Planta
Planta 133, 103--106(1977)
9 by Springer-Verlag 1977
Nature of Light Requirement for the Flowering of Chenopodium rubrum L. (Ecotype 60 ~ 47' N) II. Post-induction Light Period*
Ramma Sawhney ** Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada
Abstract. Seedlings of C. rubrum were irradiated with different light qualities and intensities following a single inductive dark period. Our results show that relatively low intensity white light (35-100 ft. c.) does not support flower development while high intensity white light (650-800 ft. c.) permits 100% flowering. We have shown that the low intensity light inhibiton of flower development is not due to suboptimal photosynthesis. Relatively low intensities of light rich in far-red or blue wavebands sustains optimum flower development, whereas red light is totally ineffective in this respect. Considering that the intensity dependent High Energy Reaction (HER) has its action maxima in the blue and far-red we propose that HER may be positively involved in the flower development of C. rubrum, Our study further suggests that there may be some flower inhibitory component at play in relatively low intensity white light conditions and HER may be required to counteract this flower inhibitory effect. Key words: Chenopodium rubrum -- Flowering -High Energy Reaction -- Photosynthesis - Phytochrome.
Introduction
Relatively low intensity white light following inductive darkness is known to inhibit the flowering of * This paper constitutes a part of a Ph.D. thesis submitted to the University of Western Ontario, London, Ontario. ** Present address: Department of Biology, University of Saskatchewan, Saskatoon S7N OWO, Canada Abbreviations." S D = s h o r t day plant; H E R = H i g h Energy Reaction ; PvR = far-red absorbing form of phytochrome ; PR = red absorbing form of phytochrome; L.I.I.=-low intensity incandescent white light; H~I.I.=high intensity incandescent white light; L.I.F. =low intensity fluorescent white light; H,I.F. = high intensity fluorescent white light; GA3 =gibberellic acid
many short-day plants (Hamner, 1940; Schwabe, 1951; ; see Searle, 1961; Carr, 1957; Mukherjee, 1969). One of the postulates, which has been presented to explain the high intensity light requirement following inductive darkness, is that, it may be required to sustain a high level of photosynthesis (Hammer, 1940; Carr, 1957; see Searle, 1961). However, some SD plants like Peril& (see De Zeeuw, 1954), C. rubrum (Cumming, 1969) and Lemna perpusilla (see Hillman, 1965) initiate flowers when exposed to continuous low intensity light of specific qualityin conditions where photosynthesis is expected to be low. On the other hand Chenopodium rubrum and Lemna perpusilla are also known to require a relatively high intensity light exposure in experimental conditions where white light is used following inductive darkness (our preliminary observations; Schuster and Kandeler, 1970, respectively). An explanation based on photosynthetic requirement alone does not seem adequate to explain these observations. We have further investigated the nature of light requirement following inductive darkness for the flowering of C. rubrum L (ecotype 60 ~ 47' N, selection 374).
Materials and Methods Chenopodium rubrum seeds (ecotype 60 ~ 47' N, selection 374) were used in all experiments. Following the initial 3.5-day growth period (growth techniques-described elsewhere, Sawhney, 1977), the seedlings were exposed to high intensity incandescent light, 850-900 ft. c., for 24 h and then maintained in darkness for 12 h (inductive dark treatment). Following inductive darkness the seedlings were irradiated with different light qualities and intensities for 10 days and then assayed for flowering. The number of replicates used, procedure of observing the plants, calculations and presentation of the results are similar to those described in Sawhney (1977). Each final experiment was repeated at least twice to ensure the validity of the results. Light sources. Descriptions and light curves are similar to those in Sawheny (1977).
