Effects of temperature on the light quality effect on bud set

None of the seedlings exposed to B and FR treatments developed buds under 18°C in the second experiment, which reflects the result from the first experiment (fig.3).

However, in 22°C about 5% of the B-treated seedlings developed buds. The first bud appeared after 9 days in this treatment. 16% of the FR-22°C–treated seedlings developed buds, where the first bud appeared after 12 days in treatment. Further analysis of bud development using cumulative link model in R and bud burst from these treatments were not conducted in the second experiment because of the low percentage of seedlings in buds.

In 18°C under SD treatment, all seedlings had developed buds after 9 days (fig. 9). All SD-treated seedlings at 22°C also developed buds, with the first bud appearance already at day 3. This indicated that this seedling had already started bud development under pre-growth. Totally 95% of the 18°C -R-treated seedlings

developed buds after 27 days, while 94% of the 22°C –R-treated seedlings developed buds after 27 days in treatment. The first bud appearance was observed after 12 days in both cases (fig.9).

Fig. 9 Percentage buds in seedlings of Norway spruce exposed to day extension with different light quality treatments from light emitting diodes, i.e. blue (B), far-red (FR) and red (R) (7 W m-²) as compared to short days (SD) without day-extension, at a temperature of 18 or 22°C in the second growth experiment. Results are mean of 19 plants in each treatment.

The average stage of the buds in SD-18°C-treated seedlings resembled the average stage found in SD-22°C-treated seedlings, which after 51 days in treatment had developed brown buds referred as 0. The same trend were found in the R-exposed treatments, although not all of the seedlings had fully developed, brown buds after 51 days (fig. 10). Thus, there was no significant effect of temperature on R-treated seedlings (0.03541) (fig. 10, tab. 6). However, in response to the R light treatment the bud development was significantly delayed compared to SD (tab. 6). Whereas after 27 days of treatment all SD-exposed plants had brown buds (stage 0), while the

R-exposed plants had green buds (0.5).

0  

Fig. 10 Stages of bud set in seedlings of Norway spruce exposed to day extension with different light qualities from light emitting diodes, i.e. red (R) (7 W m-²) as compared to short days (SD) without day extension at a temperature of 18 or 22°C in the second growth experiment. 1 = growth, 0.5 = green bud, 0 = brown bud. Results are mean ± SE of 19 plants in each treatment.

Tab.  6  Results from cumulative link model in R run to investigate the effects of temperature (T; 18 and 22°C) and red (R) light (7 W m-²) as day extension on bud development in seedlings of Norway spruce compared to short days (SD) without  day  extension in the second experiment. Positive estimated coefficient indicates an increase probability of bud set, while negative estimated coefficient indicates a delay in bud set  

Treatment Coefficient SE Z

R * -4.00942 1.73062 -2.317

T -0.06242 0.05921 -1.054

Day *** 0.42853 0.02008 21.341

R x T 0.03541 0.08626 0.410

Significance levels: * P<0.05, ** P<0.01, *** P<0,001

There was no significant effect of temperature on bud development in seedlings exposed to day extension with B- and FR light in the third experiment (p≥0.05). A total of 3% developed bud in FR-18°C-treated seedlings, whereas 6% developed bud in B-18°C-treated seedlings (fig. 11). The similar bud percentage was seen at 24°C in both FR and B treatments. All the buds developed in the FR treatments flushes, and no buds are observed after 57 days in treatment (fig. 11).

The first bud appearance in seedlings exposed to SD at 18°C occurred after 15 days of treatment, and all of the seedlings had developed buds after 24 days. The

0   0,5   1  

0   3   6   9   12   15   18   21   24   27   30   33   36   39   42   45   48   51  

Stages  

Time  (day)  

SD  (18°C)   SD  (22°C)   R  (18°C)   R  (22°C)  

same was registered for R-18°C-seedlings. SD-24°C-seedlings developed buds after 15 days, and a total of 92% buds were developed in this treatment. After 18 days the first buds appeared in 24°C-R-treated seedlings. At this temperature, not all of these seedlings developed buds, and after 36 days a total of 50% of the seedlings had developed buds and no further bud set was thereafter observed (fig.11).

Fig. 11 Percentage buds in seedlings of Norway spruce exposed to day extension with different light quality treatments from light emitting diodes, i.e. blue (B), far-red (FR) and red (R) (7 W m-²) as compared to short days (SD) without day-extension, at a temperature of 18 or 24°C in the third growth experiment. Results are mean of 36 plants in each treatment.

0  

Although there were no significant effect of temperature on bud development in seedlings exposed to FR and B light, both FR and B –light as day extension light strongly reduced the probability for bud development compared to the SD –treatment (p≤0.001, p≤0.05 respectively, tab. 7) with very few plants only showing bud set in B and FR (fig. 12).

The average stage of buds after 57 days in SD-24°C-treatment was slightly different from SD-18°C-treated seedlings in that some seedlings did not develop fully buds in SD-24°C-treatment (fig.12). The difference between the temperatures was far greater for the R than the SD treatment with brown buds in almost all plants (stage 1).

After 57 days, R-18°C-treated plants had developed more buds (stage 0.05) compared R-treated seedlings at 24°C (stage 0.58). An average higher than 0 indicates that not all seedlings had fully developed buds after 57 days (fig. 12). Furthermore, there was a significant effect of temperature on bud development under R light (p≤0.001) (fig.

12, tab. 7), i.e. R light combined with elevated temperature delayed bud development.

Fig. 12 Stages of bud set in seedlings of Norway spruce exposed to day extension with a light quality from light emitting diodes, i.e. red (R) (7 W m-²) as compared to short days (SD) without day extension at a temperature of 18 and 24°C in the third growth experiment. 1 = growth, 0.5 = green bud, 0 = brown bud. Results are mean ± SE of 36 plants in each treatment.

0   0,5   1  

0   3   6   9   12   15   18   21   24   27   30   33   36   39   42   45   48   51   54   57  

Stages  

Time  (days)   SD  (18°C)  

SD  (24°C)   B  (18°C)   B  (24°C)   FR  (18°C)   FR  (24°C)   R  (18°C)   R  (24°C)  

Tab. 7 Results from cumulative link model in R run to investigate the effects of temperature (T; 18 and 24°C) and blue (B), far-red (FR) and red (R) (7 W m-²) as day extension on bud development in seedlings of Norway spruce compared to short days (SD) without day extension in the third experiment.

Positive estimated coefficient indicates an increase probability of bud set, while negative estimated coefficient indicates a delay in bud set

Treatment Coefficient SE Z

Significance levels: * P<0.05, ** P<0.01, *** P<0,001