Responses of glasshouse roses to light conditions

Scientia Horticulturae, 6(1977)223--235 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

223

RESPONSES OF GLASSHOUSE ROSES TO LIGHT CONDITIONS

M. KHOSH-KHUI1 and R.A.T. GEORGE School of Biological Sciences, University of Bath, Bath BA 2 7A Y (Gt. Britain) 1Now at Department of Horticulture, College of Agriculture, Pahlavi University, Shiraz (Iran)

(First received 19 August 1976; in revised form 2 November 1976)

ABSTRACT Khosh-Khui, I~_ and George, R.A.T., 1977. Responses of glasshouse roses to light conditions. Scientia Hort., 6: 223--235. Effects of light intensity and duration on the vegetative and reproductive characters of the rose cultivar 'Baccara' were studied for a year. Production of unlighted 'Baccara' in S.W. England reflected the solar energy curve; the highest flower yield followed the vegetative phase receiving the most natural light. Lighting improved flower yield, decreased blind shoots and hastened flowering, in proportion to the quantity of light received, eso pecially during periods of limited natural light. Intensity was more effective than duration. Bottom breaks and axillary shoot development were stimulated by lighting, the latter being associated with higher yields. Lighting the plants in the morning and evening increased yield more than giving the same quantity of light during the day. High pressure sodium lamps (SON/T) were effective as supplementary light to glasshouse roses, especially when higher intensities were required. Lighting with sodium lamps significantly increased flower yield and decreased the number of blind shoots. Some characteristics studied were highly correlated in all experiments.

INTRODUCTION Yields o f g r e e n h o u s e roses in Britain d o n o t r e a c h t h e i r t h e o r e t i c a l p o t e n t i a l s in winter. O n e o f t h e p r i n c i p a l f a c t o r s l i m i t i n g a c t u a l yields is t h e l o w light int e n s i t y a n d f e w e r h o u r s o f n a t u r a l d a y l i g h t available d u r i n g w i n t e r m o n t h s . T h e p o s s i b i l i t y o f using s u p p l e m e n t a r y light has b e e n i n v e s t i g a t e d in t h e U.S.A. ( B i c k f o r d , 1 9 6 8 ; Mastalerz, 1 9 6 9 ; C a r p e n t e r a n d R o d r i g u e z , 1 9 7 1 ; C a r p e n t e r a n d A n d e r s o n , 1 9 7 2 ; Wiseley a n d L i n d s t r o m , 1 9 7 2 ; White a n d R i t c h e r , 1 9 7 3 ) and in N o r w a y (Moe, 1 9 7 2 ) . R e c e n t l y C o c k s h u l l ( 1 9 7 5 ) r e p o r t e d t h e r e s p o n s e o f g r e e n h o u s e roses t o s u p p l e m e n t a r y light d u r i n g 3 m o n t h s in winter. T h e r e is n o r e p o r t o n t h e e f f e c t s o f s u p p l e m e n t a r y lighting o f glasshouse roses t h r o u g h t h e w h o l e y e a r u n d e r British c o n d i t i o n s . T h e use o f light s o u r c e s o t h e r t h a n i n c a n d e s c e n t or m e r c u r y l a m p s f o r roses is s u g g e s t e d b y C a r p e n t e r a n d A n d e r s o n ( 1 9 7 2 ) a n d Moe ( 1 9 7 2 ) . A c c o r d i n g t o

224

Canham (1966) a comparison of growth rates under equal luminous intensities indicated that sodium light was only slightly less effective than mercury light and although the luminous o u t p u t of a 200 watt sodium lamp (25,000 lm) is much greater than that of a 400 watt mercury lamp (15,600 lm), the advantage of the same growth rate for a reduced current consumption has y e t to be demonstrated. The present study reports the effects of intensity, duration and timing of supplementary lighting, and the effects of high intensity light from high pressure sodium lamps on 'Baccara' roses. M A T E R I A L S AND METHODS