104
R. Sawhney: Light Requirement for Flowering. II
Experiments and Results 1. Flower Development in Low Intensity BCJ Light Chenopodium rubrum L. (60 ~ 47' N) seedlings show optimum flowering when irradiated continuously with low intensity BCJ light (35 ft. c., Cumming, 1969). BCJ bulbs emit a relatively low level of spectral energy in the blue or red regions of the spectrum and relatively high level of energy in the far-red (Fig. 3 t). As far-red has been shown to be inactive in photosynthesis (Emerson, 1958), the level of active photosynthesis is expecteed to be low in 35 ft. c. intensity of BCJ light. However our preliminary experiments showed that flowers developed at a greater rate in BCJ light (35 ft. c.) as compared to incandescent white light (800 ft. c.) or fluorescent white light (650 ft. c.) although the latter lights should theoretically sustain a much greater level of photosynthesis in plants (light curves, Fig. 31). We conducted experiments to ascertain the lower critical intensity of BCJ light which could support normal development of flowers in C. rubrum. Seedlings of C. rubrum were induced to flower by placing them in the darkness for 12 h. Different groups of seedlings were then irradiated with different intensitiess of BCJ light (5, 10, 20, 30, 35 ft. c. resp.) until assayed. The control plants were exposed to high intensity incandescent white light (800 ft. c.) following inductive darkness and these showed t00% flowering when assayed. There was 100% flowering in plants maintained in BCJ light (30-35 ft. c.) (Table 1) and the flowers were in a more advanced stage of development as compared to those maintained in high intensity incan1 Figures referred to in this paper are those of an adjoining paper (Sawhney 1977)
descent light. Seedlings irradiated with 20 ft. c. BCJ light all flowered but the rate of development of flowers was relatively slower. There was a rather low level of flowering in 10 and 5 ft. c. intensities of BCJ light. These results suggest that rate of photosynthesis occurring in seedlings placed in as low as 20 ft. c. BCJ light intensity was sufficient to sustain 100% flower development. To test if photosynthesis may have become limiting for flower development in low intensities of BCJ light (10 and 5 ft. c.), sugars were externally supplied in the medium (0.2 a n d 0.4M glucose or sucrose). All the plants maintained in 10 ft. c. BCJ flowered when sugars were supplied (Table 1). However, there was only 20-30% flowering in seedlings maintained in 5 ft. c. BCJ. These observations suggest that at least in seedlings maintained in 5 ft. c. BCJ light, some photoreaction other than photosynthesis may be limiting for flower development. A microscopic examination of the morphology of the seedlings maintained in 5 ft. c. BCJ light indicated that, the apical activity of these plants might have been low since there was virtually no epicotyl or leaf growth and the apex was buried deep within the cotyledons. The plants looked similar to those maintained in continuous darkness. Our earlier experiments (Sawhney and Cumming, 1975) showed that sucrose in conjunction with gibberellic acid (GA3) and kinetin was effective in stimulating epicotyl elongation as well as floral development in seedlings maintained in continuous darkness. We tested the effect of these compounds on the flowering of seedlings exposed to 5 ft. c. BCJ light. The compounds were supplied to the seedlings following inductive darkness (12 h), prior to transferring them into 5 ft. c. BCJ light. The seedlings treated with optimum concentra-
1. Flowering of C. rubrum seedlings exposed to different intensities of BCJ, broad band red, far-red, blue, incandescent and fluorescent white light following inductive darkness, with or without the application of sucrose, GA3 and kinetin
Table
Light treatment
BCJ 20, 30, 35 ft. c. BCJ 10 ft. c. BCJ 5 ft. c. High intensity incandescent white light (800 ft. c.) Low intensity incandescent white light (35-100 ft. c.) High intensity fluorescent white light (650 ft. c.) Low intensity fluorescent white light (35-100 ft. c.) Red (20 ft. c,) (1,17 btwatt/cm2 at 660 nm) Far-red (2 ft. c.) (8.28 gwatt]cm 2 at 725nm) Blue (35 ft. c.) (2.1 ixwatt/cm 2 at 437 nm) Blue (100 ft. c.) (5.4 lawatt/cm2 at 437 rim) Mean of 3 replicates. Figures. in brackets indicate standard error
% Flowering (Mean ~) Hoagland's
Sucrose (0.4 M)
Sucrose (0.4 M) GA3 (10 -6 M ) + kinetin (10-6 M)
lOO (o.o) 80.0 (o.o) 0.0 90.0 (0.0) 0 (0.0) 80.0 (5.7) 0 (0,0) 0.0 (0.0) 0.0 (0.0) 40.0 (0.0) 100.0 (0.0)
lOO (o.o) lOO (o.o) 30.0 (10.0) 90.0 (0.0) 10.0 (0.0) 90.0 (0.0) 0 (0.0) 0.0 (0.0) 20.0 (5.7) 80.0 (0.0) 100.0 (0.0)
lOO (o.o) lOO (o.o) 80.0 (5.7) 100 (0.0) 0 (0.0) 100 (0.0) 10 (0.0) 0.0 (0.0) 86.6 (3.3) 90.0 (10.0) 100.0 (0.0)
Sucrose (0.4 M) GA 3 (10 -7 M)+ kinetin (10-7 M)
1oo (o.o) lOO (o.o) 90.0 (0.0) 100 (0.0) 0 (0.0) 100 (0.0) 10 (0.0) 0,0 (0.0) 73.3 (6.6) 86.6 (3.3) 100.0 (0.0)
R. Sawhney:Light Requirementfor Flowering.II tions of GA3+ kinetin in combination with sucrose showed an elongated epicotyl and a high degree of flower development (Table 1). It can be inferred from the above experiments that a rather low but critical level of photosynthesis is necessary for flower development in C. rubrum. In very low,light intensities (5 ft. c.), flowering may be limited by nutritional and hormonal deficiency at the apex.
