Carotenoid composition in milo (Sorghum vulgare) shoots as affected by phytochrome and chlorophyll

The composition of coloured carotenoids in the milo shoot was investigated quantitatively (high performance liquid chromatography) during light-mediated plastidogenesis, including the time span of photodelay as caused by medium and high light fluxes. It was found that as long as only the far-red-absorbing form of phytochrome operates, the carotenoid pattern remains virtually the same as in complete darkness (violaxanthin and lutein as major constituents, traces of -carotene). On the other hand, the pattern changes dramatically in white or red light with increasing amounts of chlorophyll (lutein and -carotene dominate, -carotene showing the strongest relative increase). Photodelay during the early phase of plastidogenesis affects the carotenoid composition strongly. Increase of neoxanthin, violaxanthin and -carotene contents are diminished while lutein accumulation proves resistant towards chlorophyll-mediated photoinhibition. The photodelay can be diminished by an appropriate light pretreatment. The data indicate that light-mediated control over carotenoid accumulation is exerted at three levels: i) a coarse control through phytochrome, ii) fine tuning in connection with chlorophyll accumulation, iii) stabilization of holocomplexes against photodecomposition.Abbreviations GG14 high fluence rate green-yellow light - HPLC high-performance liquid chromatography - Chl chlorophyll - WL_w weak white light (1200 lx) - WL_m medium flux white light (12000 lx)     

Specific action of blue light on phytochrome-mediated enzyme syntheses in the shoot of milo (Sorghum vulgare Pers.)

Abstract. This paper describes the effect of prolonged treatments with red or blue light on the capacity of the milo ( Sorghum vulgare Pers.) shoot to respond to Pfr in subsequent darkness. Two groups of enzymes were studied. In group I (NADP-dependent glyceraldehyde-3-phosphate dehydrogenase, NADP-GPD. EC 1.2.1.13 and ribulose-bisphosphate carboxylase, carboxylase, EC 4.1.1.39) enzyme formation is strongly enhanced by red light pulses (operating through phytochrome) whereas in group II (NAD-dependent glyceraldehyde-3-phosphate dehydrogenase, NAD-GPD, EC 1.2.1.12 and NAD-dependent malate dehydrogenase, MDH. EC 1.1.1.37) enzyme formation hardly responds to red light pulses.
In group 1 a 24-h treatment with blue light (but not with red light) leads to a strong increase in responsivity to Pfr whereas in group II a 24-h treatment with blue or red light does not increase responsivity to Pfr in subsequent darkness.
The specific effect of blue light cannot be explained by an effect of light on gross protein synthesis. Rather, the data indicate that amplification of responsivity to Pfr by blue light is a specific process directly related to the mechanism of modulation of gene expression by phytochrome.     

Analysis of light-controlled accumulation of carotenoids in mustard (Sinapis alba L.) seedlings

Carotenoid accumulation in the cotyledons of the mustard seedling (Sinapis alba L.) is controlled by light. Besides the stimulatory function of phytochrome in carotenogenesis the experiments reveal the significance of chlorophyll accumulation for the accumulation of larger amounts of acrotenoids. A specific blue light effect was not found. The data suggest that light exerts its control over carotenoid biogenesis through two separate mechanisms: A phytochrome regulation of enzyme levels before a postulated pool of free carotenoids, and a regulation by chlorophyll draining the pool by complex-formation.Abbreviations Chl chlorophyll(s) - PChl protochlorophyll(ide) - HIR high irradiance reaction (of phytochrome) - P_fr far-red absorbing, physiologically active form of phytochrome - P_r red absorbing, physiologically inactive form of phytochrome - P_fof total phytochrome, i.e. [P_r]+[P_fr] - [P_fr]/[P_fof], wavelength dependent photoequilibrium of the phytochrome system - red red light - fr far-red light     

Responsivity amplification by light in phytochrome-mediated induction of chloroplast glyceraldehyde-3-phosphate dehydrogenase (NADP-dependent, EC 1.2.1.13) in the shoot of milo (Sorghum vulgare Pers.)