The experiments were conducted in a heated greenhouse where the thermostat was set to give a minimum day and night temperature of 17 ° C, while the ventilators c o m m e n c e d opening at 21 ° C. In each experiment adjacent 120 X 120 cm compartments were separated by dense black 200 gauge polyethylene sheets in metal frames. Thirty-six uniformly selected, 2-year old (thirty, 3-year old in experiment 3) 'Baccara' roses were used in each experiment. The roses were planted in 30 cm pots filled with potting-compost. Throughout the experiment liquid feed was given with irrigation water at the rate of 90 : 180 p.p.m. N : K, and micro-nutrients. A 2 X 2 factorial design with 2 control plots and 6 replications (5 in experiment 3) was used in all experiments. High pressure mercury lamps (MBFR/U) 400 and 900 W were used in the first 2 experiments, m o u n t e d about 110 cm above the pots to produce 2 or 3 K lux light intensity, equalling 17.2 or 25.9 J m "2 s-~, respectively. In experiment 3 the light intensities of 4800 and 9600 lux were supplied by one and t w o 400-watt high-pressure sodium lamps, respectively. Intensity was measured at c o m p o s t level. The temperature differences between compartments were negligible. Responses to treatments were recorded for 12 months and included flower number and quality, number of blind shoots, abnormal flowers, days to flowering, stem length, node number, neck node length, leaves and leaflets number, stem diameter of lowest and u p p e r m o s t internodes, the area of the terminal leaflet of the u p p e r m o s t leaf, petal size (average of 5 outer petals), petal number, and fresh and dry weights. A quality index was calculated according to the formula suggested by White and Ritcher (1973) which is Q.I. = Yield X Weight/Length. Correlation coefficients were calculated between all characteristics listed above for the different times of the year and for flowering stems and blind shoots separately. RESULTS 1. - - T h e effects of intensity and duration were studied by giving the same quantity of light in 2 different durations from 20 December 1972 until 20 December 1973. Natural daylength was extended by 4 or 8 h using 2 levels of total light; 16 K lux hours total light was given in the form of 4 K

Experiment

225

lux in 4 h or 2 K lux in 8 h, while 24 K lux hours total light was provided in the form of 6 K lux in 4 h or 3 K lux in 8 h. For conversion in light energy flux, 1 K lux = 8.6 J m -2 s -1. The additional light was divided and given equally before sunrise and after sunset. Under the natural light of S.W. England in 1973 there was a close relationship in control plants between solar energy received and the plant yield. These plants produced the highest yield as number of flowers per flowering-flush when receiving the most natural light ( Fig. 1 ) Flower number per plant was significantly higher than the controls during the whole year in those receiving supplementary lighting, and 24 K lux hours total light produced higher yields than 16 K lux hours. Flower productions in each flowering-flush of treated and non-treated plants are shown in Fig.2. The • Control 1 Control 2

2° i 18~

•

Solar

6OQ 54o

radiation

1E

.....'"

"'-..

480

,

14 LU ~12

42O

u. 1(3

~8 ~6

360

~

300

~

2 4 0 ~< -6 u 180 U)

m

120 ~ <

.,."

z

(3

6o ~

2

> <

D MONTHS OF THE YEAR

Fig, 1. The relationship b e t w e e n solar radiation o f S.W. England in 1 9 7 3 and the number of flowers of 6 'Baccara' plants. 25 23

21

Control

1

; Control 2 4 klux-4hrs • 2klux-B~rs 0 6 k Jux--4hrs

tL/ O~15 J U-13

A /

\

../'J~\ //~.~

"

~ 9

J

F

M A M J J A S 0 MONTHS OF THE YEAR

N

D

Fig. 2. Yields o f 6 'Baccara' roses in different flowering-flushes produced in a year.

226

number of blind shoots per plant was less in summer and was lower on the lighted plants than on the controls. In Fig.3 the numbers of blind shoots in each flowering-flush of treated and non-treated plants are compared. Two-way analysis of data i.e. intensity (16 and 24 K lux hours) versus duration (4 and 8 h) showed a significant difference between levels of light in their effect on flowering stems and blind shoots, b u t the duration and interaction had no significant effect (Table 1). Better grades of cut flowers were obtained from the lighted plants when the cut flowers were graded according to British Flower Industry Association recommendations. Lighted plants produced flowering shoots in fewer days than unlighted plants, which difference was significant in most flowering-flushes. Consequently control plants and those receiving 4 K lux in 4 h p r o d u c e d 6, b u t other treatments had 7 flowering-flushes during the course of the experiment (Fig.4). Significant differences in stem length between the levels of light were found when the data were analysed for the whole year (Table 1). Flowering-stem length increased in summer for the control, but decreased for lighted plants. No significant difference was obtained for the stem length of blind shoots. The number of nodes per flowering stem and blind shoot was almost unchanged, b u t internodes were longer with higher total light, especially in the periods January--April and September--December. In all seasons of the year when the same quantity of light was given over a longer period of time, neck node length decreased significantly. The number of all leaves and leaflets per flowering stem increased with a higher quantity of light (Table 1), b u t the difference was n o t significant in summer or for the whole year. The area of the terminal leaflet of flowering stems was significantly greater in lighted plants than in the controls, regardless of the time of the year, but neither total light nor longer daylength significantly reduced the size of terminal leaflets on flowering stems (Table 2). The average area of the 5 outer petals