2. Flower Development in Low Intensity white Light We next tested, whether similar to low intensity BCJ light, relatively low intensities of white light could sustain flower development in C. rubrum. Four and a half-day old seedlings were induced to flower by placing them in the darkness (12 h) and were then exposed to different intensities of incandescent and fluorescent white light (light curves Figs. 1 and 3 in Sawhney, 1977). There was no flowering in seedlings placed in 35 or 100 ft. c. intensity of incandescent or fluorescent white lights in contrast to a high level of flowering in 650 ft. c. fluorescent or 800 ft. c. incandescent lights (Table 1). Application of sugars, GA3 and kinetin had no stimulatory effect on flower development in the low intensity white light conditions. No direct measurements of the photosynthetic capacity of the seedlings maintained under BCJ or white light were taken. However, taking into account the light energies emitted in the photosynthetically active bands of the light spectrum in each light condition, plants irradiated with 35 ft. c. of incandescent or fluorescent light could be presumed to photosynthesize at a higher rate as compared to those exposed to 10 ft. c. intensity of BCJ light (Fig. 1 in Sawhney, 1977). Thus, it seems unlikely that the lack of flower development in plants exposed to relatively low intensity of incandescent or fluorescent white light could be due to limitation imposed by low photosynthetic activity.
3. Flower development in low intensity blue, red, and far-red Since floral development in induced seedlings could not proceed in relatively low intensity white incandescent or fluorescent lights, whereas higher intensities of the same light qualities resulted in a high level of flowering (Table 1), the ineffectiveness of the low intensity lights could not be due to light quality per se (in terms of relative emission spectrum), but could be due to insufficient energy in specific wavebands. It is notable that 35 ft. c. intensity of BCJ light, which allowed a high degree of floral development in our experiments, emits a high proportion of energy in
105 the far-red (Fig. 3 in Sawhney, 1977). The action spectrum of the High Energy Reaction (HER), (Borthwick et al., 1969) shows peaks in the blue and far-red. We examined the possibility of the involvement of HER in the development of flowers in C. rubrum. Following a 12-h inductive dark period, seedlings were transferred to broad band blue (2.1, 5.4 gwatt/ cmz at 437 nm), far-red (8.28 ~twatt/cmz at 725 nm) or red (1.17 ixwatt/cm z at 660 nm) radiation (light curves, Figures2 and 3 in Sawhney, 1977) until assayed. There was no flowering in the seedlings irradiated with red or far-red, but, there was 100% flowering with the high intensity of blue and 40% flowering with the lower intensity blue light (Table 1). However, when growth medium was supplemented with sugars and/or GA 3 and kinetin, there was high level of flowering in seedlings maintained in the lower intensity blue or far-red radiation (Table 1). The seedlings irradiated with red light did not develop any flowers in any experimental conditions. Even increasing the intensity of red radiation two-folds did not lead to any flowering (not given in Table). Thus, far-red and blue wavebands sustain flower development provided sufficient energy substrates are available, but red light is totally ineffective in this respect. Discussion
1. Photosynthetic Requirement during the Post-induction Phase of Flower Development In C. rubrum seedlings maintained in prolonged low energy conditions (darkness, 5 ft. c. BCJ light and far-red) there is no leaf, epicotyl or flower formation suggesting a general lack of growth rathr than a specific inhibition of flowering. However, as low as 20 ft. c. BCJ light which should theoretically sustain a rather low level of photosynthesis in plants (due to little energy in wavebands other than far-red) supports flower development in C. rubrum. Thus, a relatively low critical level of photosynthetic energy input is sufficient to sustain flower development in C. rubrum.
2. Phytochrome (PvR) Requirement during the Post-induction Phase o f F lower Development Our earlier study showed that C. rubrum (60 ~ 47' N) seedlings flower in continuous darkness (with sucrose) with a single 5-min red or far-red interruption (Sawhney and Cumming, 1975). As there was no red/far-red reversibility in the above study and farred was as effective in sustaining flowering as red, it was concluded that relatively low levels of PFR saturated this response (Sawhney and Cumming, 1975). Our present study also shows that optimum flower development occurs in C. rubrum seedlings exposed
106 to light conditions which should theoretically establish relatively low levels of Pv~ (blue-40%, Butler et al., 1964; BCJ-21%, Evans et al., 1965). Thus, flower development in C. rubrum represents a developmental system which proceeds to completion with relatively low PFR levels in the cells.
