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Responses of Alstroemeria ‘Regina’ to temperature treatments prior to flower-inducing temperatures
Scientia Horticulturae, 17 (1982) 383--390
383
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
RESPONSES OF A L S T R O E M E R I A 'REGINA' TO TEMPERATURE TREATMENTS PRIOR TO FLOWER-INDUCING TEMPERATURES
W.E. HEALY and H.F. WILKINS
Department of Horticultural Science and Landscape Architecture, University of Minnesota, Saint Paul, MN 55108 (U.S.A.) Scientific Journal Series Paper No. 11 733. Minnesota Agriculture Experiment Station (Accepted for publication 29 October 1981)
ABSTRACT Healy, W.E. and Wilkins, H.F., 1982. Responses of Alstroemeria 'Regina' to temperature treatments prior to flower-inducing temperatures. Scientia Hortic., 17: 383--390. Days to flower (DTF) were inversely related to the number of weeks (0--8) that
Alstroemeria 'Regina' plants remained at 5°C, a vernalizing temperature, before being moved to 13°C, a vernalizing as well as a forcing temperature. However, when the number of weeks at 5°C was added to the DTF, no difference in the total time to flower was observed between plants treated at 5° C or those grown continuously at 13° C, as they both induced flowering. One-year-old plants maintained at 21 ° C, a non-inductive temperature, and not divided prior to the 5°C treatments, showed an increase in total shoot production, and delayed DTF; compared to plants which were divided. When divided plants were maintained for 16 weeks at 21°C prior to 5°C treatments, total shoot production was reduced but flowering was accelerated compared to plants maintained for 8 weeks at 21°C after dividing. Total shoot and flowering-shoot production was not affected by increasing the durations of time at 5°C when plants were grown at 21°C and divided prior to this treatment. Thus, the pre-treatment of dividing or maintaining plants at 21°C prior to a 5° C treatment affected subsequent shoot production and DTF.
INTRODUCTION
Alstroemeria has recently received considerable attention as a potential new cut flower for glasshouse production in the northern United States because of its low fuel requirements, high yield per square meter, consumer interest and wholesale value. Researchers working with Alstroemeria during the past 10 years have concentrated on devising methods that could be used to promote earlier flowering. Promotion of earlier flowering is the primary method by which flower production could be increased since, regardless of previous treatment, all plants ceased flowering at the same time in the summer (Vonk Noordegraaf, 1975a). Extending the natural photoperiod during the winter to 13 h was shown to stimulate earlier 0304-4238/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
384 flowering (Vonk Noordegraaf, 1974b, 1975b; Heins and Wilkins, 1976, 1979). Our unpublished work from the forcing-year 1977--78 indicated that Alstroemeria 'Regina' grown at temperatures above 22°C failed to flower, regardless of photoperiod treatments. Subsequent work at the University of Norway verified our findings that 'Regina' would not flower at 21°C, although the photoperiods were not disclosed (Mourn, 1979). Other techniques used to increase flower production include earlier planting-dates (Baas et al., 1974; Wilkins et al., 1980b) and shoot thinning (Vonk Noordegraaf, 1975a; Heins and Wilkins, 1976). Stimulation of flower production by shoot thinning may decrease apical dominance and promote lateral rhizome growth (Heins and Wilkins, 1976). The increased flower production observed by earlier dividing of the rhizomes (Baas et al., 1974) and earlier planting (Wilkins et al., 1980b) may provide the plants with an adequate cold treatment prior to the start of the photoperiod treatment. Stinson (1942) reported that exposing Alstroemeria plants to natural cool temperatures in cold frames promoted earlier flowering. Vonk Noordegraaf (1975b) demonstrated that 9°C growing-temperatures promoted a greater ratio of flowering shoots to vegetative blind shoots compared to 25°C, but did not indicate the influence of the forcing-temperature on days-to-flower. Wilkins et al. (1980b) reported that rapid flowering occurred when vegetative 'Regina' plants were treated at 5°C for a minimum of 5 weeks when compared to the controls. Plants cooled for 5, 10 or 15 weeks had similar flowering-responses. The present experiments were undertaken to determine the minimum number of weeks (0--8 weeks) at 5°C needed to induce flowering in vegetative 'Regina' plants. These experiments were also _designed to determine the effect of one-year-old plants and the effect of a 21°C pre-treatment, which promotes vegetative growth, on the response of the plant to increased durations of 5°C treatments. MATERIALS AND METHODS Alstroemeria hybrida 'Regina' plants were grown under normal daylengths (ND) at temperatures above 21°C day/night (D/N) for 12 months to ensure that both shoots and rhizomes were in the vegetative stage. Plants for Experiments I and II were divided on 21 June 1978. Each division consisted of a single rhizome with attached storage roots and vegetative shoots. Divisions were placed singly in a 15-cm plastic pot filled with 1 p e a t : l perlite:l soil (v/v/v) medium. The plants were grown at 21°C in a glasshouse under ND (45 ° north parallel) until treatments began. Weak and old vegetative shoots were removed monthly to promote rhizome branching and thus new shoot formation (Heins and Wilkins, 1979). Soil tests were taken weekly throughout the course of the experiment and plants were fertilized to maintain the recommended nutrient levels.