t3f 12 u) 11

A Control 1 • Control 2 {3 4 k l u x - 4 hrs • 2klux-Shrs 0 6klux-4hrs ~ • 3klux-Bhrs

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,

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:

,

,

,

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J A S O N D

MONTHS OF THE YEAR

Fig.3. Number of blind shoots of 6 'Baccara' plants in different flowering-flushe~.

11.80

10.10

11.00

24 K lux hours

4h

8h 1.77 2.40

5.90

6.75

5.40

7.20

7.40

Number of blind shoots/ plant/year

5.54 7.53

66.9

63.0

67.1

62.7

62.2

Stem length with flower (cm)

18.7

17.1

8h

3.31

17.3

24 K lux hours

4h

1%

18.8

16 K lux hours

2.43

16.4

Average of controls

LSD 5 %

Terminal leaflet area/ flowering stem (era 2 )

Treatments

0.46

0.34

5.52

5.59

5.64

5.46

5.24

Petal size (cm 2)

4.56

3.25

21.50

24.70

23.70

22.40

20.80

(g)

Fresh wt/ flowering stem

Changes in some characteristics of 'Baccara' under different total light and duration.

TABLE 2

1.81 2.47

9.33

16 K lux hours

LSD 5 % 1%

6.50

Number of flowers/ plant/year

Average of controls

Treatments

Effects of total light and duration of supplementary lighting on 'Baccara' plants.

TABLE 1

10.70

1.23

0.90

6.51

7.52

7.20

6.83

6.05

(g)

Dry wt/ flowering stem

9.68 13.20

47.8

10.50

1.86 2.53

55.0 49.7

9.29 11.90

38.3 42.6

Number of leaflets/ flowering stem

7.94

Number of leaves/ flowering stem

t~ t~

228

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240

200

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160

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Fig.4. Cumulative number of days to flower in all flowering-flushes of 'Baccara' in a year for controls and treated plants.

removed from lighted plants was significantly greater than that of unlighted plants (Table 2). The n u m b e r of petals per flower from plants which received artificial light was significantly greater than controls in the period at low natural light. Both fresh and dry weights per flowering stem were significantly higher in the period of low natural light from January to April than in the controls and also higher for lighted than unlighted plants during a year (Table 2 ). Highly significant correlation coefficients, between 0.72 and 0.94 with 0.38 as an exception, were obtained between the characteristics recorded. The quality index increased in proportion to the levels of light retained by the plants (Fig.5). Analysis of the data for the whole year indicated that lighting improved flower yields by increasing b o t t o m breaks and stimulating axillary shoot development. The development of axillary buds was the principal factor in the improved branching of lighted plants (Table 3). Although the petal n u m b e r per flower of bullhead flowers in a whole year was slightly greater (~- = 66.00 + 9.44) than that of normal flowers (~- = 58.90 + 6.72), the difference was negligible. Experiment 2. - - Growth o f roses under 7 or 14 h natural light, and growth with the same quantity of light given equally in the morning and evening, were compared. From 15 April to 30 September 1973, when the natural day-length was equal to, or more than, 14 h, 3 compartments were covered by black cloth from 1630 to 0930 to produce a 7 h photoperiod. One c o m p a r t m e n t was considered as control and the other 2, in addition to 7 h natural light,

229

26 m

24

m

22 2C le

,x,, ~e m l

J lO ~

8

? Control Control 4 k l u x 2klux 6 k l u x 1 2hr$ 4hrs 8hrs 4hrs TREATMENTS

3klux 8hrs

Fig. 5. Quality index (yield × weight/length ) of unlighted and lighted 'Baccara' plants in a

whole year.