Abstract. A prolonged light treatment strongly increases responsivity to Pfr in many instances of phytochrome-controlled biogenesis of flavone or cyanidin glycosides. The present investigation deals with the question of whether light also leads to a corresponding increase of responsivity towards Pfr in such photoresponses which are not related to synthesis of flavonoid pigments in outer tissue layers of seedlings. Phytochrome-mediated accumulation of the chloroplast GPD (glyceraldehyde-3-phosphate dehydrogenase, EC 1.2.1.13) was chosen as a response and the milo shoot ( Sorghum vulgare Pers. cv. Weider, hybrid) as an appropriate subject. It was found that responsivity towards Pfr is extremely weak in a dark-grown shoot while prolonged light pretreatments lead to a dramatic increase in responsivity. Blue and UV light are far more effective than red light in eliciting this effect within a few hours. High responsivity is only maintained in the light. When the seedlings are placed in darkness the level of responsivity drops rapidly with a half-life of the order of 2 h. The data allow more complete explanations for intriguing phenomena of plant life under natural light/dark conditions such as shade detection or sensing of light → dark transitions.     

External calcium requirements for light induction of chlorophyll accumulation and its enhancement by red light and cytokinin pretreatments in excised etiolated cucumber cotyledons

Excised etiolated cucumber (Cucumis sativus L.) cotyledons that were depleted of external Ca²⁺ by equilibration with a Ca²⁺ buffer, which maintained the free Ca²⁺ concentration at 10^–8 M, failed to accumulate chlorophyll upon a 2-h exposure to white light. Increasing amounts of chlorophyll accumulation occurred at increasing external Ca²⁺ concentrations within the range of 10^–7-10^–3 M. Preillumination with red light or pretreatment with benzyladenine, which enhanced the rate of light-induced chlorophyll accumulation in control cotyledons, did not overcome the block to light-induced chlorophyll accumulation caused by the depletion of external Ca²⁺. Etiolated cotyledons that were treated with the Ca²⁺ ionophore, A23187, and then equilibrated with 10^–5 M Ca²⁺, accumulated significantly more chlorophyll during exposure to light than did untreated cotyledons. The enhancing effect of A23187 was approximately equal to that caused by red-light pretreatment. Etiolated cotyledons that were exposed to the Ca²⁺ channel-blocking agent, Nd³⁺ (neodymium), in the presence of 10^–5 M Ca²⁺, did not exhibit an enhancement of chlorophyll accumulation by red-light pretreatment, although they accumulated control levels of chlorophyll upon exposure to light and showed control levels of enhancement of chlorophyll accumulation by cytokinin pretreatment. Conversely, etiolated cotyledons that were equilibrated with 10^–5 M Ca²⁺ in the presence of nifedipine, a blocker of some Ca²⁺ channels, did not exhibit an enhancement of chlorophyll accumulation by cytokinin pretreatment, although they accumulated control levels of chlorophyll upon exposure to light and showed control levels of enhancement of chlorophyll accumulation by red-light pretreatment. These results indicate that external Ca²⁺ is required for chlorophyll accumulation by excised etiolated cucumber cotyledons during the first 2 h of light exposure, and that an influx of external Ca²⁺ is required for the enhancing effect of redlight and cytokinin. The differential abilities of Nd³⁺ and nifedipine to block the effects of red-light and cytokinin pretreatments suggests that enhancement of chlorophyll accumulation by red-light and cytokinin may involve different classes of Ca²⁺ channels.Abbreviations A23187 antibiotic 23187 calcium ionophore - Chl chlorophyll - nifedipine 1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinedicarboxylic acid dimethyl ester We thank Randy Wayne for advice and encouragement.     

Action of red light on indole-3-acetic-acid status and growth in coleoptiles of etiolated maize seedlings