R. Sawhney: Light Requirement for Flowering. II these conditions. The red reaction may or may not be PFR mediated. The effects of red light distinct from PFR have been suggested earlier (Hoshizaki and H a m mer, 1969; Andreae and Hopkins, 1973). To our knowledge, involvement of H E R in the development of flowers in SD plants has never been reported before. The above study was supported by NRC grant to Dr. B.G. Cum-
3. The High Energy Reaction Requirement during the Post-induction Phase o f Flower Development Our results suggest a positive involvement of H E R during the post-inductive phases of flower development in C. rubrum. The lack of flower development in the seedlings exposed to relatively low intensities of incandescent anf fluorescent white light may be caused by suboptimal H E R due to insufficient energy in the blue and/or far-red. Our earlier studies have shown that flowering can occur in continuous darkness, provided energy substrates and growth hormones are available (Sawhney and Cumming, 1975). In contrast, present study shows that there is a lack of flowering in low intensity white light (35-100 ft. c.), even with the application of sugars and/or growth hormones. This suggests that low intensity white light m a y be actively inhibitory to flowering of C. rubrum L. Since far-red and blue are p r o m o t o r y wavelengths, it seems possible that red radiation may be the inhibitory one. On the other hand, there is a high level of flowering when high intensity incandescent or fluorescent white lights follows inductive darkness, despite the fact that these light sources emit relatively high level of energy in the red (Fig. 3 in Sawhney, 1977). Also, although BCJ light (35 ft. c.) has approximately equal energy at 660 n m wavelength (same Fig.) as low intensity fluorescent white light (Fig. 1 in Sawhney, 1977), BCJ light sustains a high level of flowering in contrast to little flowering with low intensity fluorescent white light. We suggest that flower development may be regulated by interaction between some red-driven, flower inhibitory, and far-red or blue-driven (HER), flower p r o m o t o r y , photoreactions. All our results can be explained on the basis of the above proposal. In high intensity white light or BCJ light conditions, H E R may overcome the inhibitory effect of red radiation and thus lead to high level of flowering. Since, in darkness, there is no flower inhibitory effect in play, H E R is not necessary and flower development can proceed in the plants if optimal supply of sugars and growth hormones is available to allow sufficiently high apical activity (Sawhney and Cumming, 1975). Similarly, the high level of flowering with blue and far-red following darkness, m a y be due to lack of red inhibition. H E R may not even be required in
ming of Western Onatrio, London, Ontario. The author is grateful to him for support and helpful discussions. The author is indebted to Dr. V.K. Sawhney and Mrs. S. Andreae for very stimulating discussions and encouragement at all the stages of the above work. References Andreae, S., Hopkins, W.G. : Interaction of abscicic acid and red light during induction of flowering of Chenopodium rubrum. Plant Physiol. 51, Suppl. 158, 29 (1973) Borthwick, H.A., Hendricks, S.B., Schneider, M.J., Taylorson, H.B., Toole, B.K.: The high energy light action controlling plant responses and development. Proc. nat. Acad. Sci. (Wash.) 64, 479-486 (1969) Butler, W.L., Hendricks, S.B., Siegelman, H.W.: Action spectra of phytochrome in vitro. Photochem. Photobiol. 3, 521-528
(1964) Carr, D.J. : On the nature of photoperiodic induction IV. Preliminary experiments on the effect of light following the inductive long dark period in Xanthium pennsylvanicum. Physiol, Plant. 10, 249-265 (1957) Cumming, B.G.: Photoperiodism and rhythmic flower induction: complete substitution of inductive darkness by light. Canad. J. Bot. 47, 1241-1250 (1969) Emerson, R.: The quantum yield of photosynthesis. Ann. Rev. Plant Physiol. 9, 1-24 (1958) Evans, L.T., Hendricks, S.B., Borthwick, H,A.: The role of light in suppressing hypocotyl elongation in lettuce and petunia. Planta 64, 201-218 (1965) Hamner, K.C. : Interrelation of light and darkness in photoperiodic induction. Bot. Gaz. 101,658-687 (1940) Hillman, W.S. : Red light, blue light and copper ion in the photoperiod control of flowering in Lemna perpusitta 6746. Plant and Cell Physiol. 6, 499-506 (1965) Hoshizaki, T., Hamner, K.C.: Interactions in plant photoperiodism. Photochem. Photobiol. 10, 87-96 (1969) Mukherjee, I. : Effect of sucrose and gibberellic acid on floral induction of Xanthium. Physiol. Plant. 22, 694-700 (1969) Sawhney, R.: Nature of light requirement for the flowering of Chenopodium rubrum L. (ecotype 60~ 47' N) I. Pre-induction light period. Planta 133, 97-102 (1977) Sawhney, R., Cumming, B.G.: Non-photosynthetic light requirment for the development of flowers in Chenopodium rubrum, a short-day plant. Canad. J. Bot. 53, 512-516 (1975) Schuster, M., Kandeler, R.: The significance of photosynthesis for long-day flowering of the short-day plant Lemna perpusilIa 6746. Z. Pflanzenphysiol. 63, 308-313 (1970) Schwabe, W.W,: Factors controlling flowering in the Chrysanthemum. II. Day length effects on the further development of influorescence buds and their experimental reversal and modification. J. expt. Biol. 2, 223-237 (1951) Searle, N.E.: Persistence and transport of flowering stimulus in Xanthium. Plant Physiol. 36, 656 662 (1961) Zeeuw, D. de. : De invloed van het blad op de bloei. Meded Landbouwhogeschool, Wageningen 54, I (1954) Received 10 February; accepted 16 August I976
Planta 133, 103--106(1977)
9 by Springer-Verlag 1977
Nature of Light Requirement for the Flowering of Chenopodium rubrum L. (Ecotype 60 ~ 47' N) II. Post-induction Light Period*
Ramma Sawhney ** Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada
Abstract. Seedlings of C. rubrum were irradiated with different light qualities and intensities following a single inductive dark period. Our results show that relatively low intensity white light (35-100 ft. c.) does not support flower development while high intensity white light (650-800 ft. c.) permits 100% flowering. We have shown that the low intensity light inhibiton of flower development is not due to suboptimal photosynthesis. Relatively low intensities of light rich in far-red or blue wavebands sustains optimum flower development, whereas red light is totally ineffective in this respect. Considering that the intensity dependent High Energy Reaction (HER) has its action maxima in the blue and far-red we propose that HER may be positively involved in the flower development of C. rubrum, Our study further suggests that there may be some flower inhibitory component at play in relatively low intensity white light conditions and HER may be required to counteract this flower inhibitory effect. Key words: Chenopodium rubrum -- Flowering -High Energy Reaction -- Photosynthesis - Phytochrome.