were given 2 K lux supplementary lighting during 7 h, either during the day (from 0930 to 1630) or in the morning and evening (from 0600 to 0930 and 1630 to 2000). Three other compartments received 14 h natural light; one was the control of 14 h natural light (LD) and the other 2 received 2 supplementary lighting treatments as described above for 7 h natural light (SD). A significant increase in flower number per plant and decrease in the number of blind shoots per plant occurred when plants received 14 h rather than 7 h natural light, but the effect of the time of lighting was non-significant (Fig.6). Plants under 14 h natural light produced better grades of flowers than plants under 7 h natural light. Under SD, supplementary lighting was more effective than under LD. Plants under LD produced significantly earlier flowers than those under SD, but the differences between controls and morning and TABLE 3

Number of flowering sterns, b o t t o m breaks and axillary shoots per 'Baccara' plant in a whole year. Treatments

Number of flowering stems/plant

Number of b o t t o m breaks/plant

Number of axillary shoots/plant

Average of controls

6.50

0.58

5.92

16 K lux

9.33

0.91

8.42

24 K lux

11.80

1.25

10.55

LSD 5 %

1.81

0.71

1.38

1 %

2.47

0.85

2.10

230

• Short day o Long day <

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I CONTROL

M lid DAY LIGHTING

I &, MORN. EVEN, LIGHTING

Fig.6. Effects of SD (7h) and LD ( 1 4 h ) a n d time of supplementary lighting on the number of flowers and blind shoots of 'Baccara'.

evening lighting in both day lengths were not significant. Stem lengths with or without flowers were not significantly larger under LD and morning and evening lighting than under SD and midday lighting (Table 4). The node number per flowering stem was not influenced (Table 4), neither was the number of blind shoots. Average internode length of SD plants was not significantly different from LD plants, but morning and evening lighting under SD significantly increased this response. Under LD treatment the terminal internode per flowering stem was not significantly reduced by lighting. Stem diameter was significantly larger when the treatments were compared with the SD control (Table 4). The size of the terminal leaflet and the average area of the 5 outer petals were significantly greater under LD than under SD. Supplementary lighting in SD increased petal size, but in LD it reduced it. Both fresh and dry weights of flowering stems significantly increased on plants which received LD treatment, compared with SD with natural light. Supplementary lighting under SD increased, and under LD decreased, fresh and dry weight. A two-way analysis of data showed that the difference between day lengths was significant, but the difference between time of lighting was not. Correlation coefficients between characteristics recorded varied from 0.54 to 0.95. Plants under LD had a higher quality index than those under SD. Morning and evening lighting achieved a better quality index than mid-day lighting (Fig. 7 ). Experiment 3. - - Natural day light was extended by 4 or 8 h supplementary lighting with 4800 and 9600 lux light intensities measured at compost level, from 18 October to 10 June 1974. The 4 and 8 h supplementary lighting

Stem with flower (cm)

46.8

61.4

54.9

56.2

52.2

58.9

17.9 24.4

Treatments

Control (SD)

Control (LD)

SD

LD

Midday lighting

Morning and evening lighting

LSD 5 % 1%

9.27 12.60

47.7

44.0

47.7

44.0

49.1

36.1

Stem without flower (cm)

2.54 3.44

9.57

9.50

9.74

9.33

9.46

8.53

Node number/ flowering stem

0.51 0.70

4.35

4.27

4.39

4.24

4.93

3.74

Stem diameter (lowest internode ) (mm)

Effects of 7 or 14 h natural light, indicated as SD (short day of 7 h ) o r LD (long day of 14 h), and time of supplementary lighting on some characteristics of 'Baccara' plants.

