Terminal bud set of different latitudinal provenances of Norway spruce as affected by 24 h photoperiod with day extensions with light emitting diodes of different light qualities: red (◊), far-red (▵) (3.3 W m−2) or a mixture of monochromatic R and FR (R : FR 1 : 1; ●) of total irradiance 6.6 W m−2. The PAR light period was 12 h. No day extension (▪). n= 8 pots with five seedlings per pot.

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image

Figure 3. Terminal bud set of different latitudinal provenances of Norway spruce grown in 24 h photoperiod with day extensions with light emitting diodes of different light qualities at irradiance 1.8 W m−2: red (◊), far-red (▵) or a mixture of monochromatic R and FR (R : FR 1 : 1; ●). The PAR light period was 12 h. No day extension (▪). n = 8 pots with five seedlings per pot.

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FR enhances elongation growth in Norway spruce

FR light stimulates stem elongation in woody angiosperms (e.g. Olsen et al. 1997; Olsen & Junttila 2002). To examine this in a gymnosperm species, growth was compared between seedlings of the three latitudinal populations grown in different combinations of R : FR (Table 2). The R : FR ratios used (1, 2.2 or 3.7) did not produce any bud set within 6 weeks of treatment for the southern and intermediate population, and 2.5% bud set in the northern population after 5 weeks at the highest R : FR ratio 3.7. The growth of all three populations showed a similar non-significant trend of increasing stem elongation with increasing FR content (data not shown). When the data for all three populations were pooled, stem elongation showed a significant increase with increasing FR content (Table 2).

Table 2.  Average elongation growth of Norway spruce seedlings after 2 weeks in different combinations of 24 h monochromatic red (R) and far-red (FR) light
R/FR ratioIrradiance (W m−2) Growth (mm)
RFR
  1. Treatments were given both as day extension and during the 12 h PAR light period (30 W m−2). n = 24 pots with five seedlings per pot. All treatment means were significantly different from each other (P = 0.012).

11.11.111.9
2.21.10.510.4
3.71.10.3 9.1

R and FR light stimulates bud break in a 24 h photoperiod

Juvenile gymnosperms in photoperiods close to the critical photoperiod, display an intermediate growth pattern resembling both free and determinate growth (Downs & Borthwick 1956). In this growth pattern, a cycling between terminal bud formation and bud break followed by elongation is observed. This was observed for some of the treatments in our experiments, and bud break of the newly formed terminal buds was recorded (Table 3). Treatments with R, FR and R : FR as day extension, induced bud break differently in the latitudinal populations. FR light was most effective causing 100% bud break in the intermediate population, whereas the northern population showed less bud break in these treatments. The southern and intermediate population broke buds in R light, whereas the northern population did not respond to R light with bud break (Table 3).

Table 3.  Percentage bud break of buds induced in 6 weeks of 24 h photoperiod containing different light qualities of: red (R), far-red (FR), blue (B) and mixtures of red and far-red (R : FR) LED light as day extensions
 Halden (59°N)Snåsa (64°N)Rana (66.5°N)
  1. The main photosynthetic light period consisted of 12 h with fluorescent light. The photoperiod of short day (SD) was 12 h. The seedlings were from three Norwegian latitudinal populations of Norway spruce. The number of seedlings with terminal buds is indicated in brackets, and the lines indicate treatments without any bud set. n = 8 pots with five seedlings per pot.

R : FR 1 (Fig. 2)
R : FR 1 (Fig. 3)
R : FR 1 (Table 2)
R : FR 2.5 (Table 2)
R : FR 5 (Table 2) 0 (1)
FR (Fig. 2)0 (3)  0 (1)33 (3)
FR (Fig. 3)100 (1)81 (16)
R (Fig. 2)7 (14) 18 (27) 0 (40)
R (Fig. 3)3 (34)  0 (40) 0 (40)
B (Fig. 4)0 (40)  0 (40) 0 (40)
SD (Fig. 2)0 (40)  0 (40) 0 (40)

Day extension with blue light delays bud set

To investigate the effect of B light on bud set in Norway spruce, seedlings were grown in a 24 h photoperiod with monochromatic B light. B light for 24 h combined with 12 h photosynthetic active radiation was not able to prevent complete bud set in any of the latitudinal populations. The rate of bud set in B light was slightly slower in comparison with SD in all populations, but bud set was complete in all seedlings of the northern and intermediate population after 21 d, and 1 week later in the southern population (Fig. 4).

image

Figure 4. Terminal bud set of different latitudinal provenances of Norway spruce grown in a 24 h photoperiod with monochromatic blue light (B) (1.5 W m−2) or a 12 h photoperiod (D). Both treatments received 12 h photosynthetic active radiation (30 W  m−2). n = 2 experiments with eight pots per treatment containing five seedlings per pot.