Introduction
Relatively low intensity white light following inductive darkness is known to inhibit the flowering of * This paper constitutes a part of a Ph.D. thesis submitted to the University of Western Ontario, London, Ontario. ** Present address: Department of Biology, University of Saskatchewan, Saskatoon S7N OWO, Canada Abbreviations." S D = s h o r t day plant; H E R = H i g h Energy Reaction ; PvR = far-red absorbing form of phytochrome ; PR = red absorbing form of phytochrome; L.I.I.=-low intensity incandescent white light; H~I.I.=high intensity incandescent white light; L.I.F. =low intensity fluorescent white light; H,I.F. = high intensity fluorescent white light; GA3 =gibberellic acid
many short-day plants (Hamner, 1940; Schwabe, 1951; ; see Searle, 1961; Carr, 1957; Mukherjee, 1969). One of the postulates, which has been presented to explain the high intensity light requirement following inductive darkness, is that, it may be required to sustain a high level of photosynthesis (Hammer, 1940; Carr, 1957; see Searle, 1961). However, some SD plants like Peril& (see De Zeeuw, 1954), C. rubrum (Cumming, 1969) and Lemna perpusilla (see Hillman, 1965) initiate flowers when exposed to continuous low intensity light of specific qualityin conditions where photosynthesis is expected to be low. On the other hand Chenopodium rubrum and Lemna perpusilla are also known to require a relatively high intensity light exposure in experimental conditions where white light is used following inductive darkness (our preliminary observations; Schuster and Kandeler, 1970, respectively). An explanation based on photosynthetic requirement alone does not seem adequate to explain these observations. We have further investigated the nature of light requirement following inductive darkness for the flowering of C. rubrum L (ecotype 60 ~ 47' N, selection 374).
Materials and Methods Chenopodium rubrum seeds (ecotype 60 ~ 47' N, selection 374) were used in all experiments. Following the initial 3.5-day growth period (growth techniques-described elsewhere, Sawhney, 1977), the seedlings were exposed to high intensity incandescent light, 850-900 ft. c., for 24 h and then maintained in darkness for 12 h (inductive dark treatment). Following inductive darkness the seedlings were irradiated with different light qualities and intensities for 10 days and then assayed for flowering. The number of replicates used, procedure of observing the plants, calculations and presentation of the results are similar to those described in Sawhney (1977). Each final experiment was repeated at least twice to ensure the validity of the results. Light sources. Descriptions and light curves are similar to those in Sawheny (1977).