385 Treatments for Experiment I began 8 weeks later, on 18 August 1978, when 25 plants were placed in a 5°C cooler and irradiated with 3.5 pE m -2 of fluorescent light (08.00 to 16.00). Five plants were removed every 2 weeks for the n e x t 8 weeks. The 0-week at 5°C control plants remained at 21°C D/N until 1 September, when they were transferred to the 13°C D/N glasshouse along with the 2-week 5°C-treated plants. Experiment II was identical to Experiment I except that the cold treatments began 8 weeks later, on 13 October 1978. This was done to determine if the time span between division and the start of the 5°C treatment would alter perception of the 5°C treatment. Three plants per t r e a t m e n t were used in Experiment II. The 0-week control plants were placed in the glasshouse at 13°C under ND on 27 October 1978, along with the first group of 5°C-treated plants. Experiment III dealt with age o f the plant prior to receiving the 5°C exposure treatments. Plants were grown under ND at 21°C for 12 months in 37.5-cm plastic pots. Plants were not divided prior to the 5°C treatments, which were identical to Experiment I. Treatment of 5°C began on 1 September 1978, with 1 plant per 2-week exposure. The 0-week control was placed in the 13°C glasshouse under ND on 15 September 1978, along with the first 5°C-treated plant. In all experiments, the n u m b e r of vegetative shoots present per plant was recorded when the plants were placed in the 13°C glasshouse. The n u m b e r of weak or old vegetative shoots removed m o n t h l y was recorded. All plants in Experiments I and II were replanted at the end of the 5°C t r e a t m e n t into 37.5-cm plastic pots using the same planting-medium. When plants in Experiments I, II and III were placed in the 13°C glasshouse, an incandescent (Inc) night interruption (NI) from 22.00 to 02.00 (10 #E m -2) was used to extend the photoperiod. Lighting was discontinued on 15 April 1979, when the ND was 13.5 h (Anonymous, 1945). Bottom heat was supplied as needed, so that a minimum soil temperature of 10°C and air temperature of 13°C were maintained. Experiments I, II and III were all conducted as a completely randomized design. Stems were harvested when the primary flowers on the c y m e were fully opened. Days to flowering (DTF) were calculated from the beginning of the 13°C forcingtemperature by averaging the date that the first 5 shoots of any treated plant flowered. The total n u m b e r of vegetative and flowering shoots per plant was recorded. Data collected, but not reported here, and available upon request, included n u m b e r of primary, secondary, tertiary and quartenary flowers per cymous whorl, stem length and number of leaves per stem. RESULTS A N D DISCUSSION
D a y s t o f l o w e r . - - In all experiments DTF decreased as the n u m b e r of weeks at 5°C increased (Table I). The correlations for DTF with the n u m b e r of
386 weeks at 5°C were -0.97, -0.86 and -0.76 for Experiments I, II and III, respectively. This inverse relationship of DTF with increasing exposureduration at 5°C is similar to that previously observed with Dicentra long~florum (Lopes and Weiler, 1977), Ldzurn "" " ~" (Wllkms, " " 1980), and the vernalization response in wheat (Levy and Peterson, 1972) and rye (Gott et al., 1955). Furthermore, in all of these genera, the cold treatment, when given in combination with a long photoperiod treatment, further decreased the DTF. The photoperiodic effect on DTF was also observed in Alstroemeria when plants are grown at 13°C (Vonk Noordegraaf, 1975b; Heins and Wilkins, 1979). As with Liliurn longiflorum (Wflkins, 1980), Alstroemeria flowered without additional low-temperature treatments provided the plants were forced at temperatures below 21°C, in this case 13°C (Table I). This suggests that the low temperatures necessary for flowering of Alstroemeria could be acquired at either 5 or 13°C. This hypothesis was further supported by manipulation of the DTF data. When DTF were calculated from the beginning of the 5 ° C-treatment, rather than from the time the plants were removed from the 5°C treatments, DTF for all treatments within an experiment were not statistically different. Commercially, Alstroemeria growers could decrease growing-temperatures to 5°C without adversely affecting DTF. In Europe, the present commercial schedule is to decrease temperatures to 10°C during December and January to enhance flowering in April--June (Verboom, 1979). This schedule could be modified for U.S. growers; 5°C growing-temperatures could be used during December and January, which would save energy and not affect DTF. TABLE I M e a n d a y s to f l o w e r for the first 5 f l o w e r i n g s h o o t s of AZstroemeria plants treated at 5°C for 0--8 weeks
Weeks at 5°C
Experiment I S t a r t e d 5°C t r e a t m e n t 8 w e e k s after division E x p e x ~ n e n t II S t a r t e d 5°C t r e a t m e n t 16 w e e k s after division E x p e r i m e n t III N o d i v i s i o n p r i o r to starting
HSD
0
2
4
6
8
204.8
205.6
189.6
172.6
155.4
2.7
188.2
--
163.2
156.6
165.2
6.0
220.2
215.6
201.8
191.8
203.4
7.7
5°C t r e a t m e n t -- = Treatment not included.