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DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Using LEDs, the activities of defined wavelengths of R, FR and B light in maintaining growth were documented in the gymnosperm tree species, Norway spruce. We found an increasing clinal requirement for both R and FR light with increasing latitude of origin, and FR light as day extension was more effective in maintaining growth than R in all three populations (Table 1, Figs 2 & 3). This is in line with results from previous studies of responses to different light qualities for downy birch, bay willow and Norway spruce, documenting a clinal increase in requirement for FR light with increasing latitude of origin (Håbjørg 1972a; Junttila & Kaurin 1985; Clapham et al. 1998a). A clinal response to defined R light has not been documented previously for trees, although northern populations of Norway spruce and downy birch have been shown to require higher light intensities of photosynthetic active radiation (deficient in FR) to maintain growth than southern ones (Håbjørg 1972a; Johnsen & Skrøppa 2000). The ecological significance of clinal variations in sensitivity to FR and R light in perennial species such as Norway spruce, could be related to precise timing of developmental processes in places with short growing seasons in which day-length/night-length may not be the best indicator of seasonal time. Requirements for higher irradiances by populations originating at higher latitudes will result in earlier growth cessation and bud formation in preparation for winter hardiness and dormancy. Populations growing near or above latitudes of the arctic circle (66°33′38′′N) need to stop growth and set buds in photoperiods without a distinct dark period, and monitoring the light intensity at defined wavelengths provides an alternative to the dark-timekeeping of photoperiodism. Both systems of photo-regulation of seasonal processes must therefore exist in woody species with a wide latitudinal distribution as exemplified by Norway spruce.

In addition to the clinal increase in requirement for FR and R light, we found that, at least for the northernmost population, R : FR light at 1 : 1 ratio was more effective as day extension maintaining growth than either R or FR light alone (Figs 2 & 3). This indicates that there is an interaction between R and FR light that maintains growth in Norway spruce. However, the evidence for this can be argued to be weak in the first experiment with small differences between FR and R:FR (1 : 1) (Fig. 2), making it difficult to rule out inherent variability of the system or experimental errors. However, when repeated at lower irradiances (and R : FR at equal irradiance sum), bud set was significant at 20% after 4 weeks in FR for the northern population compared with no bud set in R : FR (1 : 1) (Fig. 3). The observed later reversion of bud set by bud break is a characteristic of photoperiodism in gymnosperms close to the critical parameters for induction (Downs & Borthwick 1956). The fact that not all northern buds reverted to growth in FR (Fig. 3), indicates a stable induction at FR for some of the seedlings. It is therefore possible that both R and FR are needed to maintain growth at low irradiances close to the critical.

The activity of both R and FR light in maintaining growth may indicate that different phytochromes, or alternatively different photoisomers of the same phytochrome type, regulate growth in Norway spruce. Responses to continuous R have been linked to phyB function, and responses to continuous FR have been linked to phyA function in Arabidopsis (Whitelam & Devlin 1997). It is possible that activity of both R and FR light in Norway spruce (Figs 2 & 3) involves distinct phytochrome types similar to phyA and phyB in angiosperms. The wavelength peak of our R and FR light (Fig. 1) corresponds to the absorption maxima of the phytochrome photoisomer Pr and Pfr, respectively (Borthwick et al. 1952), and it is conceivable that different photoisomers of the same type could be involved in regulating growth. Gymnosperm phytochromes are not well characterized, but we have data from Norway spruce documenting the presence of a light-labile phytochrome similar to phyA (unpublished results). It is also possible that a type I or a phyA-like receptor in Norway spruce could be a single sensor for R and FR in HIR responses maintaining growth in 24 h photoperiods.

We found that stem growth was stimulated significantly in Norway spruce by increasingly lower R : FR ratios (Table 2). Low R : FR ratio is an indication of shading by canopy of other plants, and stimulation of elongation growth in low R : FR ratios is thought to be of adaptive advantage. However, Norway spruce is a late successional species with relatively slow seedling growth, and the selective advantage of shade-avoidance in late successional species may be limited (Gilbert, Jarvis & Smith 2001). We were unable to detect any significant difference between the populations in growth after 2 weeks of R : FR treatments (Table 2). A study of R/FR control of growth in latitudinal populations of silver birch (Betula pendula Roth.) of similar origin as in this study also showed a non-significant difference in internode elongation between the populations, although low R : FR ratios stimulated elongation (unpublished results). In Scots pine an increasing requirement for FR in secondary needle elongation has been documented (Clapham et al. 2002). Other examples of stem elongation mediated by FR light in trees are end-of-day FR responses, shown in Populus (Olsen et al. 1997; Olsen & Junttila 2002). End-of-day FR responses have been shown to mediated by light stable phytochrome types phyB, phyD and phyE in Arabidopsis (Franklin et al. 2003).

