ISHS Acta Horticulturae 218:
XII African Symposium on Horticultural Crops, Nairobi, Kenya, 1988 – p. 121-130+

GROWTH AND FLOWERING IN ROSES

Roar Moe

Abstract

The effects of temperature and light quality on apical dominance (bud sprouting) and flowering in some greenhouse rose cultivars were studied.

Increasing temperature hastened flowering and the number of flowers in each flush was increased. At 18°C, 3–3.5 times more flowers per plant were harvested, compared with a constant temperature of 12°C, due to lower blind shoot formation at high temperature. With fluctuating day and night temperatures, an average temperature of 18°C produced approximately the same yield as a constant temperature of 18°C. Low temperatures released apical dominance and stimulated lateral shoot branching.

Day extension lighting with incandescent lamps (low R:FR ratio) inhibited lateral bud growth and induced blind shoot formation, while lighting with fluorescent lamps (high R:FR ratio) had the opposite effect. The use of red (R) and far-red (FR) lighting showed that FR inhibition of bud growth and flowering could be overcome by subsequent R lighting. These findings indicate that blind shoot formation is controlled by the phytochrome. The role of temperature and light quality on the assimilate supply to the buds and young shoots for optimum bud sprouting and flowering will be discussed.

1. Introduction

The yield of cut roses is influenced by three main factors: number of lateral shoots per plant, percentage of flowering shoots and cultivar.

Differences in the ability of different buds to sprout and grow can be related to the bud position on the branch (Zieslin et al., 1976), light (Wilkins, 1982), temperature (Moe, 1974; Zieslin and Moe, 1985) and nutrient factors such as assimilate supply to the bud (Mor and Halevy, 1984).

The environmental control of apical dominance in nature appears to be influenced by temperature, light intensity and the balance of red and far-red wavelengths (Tucker, 1977, Hagen and Moe, 1981). Reduction of light intensity and irradiation with incandescent lamps or metal halide lamps (HPI/T) or filtering of sunlight through the canopy, inhibited bud sprouting in poinsettia (Hagen and Moe, 1981), chrysanthemum (Hems and Wilkins, 1979, Moe, 1985) and some other plants (Wilkins, 1982). This indicates that increased far-red level of light inhibits lateral bud growth in both short day plants (SDP) (for example chrysanthemum and poinsettia) and long day plants (LDP) (for example carnation) and day neutral plants (NDP) (for example tomatoes). Treatments which could improve bud sprouting and growth might increase the yield of roses since all rose shoots nave the potential to flower. However, the shoots often fail to flower, due to flower abortion (Moe, 1971). Low temperature or low light intensity during the early stages of shoot growth, before the differentiation of floral parts has been completed, results in flower abortion (blind shoots). This partly explains why rose plants produce a high percentage of blind shoots during the winter months in Norway when poor light condition (short days and low light intensity) exist. Lower temperatures have been observed to cause more blind shoots to develop. The seasonal variation in blind shoots might also be partly related to variation in the quality of the natural light. In winter time in Norway, a lower red:far-red ratio of the sunlight is present. Therefore a change in the ratio of red and far-red light might be important for both bud sprouting and the production of flowering shoots. It is also questionable whether the night temperature can be lowered without any harmful effect on the yield and quality of the roses, even when the day temperature is increased to give a mean daily temperature of about 18°C. In order to answer these questions experiments were carried out to compare the effects of different day/night temperatures and constant temperatures on growth rate, yield and quality of cut roses. The influence of light quality in the form of supplementary lighting on bud sprouting and flower abortion was also studied.

2. Experiments and results

Rose plants propagated by stem cuttings were raised in a greenhouse with a temperature maintained at 18°C. One day before the initiation of the various treatments, the plants were cut back above 5 leaves of single stem plants (Exp. 2, 3 and 14) or above dormant buds of the main branches (Exp. 1). Experiments were carried out in a phytotron with natural daylight or in growth rooms with good temperature control (± 0.5°C from the desired temperature). The plants were either grown in a medium which consisted of 3 parts of perlite and 1 part of sphagnum peat (Exp. 1) or in limed and fertilized peat (Floralux, Exp. 2, 3, and 14). All plants were regularly watered with a complete nutrient solution.

Exp. 1. Effect of temperature on yield and flower quality

One-year old dormant plants of 'Sonia', 'Belinda' and 'Red Garnette', over-wintered in a greenhouse maintained at a temperature of, 2-3°C for 2.5 months, were placed in the phytotron on February 12 at constant 12, 15, 18 and 21°C or at 21/15°C or 214/12°C day/night temperatures. The day temperature was maintained between 07.00 to 19.00 (12 h) and the night temperature from 19.00 to 07.00. Each treatment comprised two replications of 8 plants in each replicate. Days from cut-back to flowering (50% flowering) and number of cut roses per plant before termination of the experiment on October 12 were recorded.