Shoot production. -- Vonk Noordegraaf (1975b) reported that Alstroemeria produced fewer shoots when grown at cooler temperatures, but an increased percentage of them flowered. Therefore, the effect of the 5°C treatments on total shoot production, as well as on shoots producing flowers, should
387 be of interest. When divided plants were given a 5°C treatment for 0--8 weeks, total s h o o t production was n o t affected in Experiments I or II (Fig. 1), nor was the number of flowering shoots affected. Therefore, 5°C treatments can be used to reduce fuel consumption w i t h o u t sacrificing flower yield. When undivided one-year-old plants were used (Experiment III) to determine the interaction of plant age with the 5°C treatments, different production statistics were observed (Fig. 1). Total shoot production was reduced as the duration of the 5°C treatment increased. This same response was observed with tiller growth in spring and winter wheat (Levy and Peterson, 1972). Thus, when Alstroemeria plants were n o t divided prior to 5°C treatments, total s h o o t production was reduced, contrary to what was observed when plants were divided. Previous w o r k b y Vonk Noordegraaf (1974a) indicated that maintaining non-inductive temperatures (25 ° C) for 8 additional weeks increased shoot production by 62%. He did n o t specify whether the increase in production was due to an increase in flowers or total shoots. We observed a decrease Experiment X 200
IOC ,sT J f J f - j
Experiment "n-
0~200 0 I (/3
'~ I00 rY LIJ m
Z 4O0
Experiment "nT
200
0
2
4
e
8
WEEKS of 5 °
Fig. 1. Total shoot (clear bar) and flower production (hatched bar) of Alstroemeria plants treated at 5°C for increasing durations of time (weeks) in Experiments I, II and III. Vertical lines are HSD 5% for Experiments I and II. No HSD for Experiment III.
388 in total shoot production between Experiments I and II, but no effect on flower production. Since flower production was not affected, the effect of an additional 8 weeks at 21°C (Experiment II) was to reduce the number of vegetative shoots prior to the start of flowering. However, giving plants 21°C for 16 weeks could be used to decrease the frequency of blind shoots which must be removed to stimulate flower production (Heins and Wilkins, 1976). The data obtained on DTF did not clearly resolve the question as to which factors interact to induce an Alstroemeria shoot to flower. Although the DTF data in Table I indicate a decrease in DTF with increasing exposure to 5°C, flowering occurs, for example, 155.4--205.6 days (Table I, Experiment I) after the treatment application, so that uncontrolled environmental factors may be influencing the results. For example, the forcingtemperature of 13°C may be retarding the rate of shoot elongation and growth (Healy and Wilkins, 1982) as was observed in Lilium (Wilkins et al., 1980a,c). They demonstrated that in lilies, increasing the forcing-temperature to 18°C reduced DTF and flower production. The naturally lengthening photoperiod and light intensity when Alstroerneria flower production begins may be promoting an increase in root growth, as shown by Weaver and Himmel (1929) in various genera, or in metabolite storage, as demonstrated by Milford and Lenton (1976) in sugar beets. The increased root growth and metabolite storage observed at cool soil temperature (Healy and Wilkins, 1979) could be involved with the flowering mechanism. Heins and Wilkins (1979) postulated that carbon storage would be necessary for flowering to occur, due to the accelerated rate of flower-shoot development. From the data presented here, carbon storage that may be utilized for flowering apparently comes from the carbon stored after the inductive temperatures have been given. When DTF from Experiments I or II are compared to Experiment III (Table I), one year of vegetative growth does not promote any earlier flowering than plants which were recently divided. We have observed that after the 5°C treatments and prior to flower-shoot development, storage-root growth and root enlargement occur. Concomitant to carbon storage, specific plant growth substances (PGS) may begin to accumulate in the plants and become conjugated