TABLE 4

b~ 50 ~a

232

Morn. & Men. llghtlng

Midday

7

• I[gl~tinq Control

6

Morn. &

× 5 uJ

i,

4I

gr

Yi g

"o! 2 ~r

T SD

SD

SD

LD

LD

LD

Fig.7. Quality index of 'Baecara' calculated for different lengths of days and times of supplementary lighting. were equally divided b e t w e e n b e f o r e sunrise and a f t e r sunset; t h e t i m e clocks were reset e v e r y 10 days. F l o w e r yields f r o m O c t o b e r t o J u n e increased, and blind s h o o t n u m b e r s decreased, w h e n t h e plants were illuminated b y high pressure s o d i u m lamps. T h e f l o w e r y i e l d d i f f e r e n c e b e t w e e n t h e c o n t r o l s and s o m e lighted plants was highly significant (Table 5). T w o - w a y analyses o f d a t a s h o w e d t h a t f l o w e r n u m b e r per p l a n t d i f f e r e d significantly at 1 % level b e t w e e n c o n t r o l s , 4 8 0 0 and 9 6 0 0 lux light intensities, whereas d i f f e r e n c e s b e t w e e n 4 and 8 h supp l e m e n t a r y lighting were significant at 5 % level. Quality i n d e x increased in p r o p o r t i o n t o the q u a n t i t y o f light received b y t h e plants (Table 5). Grading the flowers s h o w e d t h a t t h e larger n u m b e r o f flowers o b t a i n e d f o r lighted plants i n c l u d e d b o t h high and l o w grades. T h e plants which received 4 8 0 0 lux TABLE 5 Flower and blind shoot numbers per plant and quality index (yield x weight/length) for roses illuminated with light from high pressure sodium lamps. Treatments

Flower number/ plant

Blind shoot number/ plant

Quality index

Control 1 Control 2 4800 lux--4 4800 lux--8 9600 lux--4 9600 lux--8

3.25 3. 25 3.75 4.75 5.00 6.75

4.50 4.00 3.75 3.00 2.75 2.00

4.95 5.22 6.07 7.75 8.47 11.92

1.75 2.43

1.42 1.95

LSD 5 % 1%

h h h h

---

233

CUMULATI~ NUMBER OF DAYS TO FLOWER

Fig. 8. Effects of intensity and duration of light emitted from high pressure sodium lamps on the number of days to flower in different flowering-flushes of 'Baccara'.

in 8 h received the total light equivalent to those which received 9600 lux in 4 h. It was evident that giving the same a m o u n t of total light in a longer period of time may produce better grades. Earlier flowering was achieved by lighting plants during flowering-flushes (Fig.8). Giving the same quantity of light over a longer period did n o t p r o m o t e earlier flowering. None of the many other parameters recorded was significantly influenced by intensity and duration of supplementary lighting. Highly significant correlation coefficients were obtained b e t w e e n flowering stem characteristics, varying from 0.66 to 0.99. These characteristics were correlated regardless of treatments received by the plants. For example, the correlation coefficients between dry weight and other characteristics in a flowering stem varied from 0.62 to 0.99 with one exception of 0.48. DISCUSSION

Post and Howland (1946) showed that flower production in New York was a direct function of the a m o u n t of solar energy. Stanhill et al. (1973), working in Israel, f o u n d a relationship b e t w e e n b l o o m production of the glasshouse rose and solar radiation. Cockshull (1975) reported that the data obtained

234

from cultivars 'Baccara' and 'Sonia' were similarly related to the monthly sums of solar radiation received in the Littlehampton area of West Sussex, England. The present data suggest that supplementary lighting might be benefical for increasing the flower yield of 'Baccara' between September and April with a maximum effect in the middle of this period. An increase in number of flowers per plant was obtained when the natural light was supplemented by extra light from high pressure mercury lamps. This confirms the results obtained by previous investigators using other light sources. Cockshull (1975) stated that supplementary lighting of 'Baccara' and 'Sonia' for 3 months in winter yielded a modest return. In the present investigation it was evident that supplementary lighting was effective and increased the flower production of 'Baccara' even in the summer months, although it is probably not an economic treatment under British climatic conditions. Blind shoot formation on roses has been studied by many workers (Hubble, 1934; Moe, 1971; Zieslin et al., 1973). Others have shown that all the shoots have the potential to become marketable flowers (Halevy, 1972; Horridge and Cockshull, 1974). In the present study the succession of flowers and blind shoots was followed for 12 successive months and it was confirmed that all the blind shoots on the plant have flowering-potential and that following the removal of a flowering stem a blind shoot may appear from axillary buds on the same stem, if the conditions are unfavourable for flowering. Thus the direct benefit of light in preventing the activation or destruction of endogenous stimuli of blind shoot formation was confirmed. Similarly, the presented data are in agreement with those obtained by Zieslin and Halevy (1975), who showed that reduction in light intensity increased blindness in relation to shading-intensity, while photoperiod had no effect. Earlier flowering under supplementary lighting was proportional to intensity and total light received by the plants rather than to the duration. Promotion of earlier flowering with the same quantity of light in longer periods of time may be due to full interception of light by the plants in a longer time. Length of flowering stems increased during periods of low natural light intensity, if supplementary light was given, but in summer months the stem lengths of lighted plants were less than the controls. This was probably due to the greater number of stems on lighted plants. Longer internodes were responsible for longer stems, not a greater number of nodes. Increase in dry weight of lighted plants compared with controls confirms Howland (1946), who showed that the greenhouse rose is relatively efficient in net food synthesis over a wide range of light intensities. The results for petal number of bullhead flowers are at variance with those of Lindenbaum and Ginzburg (1975), who reported a larger number of petals for malformed flowers. High pressure sodium lamps (SON/T), which produce light in the orangeyellow part of the visible spectrum, were efficient light sources for supplementary lighting of 'Baccara', especially as they produced higher intensities than high pressure mercury lamps at the same wattage. Generally, the effects