104
R. Sawhney: Light Requirement for Flowering. II
Experiments and Results 1. Flower Development in Low Intensity BCJ Light Chenopodium rubrum L. (60 ~ 47' N) seedlings show optimum flowering when irradiated continuously with low intensity BCJ light (35 ft. c., Cumming, 1969). BCJ bulbs emit a relatively low level of spectral energy in the blue or red regions of the spectrum and relatively high level of energy in the far-red (Fig. 3 t). As far-red has been shown to be inactive in photosynthesis (Emerson, 1958), the level of active photosynthesis is expecteed to be low in 35 ft. c. intensity of BCJ light. However our preliminary experiments showed that flowers developed at a greater rate in BCJ light (35 ft. c.) as compared to incandescent white light (800 ft. c.) or fluorescent white light (650 ft. c.) although the latter lights should theoretically sustain a much greater level of photosynthesis in plants (light curves, Fig. 31). We conducted experiments to ascertain the lower critical intensity of BCJ light which could support normal development of flowers in C. rubrum. Seedlings of C. rubrum were induced to flower by placing them in the darkness for 12 h. Different groups of seedlings were then irradiated with different intensitiess of BCJ light (5, 10, 20, 30, 35 ft. c. resp.) until assayed. The control plants were exposed to high intensity incandescent white light (800 ft. c.) following inductive darkness and these showed t00% flowering when assayed. There was 100% flowering in plants maintained in BCJ light (30-35 ft. c.) (Table 1) and the flowers were in a more advanced stage of development as compared to those maintained in high intensity incan1 Figures referred to in this paper are those of an adjoining paper (Sawhney 1977)
descent light. Seedlings irradiated with 20 ft. c. BCJ light all flowered but the rate of development of flowers was relatively slower. There was a rather low level of flowering in 10 and 5 ft. c. intensities of BCJ light. These results suggest that rate of photosynthesis occurring in seedlings placed in as low as 20 ft. c. BCJ light intensity was sufficient to sustain 100% flower development. To test if photosynthesis may have become limiting for flower development in low intensities of BCJ light (10 and 5 ft. c.), sugars were externally supplied in the medium (0.2 a n d 0.4M glucose or sucrose). All the plants maintained in 10 ft. c. BCJ flowered when sugars were supplied (Table 1). However, there was only 20-30% flowering in seedlings maintained in 5 ft. c. BCJ. These observations suggest that at least in seedlings maintained in 5 ft. c. BCJ light, some photoreaction other than photosynthesis may be limiting for flower development. A microscopic examination of the morphology of the seedlings maintained in 5 ft. c. BCJ light indicated that, the apical activity of these plants might have been low since there was virtually no epicotyl or leaf growth and the apex was buried deep within the cotyledons. The plants looked similar to those maintained in continuous darkness. Our earlier experiments (Sawhney and Cumming, 1975) showed that sucrose in conjunction with gibberellic acid (GA3) and kinetin was effective in stimulating epicotyl elongation as well as floral development in seedlings maintained in continuous darkness. We tested the effect of these compounds on the flowering of seedlings exposed to 5 ft. c. BCJ light. The compounds were supplied to the seedlings following inductive darkness (12 h), prior to transferring them into 5 ft. c. BCJ light. The seedlings treated with optimum concentra-
1. Flowering of C. rubrum seedlings exposed to different intensities of BCJ, broad band red, far-red, blue, incandescent and fluorescent white light following inductive darkness, with or without the application of sucrose, GA3 and kinetin
Table
Light treatment
BCJ 20, 30, 35 ft. c. BCJ 10 ft. c. BCJ 5 ft. c. High intensity incandescent white light (800 ft. c.) Low intensity incandescent white light (35-100 ft. c.) High intensity fluorescent white light (650 ft. c.) Low intensity fluorescent white light (35-100 ft. c.) Red (20 ft. c,) (1,17 btwatt/cm2 at 660 nm) Far-red (2 ft. c.) (8.28 gwatt]cm 2 at 725nm) Blue (35 ft. c.) (2.1 ixwatt/cm 2 at 437 nm) Blue (100 ft. c.) (5.4 lawatt/cm2 at 437 rim) Mean of 3 replicates. Figures. in brackets indicate standard error
% Flowering (Mean ~) Hoagland's
Sucrose (0.4 M)
Sucrose (0.4 M) GA3 (10 -6 M ) + kinetin (10-6 M)
lOO (o.o) 80.0 (o.o) 0.0 90.0 (0.0) 0 (0.0) 80.0 (5.7) 0 (0,0) 0.0 (0.0) 0.0 (0.0) 40.0 (0.0) 100.0 (0.0)
lOO (o.o) lOO (o.o) 30.0 (10.0) 90.0 (0.0) 10.0 (0.0) 90.0 (0.0) 0 (0.0) 0.0 (0.0) 20.0 (5.7) 80.0 (0.0) 100.0 (0.0)
lOO (o.o) lOO (o.o) 80.0 (5.7) 100 (0.0) 0 (0.0) 100 (0.0) 10 (0.0) 0.0 (0.0) 86.6 (3.3) 90.0 (10.0) 100.0 (0.0)
Sucrose (0.4 M) GA 3 (10 -7 M)+ kinetin (10-7 M)
1oo (o.o) lOO (o.o) 90.0 (0.0) 100 (0.0) 0 (0.0) 100 (0.0) 10 (0.0) 0,0 (0.0) 73.3 (6.6) 86.6 (3.3) 100.0 (0.0)
R. Sawhney:Light Requirementfor Flowering.II tions of GA3+ kinetin in combination with sucrose showed an elongated epicotyl and a high degree of flower development (Table 1). It can be inferred from the above experiments that a rather low but critical level of photosynthesis is necessary for flower development in C. rubrum. In very low,light intensities (5 ft. c.), flowering may be limited by nutritional and hormonal deficiency at the apex.