Previous studies of Norway spruce have shown that buds induced under 24 h photoperiods rich in FR light may break again shortly after induction under the same light conditions (Clapham et al. 1998a). We also observed this response in LED treatments of R, FR or R : FR light, where some or all of the induced terminal buds broke shortly after bud set (Table 3). The incidence of bud break varied for the populations under the different light treatments. Furthermore, the treatments not giving complete bud set in all seedlings of a particular population were the ones with highest occurrence of bud break. Accordingly, the populations exposed to light conditions close to the critical for growth, were the ones most likely to show bud break. This is similar to the situation in seedlings of another gymnosperm, Scots pine (Downs & Borthwick 1956). Such responses have not been documented for bud set in angiosperm tree species, suggesting that regulation of bud set and dormancy development differs between angiosperms and gymnosperms. Both FR and R light resulted in bud break, again indicating the importance of both R and FR controlling growth in Norway spruce.

The use of B light as day extension did not maintain growth in 24 h photoperiods in our experiments (Fig. 4). Broad spectrum blue light as day extension together with R : FR have previously been shown to promote bud set in bay willow (Junttila & Kaurin 1985). In contrast, we observed a significant delay in bud set with B light in all latitudinal populations, but still a complete bud set (Fig. 4). This delay could be due to detection of B light by either cryptochromes or phytochromes. Phytochrome A is known to detect and confer response to wavelengths in the blue region, although its activity is much higher in the R and FR region (Shinomura et al. 2000). B light detected by cryptochrome may also play an indirect role in maintaining growth in trees through regulation of photoperiodism. Blue light has been linked with the function of cryptochrome in setting endogenous rhythms controlling photoperiodism in Arabidopsis (Devlin & Kay 2000). Anyhow, the small activity of B light in maintaining growth in our results, suggests that the effect of prolonged day extension in maintaining growth, is primarily mediated by phytochromes detecting R and FR light.

We conclude from our results that FR was more effective than R in maintaining growth in different latitudinal populations of Norway spruce. Northern populations required higher irradiance of both FR and R light than southern populations, and the requirements increased in a clinal mode with increasing northern latitude of origin. Mixture of R and FR light appeared more effective in maintaining growth than either of the light qualities alone. Activity of both R and FR may indicate involvement of multiple phytochrome types or different isomers of one phytochrome in maintaining growth in Norway spruce seedlings. Blue light alone as day extension was unable to prevent growth cessation and bud set in Norway spruce.

ACKNOWLEDGMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Thanks are due to Jarle Nilsen, Leidulf Lund and Kjell Nilsen for advice and technical assistance at the phytotrone at the University of Tromsø. This work was financed by the Norwegian Research Council (Project 130159/130).