with the stored carbon. Once a critical mass of metabolites and/or PGS have accumulated, flowering occurs. During the utilization of the stored metabolites, the conjugated PGS could become free and be involved in the flowering-response. From unpublished data, we observed that when plants were pre-treated for 8 weeks at 5°C, forced at 13°C under NI, and then received 1000 mg 1-1 Pro-Gibb or 50 mg 1-1 Ancymidol drench upon removal from the cooler, the resulting leaf and shoot morphology was radically different from untreated plants. Pro-Gibb-treated plants had elongated shoots with leaves longer than they were wide, which morphologically resembled a flowering shoot. Plants receiving Ancymidol produced short shoots with broad leaves
389 w h i c h r e s e m b l e v e g e t a t i v e s h o o t s g r o w n u n d e r s h o r t days, w h i c h delays flowering. A n c y m i d o l has b e e n s h o w n t o b l o c k gibberellin s y n t h e s i s (Coolbaugh and Hamilton, 1976). Thus, the Ancymidol and Pro-Gibb r e s p o n s e s s e e m r e a s o n a b l e . N e i t h e r P r o - G i b b - n o r A n c y m i d o l - t r e a t e d plants f l o w e r e d a n y earlier t h a n t h e c o n t r o l s , suggesting t h a t GA3 m a y b e i n v o l v e d w i t h f l o w e r - s h o o t e l o n g a t i o n b u t n o t f l o w e r i n i t i a t i o n , as was discussed b y C h o u a r d ( 1 9 6 0 ) . F u r t h e r w o r k is n e e d e d to d e t e r m i n e t h e i n v o l v e m e n t o f o t h e r P G S a n d s t o r e d c a r b o n in t h e f l o w e r i n g - m e c h a n i s m o f A I s t r o e m e r i a as t h e y i n t e r a c t w i t h l o w t e m p e r a t u r e s . ACKNOWLEDGEMENTS T h e a u t h o r s wish t o a c k n o w l e d g e t h e financial s u p p o r t o f t h e F r e d Gloeckner Foundation, a computing-grant from the University of Minnesota C o m p u t e r C e n t e r a n d t h e t e c h n i c a l assistance o f R a e q u e l R o b e r t s .
REFERENCES Anonymous, 1945. Tables of sunrise and sunset and twilight (supplement to American Ephemeris, Nautical Almanac Office, U.S. Naval Observatory, 1946). U.S. Government Printing Office, Washington, DC. Baas, A., Vonk Noordegraaf, C. and Leliveld, H.P.J., 1974. Alstroemeriaproef op de proeftuin te Vleuten. Invloed van planttijd en belichting. Proefstation voor de Bloemisterij in Nederland te Aalsmeer. Aalsmeer, The Netherlands, pp. 21--24. Chouard, P., 1960. Vernalization and its relations to dormancy. Annu. Rev. Plant Physiol., 10! 191--238. Coolbaugh, R.C. and Hamilton, R., 1976. Inhibition of ent-kaurene oxidation and growth by ~ ~yclopropyl~-(emethoxyphenyl)-5-pyrimidine methyl alcohol. Plant Physiol., 57 : 245--248. Gott, M.B., Gregory, F.G. and Purvis, O.N., 1955. Studies in vernalization of cereals. XIIL Photoperiodic control of stages in flowering between initiation and ear formation in vernalized and unvernalized Petkus winter rye. Ann. Bot., 21: 87--126. Healy, W.E. and Wilkins, H.F., 1979. Flowering requirements of Alstroemeria hybrida 'Regina'. HortScience, 1 4 : 3 9 5 (abstract). Healy, W.E. and Wilkins, H.F., 1982. The interact~ion of temperature on flowering of Alstroemeria 'Regina'. J. Am. Soc. Hortic. Sci., Vol. 107. Heins, R.D. and Wilkins, H,F., 1976. Alstroemeria cultural research, 1975--1976, University of Minnesota. The influence of night interruption with incandescent light and shoot-pruning techniques. Florist Rev., 159(4123): 34--35, 80--81. Heins, R.D. and Wilkins, H.F., 1979. Effect of soil temperature and photoperiod on vegetative and reproductive growth of Alstroemeria 'Regina'. J. Am. Soc. Hortic. Sci., 104: 359--365. Levy, J. and Peterson, M.L., 1972. Responses of spring wheats to vernalization and photoperiod. Crop Sci., 12: 487--490. Lopes, L.C. and Weiler, T.C., 1977. Light and temperature effects on the growth and flowering of Dicentra spectabilis (L.) Lem. J. Am. Soc. Hortic. Sci., 102: 388--390. Milford, G.F.J. and Lenton, J.R., 1976. Effect of photoperiod on growth of sugar beet. Ann. Bot., 40: 1309--1315. Mourn, P.O., 1979. Virkning av temperatur og daglengde pa vekst og utvikling nos "Vasa-Alstroemeria". MS Thesis, Agriculture University of Norway, Aas, Norway.