When the temperature was increased from a constant 12 to 18°C, the time from cut back to first flush was reduced by 50% or about 50 days. A constant temperature of 18°C or an average temperature of 18°C by , day/night fluctuations (21/15 or 24/12°C) resulted in same rate of flower development in all 3 cultivars. The cvs. 'Sonia' , 'Belinda' flowered earlier than 'Red Garnette' at all temperature levels. The yield of cut roses increased dramatically with increased growing temperatures. At a constant 18°C the yield was 2.5-3.0 times higher, com- pared with a constant temperature of 12°C. A 21/15°C or 24/12°C day/night temperature resulted in a yield similar to that of the constant 18°C treatment in 'Red Garnette' and 'Belinda', while day/night temperature fluctuation was more favourable than the constant temperature treatment  in 'Sonia'.

Figure 1 shows the production of cut roses in 'Sonia' during a owing period of 8 months. At 18°C, almost 5 flushes of flowers were harvested, while only 2 were produced at the 12°C treatment. Each flush as also considerably greater at high than at low temperatures. Larger flushes were produced during the summer months than in early spring.

The growing temperature has a positive influence on both yield and cut rose quality (Fig. 2). At temperatures below 15°C, a large number of malformed flowers and blind shoots are formed, and some rose cultivars produced poor flower colours ("blackening" or "greening" of petals). Too high temperatures (21°C or above) resulted in shorter and weaker flower stems, long upper internodes ("neck") and small flower size and pale flower colour. Under prevailing light conditions in Scandinavia, the optimum temperature is about 18°C. It might be increased by 3-4°C during the day when CO 2 is applied or in summer time with higher solar irradiation.

Exp. 2. Effect of temperature and shoot orientation on apical dominance

Approximately 3-month old single stem plants of 'Red Garnette', raised in a greenhouse at 18°C, were placed in the phytotron under natural daylight at 9°C on March 6. After 5 or 8 weeks of cooling, the plants were moved back to the 18°C regime. The control plants were kept continuously at 18°C. The shoots of one half of the plants were bent down, while the other plants were allowed to grow normally. Bud sprouting and shoot growth from the basal parts of the main shoot of the plants and the number of flowering shoots per plant in the first flush, were recorded.

Cooling at 9°C stimulated bud sprouting and the growth of shoots from lateral branches of the plants (Table 2). A significantly higher total number of lateral shoots and flowering shoots per plant was obtained when the main shoot was bent down. A combination of bending down and cooling resulted in the best lateral shoot branching and flower production Lateral shoot branching and flowering was stimulated more by 8 than by 5 weeks of cooling.

The average length of the lateral shoots was about 29 cm in cooled or bent down plants, while the shoots of plants which were cut back above 5 leaves were about 5 cm shorter in the first flush (data not shown).

The growing temperature has also a significant effect on the formation of the lateral flowering shoots. Rose plants grown at low temperatures formed shoots with many terminal flowers compared with those grown at higher temperatures (Fig. 3). The hybrid tea cultivar 'Baccara' produced one or few terminal flowers while the Floribunda cultivar 'Fire King' produced a cluster of many flowers, especially at low temperatures.

Exp. 3. Effect of lamp types as day extension lighting

Approximate 3-month old plants of 'Red Garnette' were cut back above 5 leaves and placed in growth rooms at different temperatures (15, 18, 21, and 24°C) in combination with short days (8 h) or long days (24 h) with extended day lighting for 16 hours. Fluorescent lamps (Philips TL33) or incandescent lamps were used as extended day lighting. All plants were illuminated for 8 h per day with fluorescent light (Philips TL33) with an irradiation of 29 W/m2 (400-700 ) at plant level.

The main effects of temperature and lamp types are given in Table 3. The highest number of bud breaks and lowest percentage of blind shoots were produced at 18°C. Increasing temperature inhibited bud growth. Low temperatures (15°C) caused the highest number of blind shoots.

Short days (8 h) stimulated bud sprouting but resulted in flower abortion. Extended day lighting with incandescent lamps strongly inhibited bud sprouting, and did not reduce the percentage of blind shoots compared with the SD treatment. Fluorescent lighting, however, resulted in good bud sprouting and flowering (a very low percentage of blind shoots was produced).