2. Flower Development in Low Intensity white Light We next tested, whether similar to low intensity BCJ light, relatively low intensities of white light could sustain flower development in C. rubrum. Four and a half-day old seedlings were induced to flower by placing them in the darkness (12 h) and were then exposed to different intensities of incandescent and fluorescent white light (light curves Figs. 1 and 3 in Sawhney, 1977). There was no flowering in seedlings placed in 35 or 100 ft. c. intensity of incandescent or fluorescent white lights in contrast to a high level of flowering in 650 ft. c. fluorescent or 800 ft. c. incandescent lights (Table 1). Application of sugars, GA3 and kinetin had no stimulatory effect on flower development in the low intensity white light conditions. No direct measurements of the photosynthetic capacity of the seedlings maintained under BCJ or white light were taken. However, taking into account the light energies emitted in the photosynthetically active bands of the light spectrum in each light condition, plants irradiated with 35 ft. c. of incandescent or fluorescent light could be presumed to photosynthesize at a higher rate as compared to those exposed to 10 ft. c. intensity of BCJ light (Fig. 1 in Sawhney, 1977). Thus, it seems unlikely that the lack of flower development in plants exposed to relatively low intensity of incandescent or fluorescent white light could be due to limitation imposed by low photosynthetic activity.
3. Flower development in low intensity blue, red, and far-red Since floral development in induced seedlings could not proceed in relatively low intensity white incandescent or fluorescent lights, whereas higher intensities of the same light qualities resulted in a high level of flowering (Table 1), the ineffectiveness of the low intensity lights could not be due to light quality per se (in terms of relative emission spectrum), but could be due to insufficient energy in specific wavebands. It is notable that 35 ft. c. intensity of BCJ light, which allowed a high degree of floral development in our experiments, emits a high proportion of energy in
105 the far-red (Fig. 3 in Sawhney, 1977). The action spectrum of the High Energy Reaction (HER), (Borthwick et al., 1969) shows peaks in the blue and far-red. We examined the possibility of the involvement of HER in the development of flowers in C. rubrum. Following a 12-h inductive dark period, seedlings were transferred to broad band blue (2.1, 5.4 gwatt/ cmz at 437 nm), far-red (8.28 ~twatt/cmz at 725 nm) or red (1.17 ixwatt/cm z at 660 nm) radiation (light curves, Figures2 and 3 in Sawhney, 1977) until assayed. There was no flowering in the seedlings irradiated with red or far-red, but, there was 100% flowering with the high intensity of blue and 40% flowering with the lower intensity blue light (Table 1). However, when growth medium was supplemented with sugars and/or GA 3 and kinetin, there was high level of flowering in seedlings maintained in the lower intensity blue or far-red radiation (Table 1). The seedlings irradiated with red light did not develop any flowers in any experimental conditions. Even increasing the intensity of red radiation two-folds did not lead to any flowering (not given in Table). Thus, far-red and blue wavebands sustain flower development provided sufficient energy substrates are available, but red light is totally ineffective in this respect. Discussion
1. Photosynthetic Requirement during the Post-induction Phase of Flower Development In C. rubrum seedlings maintained in prolonged low energy conditions (darkness, 5 ft. c. BCJ light and far-red) there is no leaf, epicotyl or flower formation suggesting a general lack of growth rathr than a specific inhibition of flowering. However, as low as 20 ft. c. BCJ light which should theoretically sustain a rather low level of photosynthesis in plants (due to little energy in wavebands other than far-red) supports flower development in C. rubrum. Thus, a relatively low critical level of photosynthetic energy input is sufficient to sustain flower development in C. rubrum.
2. Phytochrome (PvR) Requirement during the Post-induction Phase o f F lower Development Our earlier study showed that C. rubrum (60 ~ 47' N) seedlings flower in continuous darkness (with sucrose) with a single 5-min red or far-red interruption (Sawhney and Cumming, 1975). As there was no red/far-red reversibility in the above study and farred was as effective in sustaining flowering as red, it was concluded that relatively low levels of PFR saturated this response (Sawhney and Cumming, 1975). Our present study also shows that optimum flower development occurs in C. rubrum seedlings exposed
106 to light conditions which should theoretically establish relatively low levels of Pv~ (blue-40%, Butler et al., 1964; BCJ-21%, Evans et al., 1965). Thus, flower development in C. rubrum represents a developmental system which proceeds to completion with relatively low PFR levels in the cells.