390 Stinson, H.L., 1942. Harvesting, storage and forcing of Alstroemeria. Herbertia, 9: 223--225. Verboom, H., 1979. Alstroemeria and some other flower crops for the future. Sci. Hortic., 31: 33--42. Vonk Noordegraaf, C., 1974a. Alstroemeria: Invloed van een temperatuurbehandeling op jonge planten. Proefstation voor de Bloemisterij in Nederland te Aalsmeer, Aalsmeer, The Netherlands, p. 22. Vonk Noordegraaf, C., 1974b. Alstroemeria: Plantafstand en belichting. Proefstation voor de Bloemisterij in Nederland te Aalsmeer, Aalsmeer, The Netherlands, pp. 18--23. Vonk Noordegraaf, C., 1975a. Alstroemeria: Invloed van dunnen bij 'Regina'. Bloemisterij onderzoek in Nederland, Aalsmeer, The Netherlands, pp. 15--16. Vonk Noordegraaf, C., 1975b. Temperature and daylength requirements of Alstroemeria. Acta Hortic., 51: 267--274. Weaver, ,I.E. and Himmel, W.J., 1929. Relation between the development of root system and shoot under long and short-day illumination. Plant Physiol., 4: 435--457. Wilkins, H.F., 1980. Easter lilies. In: R.A. Larson (Editor), Introduction to Floriculture. Academic Press, New York, pp. 327--352. Wilkins, H.F., Healy, W.E. and Pemberton, H.B., 1980a. Flower bud growth model for Lilium longiflorum. HortScience, 1 5 : 3 8 6 (abstract). Wilkins, H.F., Healy, W.E. and Gilbertson-Ferriss, T.L., 1980b. Comparing and contrasting the control of flowering in Alstroemeria 'Regina', Freesia x hybrida and Lilium longiflorum. In: C. Brickell, P.F. Cutler and M. Gregory (Editors), Petaloid Monocotyledons. Horticultural and Botanical Research. Academic Press, London, pp. 51--63. Wilkins, H.F., Pemberton, H.B., Heins, R.D. and Healy, W.E., 1980c. Some effects of photoperiod and light levels on the growth and development of Lilium longiflorum. HortScience, 15:386 (abstract).
383
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
RESPONSES OF A L S T R O E M E R I A 'REGINA' TO TEMPERATURE TREATMENTS PRIOR TO FLOWER-INDUCING TEMPERATURES
W.E. HEALY and H.F. WILKINS
Department of Horticultural Science and Landscape Architecture, University of Minnesota, Saint Paul, MN 55108 (U.S.A.) Scientific Journal Series Paper No. 11 733. Minnesota Agriculture Experiment Station (Accepted for publication 29 October 1981)
ABSTRACT Healy, W.E. and Wilkins, H.F., 1982. Responses of Alstroemeria 'Regina' to temperature treatments prior to flower-inducing temperatures. Scientia Hortic., 17: 383--390. Days to flower (DTF) were inversely related to the number of weeks (0--8) that
Alstroemeria 'Regina' plants remained at 5°C, a vernalizing temperature, before being moved to 13°C, a vernalizing as well as a forcing temperature. However, when the number of weeks at 5°C was added to the DTF, no difference in the total time to flower was observed between plants treated at 5° C or those grown continuously at 13° C, as they both induced flowering. One-year-old plants maintained at 21 ° C, a non-inductive temperature, and not divided prior to the 5°C treatments, showed an increase in total shoot production, and delayed DTF; compared to plants which were divided. When divided plants were maintained for 16 weeks at 21°C prior to 5°C treatments, total shoot production was reduced but flowering was accelerated compared to plants maintained for 8 weeks at 21°C after dividing. Total shoot and flowering-shoot production was not affected by increasing the durations of time at 5°C when plants were grown at 21°C and divided prior to this treatment. Thus, the pre-treatment of dividing or maintaining plants at 21°C prior to a 5° C treatment affected subsequent shoot production and DTF.