Exp. 4. Effect of red (R) and far-red (FR) lighting

The same plant type and cut back procedures were used as in Exp. 3.  After a 8 or 9 hour daily irradiation of 29 W/m2 (400-700 nm) at plant level with fluorescent lamps (Philips TL33), the plants were given weak lighting with red (R) or far-red (FR) light (Fig. 4). R or FR lighting was applied as a 15 h day extension, as a 3 h night interruption or as 3 h periods of R and FR lighting throughout the night period.

Table 4 shows that short days (9 h) or FR lighting during the night stimulated flower abortion, while R lighting resulted in a high percentage of flowering shoots. Night interruption did not prevent flower abortion. The FR stimulation of flower abortion could be reversed by R light.

The number of sprouts per plant was about 30% lower with FR than with R lighting (Table 5). R lighting resulted in the production of a high percentage of flower shoots while FR had the opposite effect. FR promoted blind shoot formation in all bud positions on the main shoot, while R lighting showed no blind shoot production in the upper position (position 1) and very little in bud position 2.

3. Discussion

When the growing temperature was increased from 12 to 18°C the growth rate and flowering was improved by about 50%.  Thus, rose plants grown for a period of 8 months produced about 2 flushes of flowers at 12°C and almost 5 flushes at 18°C (Fig. 1). The rate of flower development is controlled by the mean daily temperature. A higher day temperature can compensate for a slower growth rate when the night temperature is reduced (Table 1). These findings seem to be the general pattern for: all rose cultivars (Moe and Kristoffersen, 1969; Moe, 1972). Rose plants grown at higher temperatures resulted in a total higher yield flowers due to a more rapid rate of flower development and a higher : number of roses produced in each flush. The lower number of roses per flush at low temperatures is caused by an increased number of produced blind shoots (Moe and Krlstoffersen, 1969; Moe 1971). The reduction yield due to the induction of flower abortion when the night temperature is lowered (Zieslin and Moe, 1985), seems to be eliminated when the day temperature is increased to a mean daily temperature of 18°C (Table 1). Recently, Khayat and Zieslin (unpublished data) have found that temperature alternation between 12 and 18°C during the night gave almost the same yield of roses as a constant 18°C night temperature. These findings indicate the possibility of energy savings without a reduction in yield.

The quality of the cut roses was strongly influenced by the temperature. Low night temperatures (below 12°C) or mean temperatures below 15°C caused the production of a high number of malformed flowers  ("bullhead") or "blackening" of the petals (Zieslin and Moe, 1985). Too high temperatures (mean temperatures of above 21°C) resulted in the development of shorter and weaker flower stems and a pale colour of the petals (Moe 1971, 1972). In some cultivars (especially hybrid tea types) high temperatures induce an excessive elongation of the internode (neck). Optimization of the temperature levels in relation to light intensity and CO2 enrichment is essential for a high yield of good quality roses.

Temperature, light quality and shoot orientation have an influence on apical dominance. When the main shoots of the plant are bent down, bud sprouting at the basal part of the plant is stimulated since the apical meristems no longer control bud growth. Low temperatures also stimulate bud sprouting and growth. When raising young rose plants, a period of low temperature treatment, combined with shoot bending, can be an useful method for obtaining earlier flower production of newly planted roses (Moe, 1973).

Bud sprouting and flower abortion are strongly influenced by light quality. Extended day lighting with incandescent light with a low R:FR ratio (about 0.8) resulted in inhibited bud sprouting and increased flower abortion. FR light had a similar effect, while R light or fluorescent lamps with a high R:FR ratio (about 5.2) promoted bud sprouting and flowering. The FR stimulation of flower abortion could be overcome by subsequent R lighting. This indicates that blind shoot formation is controlled by the phytochrome system. Low light intensity (non-photosynthetic light) has a great influence on C14 translocation from the source leaf to the sink (buds) (Mor and Halevy, 1984). Darkening of the snoot (sink) completely inhibited the translocation of the assimilate to the buds. Moe and Khayat (unpublished data) have recently found that FR light completely inhibited the translocation of C14 assimilates from the source leaf to the buds. The FR stimulation of flower abortion and the inhibition of bud sprouting could be caused by a lack of assimilate supply to the buds. Numerous other factors may have an influence on bud sprouting and flower abortion (Table 6). Several of these factors can be related to the carbohydrate content in the plant and its supply to the bud or active growing snoots. Further studies are in progress to investigate the hypothesis that an adequate carbohydrate supply is necessary for the production of bud sprouting and flowering in roses.