Brief irradiation of intact etiolated seedlings of maize (Zea mays L.) with red light (R; 30 W cm^-2, 10 min) reduces the amounts of diffusible and free (solvent-extractable) indole-3-acetic acid (IAA) obtainable from excised coleoptile tips. The effect is transient, the lowest level (30% of the dark control) occurring at about 3 h after irradiation. The free-IAA content of the whole coleoptile and the diffusible-IAA yield from the base of the same organ are similarly reduced, whereas the conjugated-IAA content of the coleoptile is not affected. These results support the view that R inhibits the production of IAA at the coleoptile tip. It is further shown that R inhibits biosynthesis of [³H]IAA from [³H]tryptophan supplied to the coleoptile tip. The shapes of the fluence-response curves obtained for the reduction of the diffusible-IAA yield by R and far-red light (FR) indicate the participation of two photoreactive systems. One has thresholds at 10^-3 W s cm² of R, five orders of magnitude less than the minimum required for the appearance of spectrophotometrically measurable far-red-absorbing form of phytochrome (Pfr) in vivo, and 10^-1 W s cm^-2 of FR; its response is linear to the logarithm of fluence exceeding five orders of magnitude. The other system is seen above 10² W s cm^-2 as an increase in the slope of the fluenceresponse curve; its response is FR reversible and related to the Pfr level of total photoreversible phytochrome. Both systems inhibit biosynthesis of IAA from tryptophan. Elongation of the coleoptile is stimulated by R; the stimulation is most apparent in the apical region, and is saturated with a fluence at which bo detectable pfr is formed. Farred light can also saturate this response. Since the endogenous IAA concentration in the coleoptile appears not to be in the inhibitory range, it is concluded that the stimulation of coleoptile elongation is not the result of changes in free-IAA levels.Abbreviations FR far-red light - IAA indole-3-acetic acid - Pfr phytochrome in the far-red-absorbing form - Pr phytochrome in the red-absorbing form - R red light     

RFLP-based assay of Sorghum bicolor (L.) Moench genetic diversity

Sixty-two single-copy sorghum DNA clones were used to compare restriction fragment patterns of 53 sorghum accessions from Africa, Asia and the United States. Included were accessions from five morphological races of the cultivated subspecies bicolor, and four races of the wild subspecies verticilliflorum. From two to twelve alleles were detected with each probe. There was greater nuclear diversity in the wild subspecies (255 alleles in ten accessions) than in the domestic accessions (236 alleles in 37 accessions). Overall, 204 of the 340 alleles (60%) that were detected occurred in both subspecies. Phylogenetic analysis using parsimony separated the subspecies into separate clusters, with one group of intermediate accessions. Though exceptions were common, especially for the race bicolor, accessions classified as the same morphological race tended to group together on the basis of RFLP similarities. Selection for traits such as forage quality may have led to accessions genetically more similar to other races being classified as bicolors, which have a loose, small-grained panicle similar to wild races. Population statistics, calculated using four nuclear and four cytoplasmic probes that detect two alleles each, revealed a low but significant amount of heterozygosity, and showed little differentiation in alleles in the wild and cultivated subspecies. Outcrossing with foreign pollen appears to have been more important than migration via seed dispersal as a mechanism for gene flow between the wild and domestic accessions included in this study.     

Floral initiation in sorghum hastened by gibberellic acid and far-red light

Combinations of far-red light (FR) (4 min) and gibberellic acid (GA₃), given at the beginning of a daily 12-h dark period in a growth room, were used to study floral induction in four maturity genotypes of the milo group of sorghum (Sorghum bicolor (L.) Moench). The 12-h dark period without GA₃ application or FR induced flowering in only the early genotype; FR hastened initiation in the early genotype, while GA₃ hastened floral initiation in the two intermidiate-flowering genotypes. GA₃ and FR together had a strong synergistic effect, hastening floral initiation by 30 to more than 80 d in the early and intermediate genotypes. Red light (R) did not hasten flowering; FR preceded by R gave the same effect as FR alone. GA₃ promoted stem elongation equally whether floral initiation occurred or not; thus, its effect on stem elongation was independent of floral initiation. The capacity of GA₃ to induce flowering in sorghum, a short-day plant, seems to be enhanced by phytochrome being in the P_R form at the beginning of the night when GA₃ was applied.Abbreviations FR far-red light - GA(s) gibberellin(s) - GA₃ gibberellic acid - R red light     

Role of red light in hair-whorl formation induced by blue light in Acetabularia mediterranea