INTRODUCTION
Alstroemeria has recently received considerable attention as a potential new cut flower for glasshouse production in the northern United States because of its low fuel requirements, high yield per square meter, consumer interest and wholesale value. Researchers working with Alstroemeria during the past 10 years have concentrated on devising methods that could be used to promote earlier flowering. Promotion of earlier flowering is the primary method by which flower production could be increased since, regardless of previous treatment, all plants ceased flowering at the same time in the summer (Vonk Noordegraaf, 1975a). Extending the natural photoperiod during the winter to 13 h was shown to stimulate earlier 0304-4238/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
384 flowering (Vonk Noordegraaf, 1974b, 1975b; Heins and Wilkins, 1976, 1979). Our unpublished work from the forcing-year 1977--78 indicated that Alstroemeria 'Regina' grown at temperatures above 22°C failed to flower, regardless of photoperiod treatments. Subsequent work at the University of Norway verified our findings that 'Regina' would not flower at 21°C, although the photoperiods were not disclosed (Mourn, 1979). Other techniques used to increase flower production include earlier planting-dates (Baas et al., 1974; Wilkins et al., 1980b) and shoot thinning (Vonk Noordegraaf, 1975a; Heins and Wilkins, 1976). Stimulation of flower production by shoot thinning may decrease apical dominance and promote lateral rhizome growth (Heins and Wilkins, 1976). The increased flower production observed by earlier dividing of the rhizomes (Baas et al., 1974) and earlier planting (Wilkins et al., 1980b) may provide the plants with an adequate cold treatment prior to the start of the photoperiod treatment. Stinson (1942) reported that exposing Alstroemeria plants to natural cool temperatures in cold frames promoted earlier flowering. Vonk Noordegraaf (1975b) demonstrated that 9°C growing-temperatures promoted a greater ratio of flowering shoots to vegetative blind shoots compared to 25°C, but did not indicate the influence of the forcing-temperature on days-to-flower. Wilkins et al. (1980b) reported that rapid flowering occurred when vegetative 'Regina' plants were treated at 5°C for a minimum of 5 weeks when compared to the controls. Plants cooled for 5, 10 or 15 weeks had similar flowering-responses. The present experiments were undertaken to determine the minimum number of weeks (0--8 weeks) at 5°C needed to induce flowering in vegetative 'Regina' plants. These experiments were also _designed to determine the effect of one-year-old plants and the effect of a 21°C pre-treatment, which promotes vegetative growth, on the response of the plant to increased durations of 5°C treatments. MATERIALS AND METHODS Alstroemeria hybrida 'Regina' plants were grown under normal daylengths (ND) at temperatures above 21°C day/night (D/N) for 12 months to ensure that both shoots and rhizomes were in the vegetative stage. Plants for Experiments I and II were divided on 21 June 1978. Each division consisted of a single rhizome with attached storage roots and vegetative shoots. Divisions were placed singly in a 15-cm plastic pot filled with 1 p e a t : l perlite:l soil (v/v/v) medium. The plants were grown at 21°C in a glasshouse under ND (45 ° north parallel) until treatments began. Weak and old vegetative shoots were removed monthly to promote rhizome branching and thus new shoot formation (Heins and Wilkins, 1979). Soil tests were taken weekly throughout the course of the experiment and plants were fertilized to maintain the recommended nutrient levels.
385 Treatments for Experiment I began 8 weeks later, on 18 August 1978, when 25 plants were placed in a 5°C cooler and irradiated with 3.5 pE m -2 of fluorescent light (08.00 to 16.00). Five plants were removed every 2 weeks for the n e x t 8 weeks. The 0-week at 5°C control plants remained at 21°C D/N until 1 September, when they were transferred to the 13°C D/N glasshouse along with the 2-week 5°C-treated plants. Experiment II was identical to Experiment I except that the cold treatments began 8 weeks later, on 13 October 1978. This was done to determine if the time span between division and the start of the 5°C treatment would alter perception of the 5°C treatment. Three plants per t r e a t m e n t were used in Experiment II. The 0-week control plants were placed in the glasshouse at 13°C under ND on 27 October 1978, along with the first group of 5°C-treated plants. Experiment III dealt with age o f the plant prior to receiving the 5°C exposure treatments. Plants were grown under ND at 21°C for 12 months in 37.5-cm plastic pots. Plants were not divided prior to the 5°C treatments, which were identical to Experiment I. Treatment of 5°C began on 1 September 1978, with 1 plant per 2-week exposure. The 0-week control was placed in the 13°C glasshouse under ND on 15 September 1978, along with the first 5°C-treated plant. In all experiments, the n u m b e r of vegetative shoots present per plant was recorded when the plants were placed in the 13°C glasshouse. The n u m b e r of weak or old vegetative shoots removed m o n t h l y was recorded. All plants in Experiments I and II were replanted at the end of the 5°C t r e a t m e n t into 37.5-cm plastic