4. References

Table 1 - Effekt of day- and night temperature on days from cut back to flower of first flush and
number of harvested flowers per plant after 8 months (Feb. 12 - Oct. 12) in 3 rose cultivars.

Cultivar Temperature °C Days to
1. flush
Relative Number
of roses
per plant
Relative
Day Night
'Sonia' 12 12 90 100 8.6 100
15 15 60 67 14.1 164
18 18 48 53 19.3 224
21 21 42 47 23.9 278
21 15 48 53 23.3 271
24 12 45 50 24.4 284
'Belinda' 12 12 93 100 9.1 100
15 15 63 67 14.5 159
18 18 45 48 27.9 306
21 21 42 45 36.8 404
21 15 48 52 27.8 305
24 12 45 48 28.3 311
'Red Garnette' 12 12 101 100 14.1 100
15 15 69 68 22.9 162
18 18 52 51 27.9 198
21 21 48 47 46.5 330
21 15 51 50 27.0 191
24 12 52 51 27.9 198

Table 2 - The effect of cooling at 9°C for 8 weeks on lateral shoot growth
(bud breaks) in 'Red Garnette' plants with upright bent down shoots.

Temp.
treatment
Shoot
orientation
Basal shoots
per plant
Lateral shoots
per plant
Flower shoots/plant
No. Relative Total Relative No. Relative
Control 18°C Upright 0.3 100 3.7 100 2.2 100
  Bent down 0.7 233 5.3 143 3.1 141
Cooling 9°C Upright 1.0 333 6.4 173 2.9 132
  Bent down 1.8 600 6.8 184 3.7 168

Table 3 - Main effects of lamp types as day extension light on number of bud breaks
per plant (relative units) and percent blind shoots in Rosa 'Red Garnette'.

SD short days, LD = long days

  Number (Rel. units) Blind shoots %
15 °C 200 59
18 °C 220 31
21 °C 181 46
24 °C 172 53
SD (8 h) 223 69
LD (24 h) incandescent lamps 142 59
LD (24 h) fluorescent lamps 214 14

Table 4 - Effect of red (R) and far-red (FR) lighting on blind shoot formation (%) in rosa 'Red Garnette'.
All plants were given a 9 h day with fluorescent light (29 Wm2 400-700 nm).

9 h day +
15 h night period 
Blind
shoots %
Flowering
shoots %
15 h dark (D) 69 31
15 h D 9 91
15 h FR 37 63
6 h D + 3 h R + 6 h D 54 46
6 h D + 3 h FR + 6 h D 63 37
3 h R + 3 h FR + 3 h R + 3 h FR + 3 h R 26 74
3 h FR + 3 h R + 3 h FR + 3 h R + 3 h FR 56 44

Table 5 - Effect of red (R) and far-red (FR) light as day extension lighting for 16 h on bud sprouting and flower abortion
(% blind shoots) in Rosa 'Red Garnette'. All plants were given 8 h day with fluorescent light (29 Wm2 400-700 nm).

  No. of
Sprouts
Flowering Blind Blind shoots %
Bud position
1 2 3 4 5 Basal
R 2.9 74 26 0 5 75 100 100 60
FR 2.2 30 70 40 94 100 100 100

Table 6 - Factors affecting bud sprouting abortion (blind shoots) in roses.
+ stimulation
- inhibition   

    Bud Flower
1. Low light intensity   - +
  High light intensity   + -
2. Daylength SD + +
    LD - + -
3. Light quality R + -
    FR - +
4. Pruning Strong - +
    Light + -
5. Temperature Low + +
    High - -
6. Removal of leaves   + +
7. Bud position      
8. Shoot orientation      

 

Figure 1 - Number of flowers harvested per 10 days in the rose cultivar 'Sonia' during a growing period of 8 months. Each treatment comprised 8 plants. Flush-number is indicated in the figure.
Figure 2 - The responses of the rose plants to temperature on yield, flower malformation ("bullhead") , flower abortion, flower stem length, length of uppermost internode ("neck") and flower colour. Relative unit 100 = highest amount or quality.
Fig. 3. Effect of low (12 °C) and normal (18 °C) temperature on development of lateral shoots of the infloresence in 'Baccara' (hybrid tea) and 'Fire King' (floribunda). From left: 'Baccara' 12 °C, 'Baccara' 18 °C and 'Fire King' 12 °C.
Figure 4 - Spectral energy distribution for red (R) and far-red (FR) light sources measured by an UDT 1100B spectroradiometer (380-1100 nm). Total energy at plant level was 410 mW m-2 (43 lux) for R-light and 3300 mW m-2 (1.6 lux) for FR-light.