After a pre-treatment with red light, hair formation at the growing tip of the siphonaceous green alga Acetabularia mediterranea Lamour. (= A. acetabulum (L.) Silva) can be induced by a pulse of blue light. Red light is needed again after the inductive blue-light pulse if the new whorl of hairs is to develop within the next 24 h. In order to investigate the role of this red light, the duration of the red irradiation was varied and combined with periods of darkness. The response of hair-whorl formation was dependent on the total amount of red light, regardless of whether the red irradiation followed the blue pulse immediately or was separated from it by a period of darkness. Furthermore, periods of exposure to the photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1-1dimethylurea had a similar effect to darkness. Both observations indicate that this red irradiation acts as a light source for photosynthesis. Whether or not the red light had an additional effect via phytochrome was tested in another type of experiment. The dependence of hair-whorl formation on red-light irradiance in the presence of simultaneous far-red irradiation was determined for the pre-irradiation period as well as for the irradiation period after the blue pulse. In both experiments, far-red light caused a small promotion of hair-whorl formation when low irradiances of red light were used. However, these differences were attributable to a low level of photosynthetic activity (which in fact was measurable) caused by red light reflected in the growth chamber. Furthermore, lowering the proportion of active phytochrome by far-red light would be expected to suppress hair-whorl formation. The influence of far-red light was also tested in a strain of Acetabularia mediterranea that developed hair whorls in about 20% of cells even when kept in complete darkness after the blue-light pulse. Far-red irradiation had no effect. These results strongly indicate that phytochrome is not involved in hair-whorl formation. Rather it is concluded that the effects of red light are caused by photosynthesis.Abbreviation DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea     

The kinetics of type 1 phytochrome in green,light-grown wheat (Triticum aestivum L.)

The kinetics of type 1 phytochrome were investigated in green, light-grown wheat. Phytochrome was measured by a quantitative sandwich enzyme-linked immunosorbent assay using monoclonal antibodies. The assay was capable of detecting down to 150 pg of phytochrome. In red light, rapid first-order destruction of the far-red-light-absorbing form of phytochrome (Pfr) with a half-life of 15 min was observed. Following white light terminated by red, phytochrome synthesis was delayed in darkness by about 15 h compared to plants given a terminal far-red treatment. Synthesis of the red-light-absorbing form of phytochrome (Pr) was zero-order in these experiments. Phytochrome synthesis in far-red light was approximately equal to synthesis in darkness in wheat although net destruction occurred in light-grown Avena sativa tissues in continuous far-red light, as has been reported for other monocotyledons. In wheat, destruction of Pfr apparently did not occur below a certain threshold level of Pfr or Pfr/total phytochrome. These results are consistent with an involvement of type 1 phytochrome in the photoperiodic control of flowering in wheat and other long-day plants.Abbreviations ELISA enzyme-linked immunosorbent assay - FR far-red light - HIR high-irradiance response - Pfr farred-light-absorbing form of phytochrome - Pr red-light-absorbing form of phytochrome - Ptot total phytochrome (Pr + Pfr) - R red light The authors wish to thank Prof. Daphne Vince-Prue (University of Reading) for many helpful discussions regarding this work. Hugh Carr-Smith was supported by a Science and Engineering Research Council studentship and Chris Plumpton by an Agricultural and Food Research Council (AFRC) studentship. B. Thomas and G. Butcher were supported by the AFRC.     

Phytochrome action in light-grown plants: the influence of light quality and fluence rate on extension growth in Sinapis alba L.

Using light-grown plants of Sinapis alba an analysis has been made of the effect on extension growth of adding far red light to a background photosynthetic source. It has been possible to distinguish between the increase in fluence rate and the reduction of the amount of phytochrome present as Pfr, which are both consequences of the addition of supplementary far red light, and to determine that the response of increased extension growth is due only to the latter. It is shown that the degree of fluence rate dependency varies with photoequilibrium and the significance of this interaction is discussed in terms of the mode of action of phytochrome and of its role in the natural light environment.Abbreviations PAR photosynthetically active radiation - SAN 9789 4-chloro-5-(methylamino)-2-(,,-trifluoro-m-tolyl)-3(2H) pyridazinone - Pfr far-red absorbing form of phytochrome - Pr red-absorbing form of phytochrome - LER logarithmic extension rate     

Light-dependent,but phytochrome-independent,translational control of the accumulation of the P700 chlorophyll-a protein of photosystem I in barley (Hordeum vulgare L.)