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES
  • Ahmad M., Grancher N., Heil M., Black R.C., Giovani B., Galland P. & Lardemer D. (2002) Action spectrum for cryptochrome-dependent hypocotyl growth inhibition in Arabidopsis. Plant Physiology 129, 774785.
  • Borthwick H.A., Hendricks S.B., Parker M.W., Toole E.H. & Toole V.K. (1952) A reversible photoreaction controlling seed germination. Proceedings of the National Academy of Sciences, USA 38, 662666.
  • Burgin M.J., Casal J.J., Whitelam G.C. & Sánchez R.A. (1999) A light-regulated pool of phytochrome and rudimentary high-irradiance responses under far-red light in Pinus elliottii and Pseudotsuga menziesii. Journal of Experimental Botany 50, 831836.
  • Clapham D., Dormling I., Ekberg I., Eriksson G., Qamaruddin M. & Vince-Prue D. (1998a) Latitudinal cline of requirement for far-red light for the photoperiodic control of bud set and extension growth in Picea abies (Norway spruce). Physiologia Plantarum 102, 7178.
  • Clapham D., Ekberg I., Dormling I., Eriksson G., Qamaruddin M. & Vince-Prue D. (1998b) Dormancy: night timekeeping and day timekeeping for the photoperiodic control of bud set in Norway spruce. In Biological Rhythms and Photoperiodism in Plants (eds P.J.Lumsden & A.J.Millar), pp. 195209. Bios Scientific Publishers Ltd, Oxford, UK.
  • Clapham D., Ekberg I., Eriksson G., Norell L. & Vince-Prue D. (2002) Requirement for far-red light to maintain secondary needle extension growth in northern but not southern populations of Pinus sylvestris (Scots pine). Physiologia Plantarum 114, 207212.
  • Devlin P.F. & Kay S.A. (2000) Cryptochromes are required for phytochrome signalling to the circadian clock but not for rhythmicity. Plant Cell 12, 24992509.
  • Downs R.J. & Borthwick H.A. (1956) Effects of photoperiod on growth of trees. Botanical Gazette 117, 310326.
  • Eriksson M.E. (2000) The role of phytochrome A and gibberellins in growth under long and short day conditions. Studies in hybrid aspen. Acta Universitatis Agriculturae Sueciae, Silvestria 164, 155.
  • Fernbach E. & Mohr H. (1990) Coaction of blue/ultraviolet-A light and light absorbed by phytochrome in controlling growth of pine (Pinus sylvestris L.) seedlings. Planta 180, 212216.
  • Franklin K.A., Praekelt U., Stoddart W.M., Billingham O.E., Halliday K.J. & Whitelam G.C. (2003) Phytochromes B, D, and E act redundantly to control multiple physiological responses in Arabidopsis. Plant Physiology 131, 13401346.
  • Frewen B.E., Chen T.H.H., Howe G.T., Davis J., Rohde A., Boerjan W. & Bradshaw H.D.J. (2000) Quantitative trait loci and candidate gene mapping of bud set and bud flush in Populus. Genetics 154, 837845.
  • Gilbert I.E., Jarvis P.G. & Smith H. (2001) Proximity signal and shade avoidance differences between early and late successional trees. Nature 411, 792795.
  • Håbjørg A. (1972a) Effects of light quality, light intensity and night temperature on growth and development of three latitudinal populations of Betula pubescens Ehrh. Meldinger Fra Norges Landbrukshøgskole 51, 117.
  • Håbjørg A. (1972b) Effects of photoperiod and temperature on growth and development of three latitudinal and three altitudinal populations of Betula pubescens Ehrh. Meldinger Fra Norges Landbrukshøgskole 51, 127.
  • Howe G.T., Gardner G., Hackett W.P. & Furnier G.R. (1996) Phytochrome control of short-day-induced bud set in black cottonwood. Physiologia Plantarum 97, 95103.
  • Johnsen Ø. & Skrøppa T. (2000) Provenances and families show different patterns of relationship between bud set and frost hardiness in Picea abies. Canadian Journal of Forest Research 30, 18581866.
  • Junttila O. (1976) Apical growth cessation and shoot tip abscission in Salix. Physiologia Plantarum 38, 278286.
  • Junttila O. & Kaurin Å. (1985) Climatic control of apical growth cessation in latitudinal ecotypes of Salix pentandra L. In Plant Production in the North (eds Å.Kaurin, O.Junttila & J.Nilsen), pp. 8391. Norwegian University Press, Tromsø.
  • Junttila O. & Kaurin Å. (1990) Environmental control of cold acclimation in Salix pentandra. Scandinavian Journal of Forest Research 5, 195204.
  • Nilsen J. (1985) Light climate in northern areas. In Plant Production in the North (eds Å.Kaurin, O.Junttila & J.Nilsen), pp. 6272. Norwegian University Press, Tromsø.
  • Nitsch J.P. (1957) Photoperiodism in woody plants. Proceedings of the American Society of Horticultural Sciences 70, 526544.
  • Olsen J.E. & Junttila O. (2002) Far red end-of-day treatment restores wild type-like plant length in hybrid aspen overexpressing phytochrome A. Physiologia Plantarum 115, 448457.
  • Olsen J.E., Junttila O., Nilsen J., Eriksson M.E., Martinussen I., Olsson O., Sandberg G. & Moritz T. (1997) Ectopic expression of oat phytochrome A in hybrid aspen changes critical daylength for growth and prevents cold acclimatization. The Plant Journal 12, 13391350.
  • Quail P.H. (2002) Photosensory perception and signalling in plant cells: new paradigms? Current Opinion in Plant Biology 14, 180188.
  • Shinomura T., Uchida K. & Furuya M. (2000) Elementary Process of photoperception by phytochrome A for high-irradiance response of hypocotyl elongation in Arabidopsis. Plant Physiology 122, 147156.
  • Thomas B. & Vince-Prue D. (1997) Photoperiodism in Plants (eds B. Thomas & D. Vince-Prue). Academic Press, London, UK.
  • Vince-Prue D. (1984) Contrasting types of photoperiodic response in the control of dormancy. Plant, Cell and Environment 7, 507513.
  • Weiser C.J. (1970) Cold resistance and injury in woody plants. Science 169, 12691278.
  • Whitelam G.C. & Devlin P.F. (1997) Roles of different phytochromes in Arabidopsis photomorphogenesis. Plant, Cell and Environment 20, 752758.