R. Sawhney: Light Requirement for Flowering. II these conditions. The red reaction may or may not be PFR mediated. The effects of red light distinct from PFR have been suggested earlier (Hoshizaki and H a m mer, 1969; Andreae and Hopkins, 1973). To our knowledge, involvement of H E R in the development of flowers in SD plants has never been reported before. The above study was supported by NRC grant to Dr. B.G. Cum-
3. The High Energy Reaction Requirement during the Post-induction Phase o f Flower Development Our results suggest a positive involvement of H E R during the post-inductive phases of flower development in C. rubrum. The lack of flower development in the seedlings exposed to relatively low intensities of incandescent anf fluorescent white light may be caused by suboptimal H E R due to insufficient energy in the blue and/or far-red. Our earlier studies have shown that flowering can occur in continuous darkness, provided energy substrates and growth hormones are available (Sawhney and Cumming, 1975). In contrast, present study shows that there is a lack of flowering in low intensity white light (35-100 ft. c.), even with the application of sugars and/or growth hormones. This suggests that low intensity white light m a y be actively inhibitory to flowering of C. rubrum L. Since far-red and blue are p r o m o t o r y wavelengths, it seems possible that red radiation may be the inhibitory one. On the other hand, there is a high level of flowering when high intensity incandescent or fluorescent white lights follows inductive darkness, despite the fact that these light sources emit relatively high level of energy in the red (Fig. 3 in Sawhney, 1977). Also, although BCJ light (35 ft. c.) has approximately equal energy at 660 n m wavelength (same Fig.) as low intensity fluorescent white light (Fig. 1 in Sawhney, 1977), BCJ light sustains a high level of flowering in contrast to little flowering with low intensity fluorescent white light. We suggest that flower development may be regulated by interaction between some red-driven, flower inhibitory, and far-red or blue-driven (HER), flower p r o m o t o r y , photoreactions. All our results can be explained on the basis of the above proposal. In high intensity white light or BCJ light conditions, H E R may overcome the inhibitory effect of red radiation and thus lead to high level of flowering. Since, in darkness, there is no flower inhibitory effect in play, H E R is not necessary and flower development can proceed in the plants if optimal supply of sugars and growth hormones is available to allow sufficiently high apical activity (Sawhney and Cumming, 1975). Similarly, the high level of flowering with blue and far-red following darkness, m a y be due to lack of red inhibition. H E R may not even be required in
ming of Western Onatrio, London, Ontario. The author is grateful to him for support and helpful discussions. The author is indebted to Dr. V.K. Sawhney and Mrs. S. Andreae for very stimulating discussions and encouragement at all the stages of the above work. References Andreae, S., Hopkins, W.G. : Interaction of abscicic acid and red light during induction of flowering of Chenopodium rubrum. Plant Physiol. 51, Suppl. 158, 29 (1973) Borthwick, H.A., Hendricks, S.B., Schneider, M.J., Taylorson, H.B., Toole, B.K.: The high energy light action controlling plant responses and development. Proc. nat. Acad. Sci. (Wash.) 64, 479-486 (1969) Butler, W.L., Hendricks, S.B., Siegelman, H.W.: Action spectra of phytochrome in vitro. Photochem. Photobiol. 3, 521-528
(1964) Carr, D.J. : On the nature of photoperiodic induction IV. Preliminary experiments on the effect of light following the inductive long dark period in Xanthium pennsylvanicum. Physiol, Plant. 10, 249-265 (1957) Cumming, B.G.: Photoperiodism and rhythmic flower induction: complete substitution of inductive darkness by light. Canad. J. Bot. 47, 1241-1250 (1969) Emerson, R.: The quantum yield of photosynthesis. Ann. Rev. Plant Physiol. 9, 1-24 (1958) Evans, L.T., Hendricks, S.B., Borthwick, H,A.: The role of light in suppressing hypocotyl elongation in lettuce and petunia. Planta 64, 201-218 (1965) Hamner, K.C. : Interrelation of light and darkness in photoperiodic induction. Bot. Gaz. 101,658-687 (1940) Hillman, W.S. : Red light, blue light and copper ion in the photoperiod control of flowering in Lemna perpusitta 6746. Plant and Cell Physiol. 6, 499-506 (1965) Hoshizaki, T., Hamner, K.C.: Interactions in plant photoperiodism. Photochem. Photobiol. 10, 87-96 (1969) Mukherjee, I. : Effect of sucrose and gibberellic acid on floral induction of Xanthium. Physiol. Plant. 22, 694-700 (1969) Sawhney, R.: Nature of light requirement for the flowering of Chenopodium rubrum L. (ecotype 60~ 47' N) I. Pre-induction light period. Planta 133, 97-102 (1977) Sawhney, R., Cumming, B.G.: Non-photosynthetic light requirment for the development of flowers in Chenopodium rubrum, a short-day plant. Canad. J. Bot. 53, 512-516 (1975) Schuster, M., Kandeler, R.: The significance of photosynthesis for long-day flowering of the short-day plant Lemna perpusilIa 6746. Z. Pflanzenphysiol. 63, 308-313 (1970) Schwabe, W.W,: Factors controlling flowering in the Chrysanthemum. II. Day length effects on the further development of influorescence buds and their experimental reversal and modification. J. expt. Biol. 2, 223-237 (1951) Searle, N.E.: Persistence and transport of flowering stimulus in Xanthium. Plant Physiol. 36, 656 662 (1961) Zeeuw, D. de. : De invloed van het blad op de bloei. Meded Landbouwhogeschool, Wageningen 54, I (1954) Received 10 February; accepted 16 August I976