235

of light emitted from sodium lamps on 'Baccara' were almost the same as those obtained for high pressure mercury lamps (MBFR/U). The lack of response in some parameters measured may be due to the overall larger number of cut flowers from lighted plants which consisted of higher and lower grades of flowers, resulting in a similar mean as the unlighted plants. The demonstration of highly correlated characteristics for 'Baccara' might be helpful for estimating other characteristics by measuring one of them under the conditions of these experiments. Thus, for example, measuring the fresh weight of flowering stems might provide an estimate of dry weight. REFERENCES Bickford, E.D., 1968. Effect of supplementary lighting on growth flowering of roses. Roses Inc. Bull., 17--26. Canham, A. K, 1966. Artificial light in horticulture. Centex, Eindhoven. Carpenter, W.J. and Rodriguez, R.C., 1971. Supplemental lighting effects on the newly planted and cut-back greenhouse roses. HortScience, 6: 207--208. Carpenter, W.J. and Anderson, G.A., 1972. High intensity supplementary lighting increased yields of greenhouse roses. J. Am. Soc. Hortic. Sci., 97: 331--334. Cockshull, K.E., 1975. Roses II. The effects of supplementary light on winter bloom production. J. Hortic. Sci., 50: 193--206. Halevy, A.H., 1972. Phytohormones in flowering regulation of self-inductive plants. Proc. 18th Int. Hortic. Congr., 5: 187--198. Horridge, J.S. and Cockshull, K.E., 1974. Flower initiation and development in the glasshouse rose. Scientia Hort., 2: 273--284. Howtand, J.E., 1946. The rate of photosynthesis of glasshouse roses. Proc. Am. Soc. Hortic. Sci., 47: 473--481. Hubble, D.S., 1934. A morphological study of blind and flowering rose shoots with special reference to flower bud differentiation. J. Agric. Res., 48: 91--95. Lindenbaum, S. and Ginzburg, C., 1975. A morphological study of the "Bullhead" malformation in the 'Baccara' roses. Ann. Bot. (London), 39: 219--223. Mastalerz, J.W., 1969. Environmental f a c t o r s - light, temperature, carbon dioxide. In: J.W. Mastalerz and R.W. Langhans (Editors), l~ses. Pennsylvania Flower Growers. N.Y. State Flowers Assoc. Roses Inc. Moe, R., 1971. Factors affecting flower abortion and malformation in roses. Physiol. Plant., 24: 291--300. Moe, R., 1972. Effects of daylength, light intensity and temperature on growth and flowering in roses. J. Am. Soc. Hortic. Sci., 97: 796--800. Post, K. and Howland, J.E., 1946. The influence of nitrate level and light intensity on the growth and production of greenhouse roses. Proc. Am. Soc. Hortic. Sci., 47: 446--450. Stanhill, C., Fuchs, M., Bakker, J. and Moreshet, S., 1973. 2he radiation balance of a glasshouse rose crop. Agric. Meteorol., 11: 385--404. White, J.W. and Ritcher, D., 1973. Response of roses to low moisture stress at high and low light intensities. Pa. Flower Growers Bull., 262: 1--9. Wiseley, D.K. and Lindstrom, R.S., 1972. Supplemental light and growth of rose during periods of low light intensity. HortScience, 7: 292--293. Zieslin, N. and Halevy, A.H., 1975. Flower bud atrophy in 'Baceara' roses. II. The effect of environmental factors. Scientia Hort., 3: 383--391. Zieslin, N., Halevy, A.H. and Biran, I., 1973. Sources of variability in greenhouse rose flower production. J. Am. Soc. Hortic. Sci., 98: 321--324.