pots using the same planting-medium. When plants in Experiments I, II and III were placed in the 13°C glasshouse, an incandescent (Inc) night interruption (NI) from 22.00 to 02.00 (10 #E m -2) was used to extend the photoperiod. Lighting was discontinued on 15 April 1979, when the ND was 13.5 h (Anonymous, 1945). Bottom heat was supplied as needed, so that a minimum soil temperature of 10°C and air temperature of 13°C were maintained. Experiments I, II and III were all conducted as a completely randomized design. Stems were harvested when the primary flowers on the c y m e were fully opened. Days to flowering (DTF) were calculated from the beginning of the 13°C forcingtemperature by averaging the date that the first 5 shoots of any treated plant flowered. The total n u m b e r of vegetative and flowering shoots per plant was recorded. Data collected, but not reported here, and available upon request, included n u m b e r of primary, secondary, tertiary and quartenary flowers per cymous whorl, stem length and number of leaves per stem. RESULTS A N D DISCUSSION
D a y s t o f l o w e r . - - In all experiments DTF decreased as the n u m b e r of weeks at 5°C increased (Table I). The correlations for DTF with the n u m b e r of
386 weeks at 5°C were -0.97, -0.86 and -0.76 for Experiments I, II and III, respectively. This inverse relationship of DTF with increasing exposureduration at 5°C is similar to that previously observed with Dicentra long~florum (Lopes and Weiler, 1977), Ldzurn "" " ~" (Wllkms, " " 1980), and the vernalization response in wheat (Levy and Peterson, 1972) and rye (Gott et al., 1955). Furthermore, in all of these genera, the cold treatment, when given in combination with a long photoperiod treatment, further decreased the DTF. The photoperiodic effect on DTF was also observed in Alstroemeria when plants are grown at 13°C (Vonk Noordegraaf, 1975b; Heins and Wilkins, 1979). As with Liliurn longiflorum (Wflkins, 1980), Alstroemeria flowered without additional low-temperature treatments provided the plants were forced at temperatures below 21°C, in this case 13°C (Table I). This suggests that the low temperatures necessary for flowering of Alstroemeria could be acquired at either 5 or 13°C. This hypothesis was further supported by manipulation of the DTF data. When DTF were calculated from the beginning of the 5 ° C-treatment, rather than from the time the plants were removed from the 5°C treatments, DTF for all treatments within an experiment were not statistically different. Commercially, Alstroemeria growers could decrease growing-temperatures to 5°C without adversely affecting DTF. In Europe, the present commercial schedule is to decrease temperatures to 10°C during December and January to enhance flowering in April--June (Verboom, 1979). This schedule could be modified for U.S. growers; 5°C growing-temperatures could be used during December and January, which would save energy and not affect DTF. TABLE I M e a n d a y s to f l o w e r for the first 5 f l o w e r i n g s h o o t s of AZstroemeria plants treated at 5°C for 0--8 weeks
Weeks at 5°C
Experiment I S t a r t e d 5°C t r e a t m e n t 8 w e e k s after division E x p e x ~ n e n t II S t a r t e d 5°C t r e a t m e n t 16 w e e k s after division E x p e r i m e n t III N o d i v i s i o n p r i o r to starting
HSD
0
2
4
6
8
204.8
205.6
189.6
172.6
155.4
2.7
188.2
--
163.2
156.6
165.2
6.0
220.2
215.6
201.8
191.8
203.4
7.7
5°C t r e a t m e n t -- = Treatment not included.
Shoot production. -- Vonk Noordegraaf (1975b) reported that Alstroemeria produced fewer shoots when grown at cooler temperatures, but an increased percentage of them flowered. Therefore, the effect of the 5°C treatments on total shoot production, as well as on shoots producing flowers, should
387 be of interest. When divided plants were given a 5°C treatment for 0--8 weeks, total s h o o t production was n o t affected in Experiments I or II (Fig. 1), nor was the number of flowering shoots affected. Therefore, 5°C treatments can be used to reduce fuel consumption w i t h o u t sacrificing flower yield. When undivided one-year-old plants were used (Experiment III) to determine the interaction of plant age with the 5°C treatments, different production statistics were observed (Fig. 1). Total shoot production was reduced as the duration of the 5°C treatment increased. This same response was observed with tiller growth in spring and winter wheat (Levy and Peterson, 1972). Thus, when Alstroemeria plants were n o t divided prior to 5°C treatments, total s h o o t production was reduced, contrary to what was observed when plants were divided. Previous w o r k b y Vonk Noordegraaf (1974a) indicated that maintaining non-inductive temperatures (25 ° C) for 8 additional weeks increased shoot production by 62%. He did n o t specify whether the increase in production was due to an increase in flowers or total shoots. We observed a decrease Experiment X 200
IOC ,sT J f J f - j
Experiment "n-
0~200 0 I (/3
'~ I00 rY LIJ m
Z 4O0
Experiment "nT
200
0
2
4
e
8
WEEKS of 5 °
Fig. 1. Total shoot (clear bar) and flower production (hatched bar) of Alstroemeria plants treated at 5°C for increasing durations of time (weeks) in Experiments I, II and III. Vertical lines are HSD 5% for Experiments I and II. No HSD for Experiment III.
388 in total shoot production between Experiments I and II, but no effect on flower production. Since flower production was not affected, the effect of an additional 8 weeks at 21°C (Experiment II) was to reduce the number of vegetative shoots prior to the start of flowering. However, giving plants 21°C for 16 weeks could be used to decrease the frequency of blind shoots which must be removed to stimulate flower production (Heins and Wilkins, 1976). The data obtained on DTF did not clearly resolve the question as to which factors interact to induce an Alstroemeria shoot to flower. Although the DTF data in Table I indicate a decrease in DTF with increasing exposure to 5°C, flowering occurs, for example, 155.4--205.6 days (Table I, Experiment I) after the treatment application, so that uncontrolled environmental factors may be influencing the results. For example, the forcingtemperature of 13°C may be retarding the rate of shoot elongation and growth (Healy and Wilkins, 1982) as was observed in Lilium (Wilkins et al., 1980a,c). They demonstrated that in lilies, increasing the forcing-temperature to 18°C reduced DTF and flower production. The naturally lengthening photoperiod and light intensity when Alstroerneria flower production begins may be promoting an increase in root growth, as shown by Weaver and Himmel (1929) in various genera, or in metabolite storage, as demonstrated by Milford and Lenton (1976) in sugar beets. The increased root growth and metabolite storage observed at cool soil temperature (Healy and Wilkins, 1979) could be involved with the flowering mechanism. Heins and Wilkins (1979) postulated that carbon storage would be necessary for flowering to occur, due to the accelerated rate of flower-shoot development. From the data presented here, carbon storage that may be utilized for flowering apparently comes from the carbon stored after the inductive temperatures have been given. When DTF from Experiments I or II are compared to Experiment III (Table I), one year of vegetative growth does not promote any earlier flowering than plants which were recently divided. We have observed that after the 5°C treatments and prior to flower-shoot development, storage-root growth and root enlargement occur. Concomitant to carbon storage, specific plant growth substances (PGS) may begin to accumulate in the plants and become conjugated with the stored carbon. Once a critical mass of metabolites and/or PGS have accumulated, flowering occurs. During the utilization of the stored metabolites, the conjugated PGS could become free and be involved in the flowering-response. From unpublished data, we observed that when plants were pre-treated for 8 weeks at 5°C, forced at 13°C under NI, and then received 1000 mg 1-1 Pro-Gibb or 50 mg 1-1 Ancymidol drench upon removal from the cooler, the resulting leaf and shoot morphology was radically different from untreated plants. Pro-Gibb-treated plants had elongated shoots with leaves longer than they were wide, which morphologically resembled a flowering shoot. Plants receiving Ancymidol produced short shoots with broad leaves
389 w h i c h r e s e m b l e v e g e t a t i v e s h o o t s g r o w n u n d e r s h o r t days, w h i c h delays flowering. A n c y m i d o l has b e e n s h o w n t o b l o c k gibberellin s y n t h e s i s (Coolbaugh and Hamilton, 1976). Thus, the Ancymidol and Pro-Gibb r e s p o n s e s s e e m r e a s o n a b l e . N e i t h e r P r o - G i b b - n o r A n c y m i d o l - t r e a t e d plants f l o w e r e d a n y earlier t h a n t h e c o n t r o l s , suggesting t h a t GA3 m a y b e i n v o l v e d w i t h f l o w e r - s h o o t e l o n g a t i o n b u t n o t f l o w e r i n i t i a t i o n , as was discussed b y C h o u a r d ( 1 9 6 0 ) . F u r t h e r w o r k is n e e d e d to d e t e r m i n e t h e i n v o l v e m e n t o f o t h e r P G S a n d s t o r e d c a r b o n in t h e f l o w e r i n g - m e c h a n i s m o f A I s t r o e m e r i a as t h e y i n t e r a c t w i t h l o w t e m p e r a t u r e s . ACKNOWLEDGEMENTS T h e a u t h o r s wish t o a c k n o w l e d g e t h e financial s u p p o r t o f t h e F r e d Gloeckner Foundation, a computing-grant from the University of Minnesota C o m p u t e r C e n t e r a n d t h e t e c h n i c a l assistance o f R a e q u e l R o b e r t s .
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