This work reports on the regulation of synthesis of the P700 chlorophyll-a apoprotein of photosystem I in barley. The mRNA for the P700 apoprotein is almost exclusively confined to the plastid membrane-bound polysomes. However, the mRNA for the 32-kDa herbicide-binding protein of photosystem II is found in both the soluble and membrane-bound polysomes.The mRNA for the P700 apoprotein is found in similar amounts in dark-grown and light-grown wild-type as well as mutant xantha-l⁸¹ barley. The latter mutant is deficient in chlorophyll biosynthesis. However, while wild-type leaves accumulate the P700 chlorophyll-a protein only in the light, mutant leaves never accumulate the P700 apoprotein.A more sensitive approach was taken using isolated plastids to study P700 apoprotein synthesis. Etioplasts did not synthesize detectable P700 apoprotein even when the etioplasts were exposed to light. However, only a 1-min exposure of leaves to light was necessary to induce P700 apoprotein synthesis by isolated plastids.Phytochrome involvement in controlling P700 apoprotein synthesis was tested by using red/farred light treatment of leaves. These treatments showed no far-red reversibility of red-induced P700-apoprotein synthesis in isolated plastids even after 3 h of darkness after the light treatments. From these data we conclude that the accumulation of P700 apopootein is not under the control of phytochrome and that the light induction of P700 apoprotein is most likely mediated through the protochlorophyllide/chlorophyllide system. This control, however, may also involve cytoplasmic signals as the synthesis of the P700 apoprotein is not turned on in illuminated etioplasts.     

Inhibition and promotion by light of the accumulation of translatable mRNA of the light-harvesting chlorophyll a/b-binding protein of photosystem II

The amount of in-vitro translatable mRNA of the light-harvesting chlorophyll a/b-binding protein (LHCP) of photosystem II strongly increases in darkness (D) after a 5-min red-light pulse while continuous illumination of mustard seedlings with far-red (FR), red or white light leads only to a slight increase in the amount of translatable LHCP-mRNA. No increase can be observed after a long-wavelength FR (RG9-light) pulse. However, a FR pretreatment prior to the RG9-light pulse strongly increase LHCP-mRNA accumulation in subsequent D. This is not observed in the case of the mRNA for the small subunit of ribulose-1.5-bisphosphate carboxylase. The increase of LHCP-mRNA in D after a FR pretreatment can be inhibited by a reillumination of the seedlings with FR. The inhibition of LHCP-mRNA accumulation during continuous illumination with FR and the strong increase in D following a FR illumination was found to be independent of chlorophyll biosynthesis since no correlation between chlorophyll biosynthesis and translatable LHCP-mRNA levels could be detected. Even strong changes in the amount of intermediates of chlorophyll biosynthesis caused by application of levulinic acid or 5-aminolevulinic acid did not affect LHCP-mRNA levels. Therefore, we conclude that the appearance of LHCP-mRNA is inhibited during continuous illumination, even though illumination leads to a storage of a light singal which promotes accumulation of translatable LHCP-mRNA in D.Abbreviations c continuous - Chl chlorophyll - D darkness - FR far-red light (3.5 W·m^-2) - LHCP light-harvesting chlorophyll a/b-binding protein of photosystem II - NF Norfluration - PChl protochlorophyll(ide) - Pfr far-red absorbing form of phytochrome - Ptot total phytochrome - R red light (6.8 W·m^-2) - RG9-light long-wavelength FR (10 W·m^-2) - SSU small subunit of ribulose-1.5-bisphosphate carboxylase - WL white light - () Pfr/Ptot=wavelength-dependent photoequilibrium of the phytochrome system     

Inhibitory action of red light on the growth of the maize mesocotyl: evaluation of the auxin hypothesis

Brief irradiation of 3-d-old maize (Zea mays L.) seedlings with red light (R; 180 J m^-2) inhibits elongation of the mesocotyl (70–80% inhibition in 8 h) and reduces its indole-3-acetic acid (IAA) content. The reduction in IAA content, apparent within a few hours, is the result of a reduction in the supply of IAA from the coleoptile unit (which includes the shoot apex and primary leaves). The fluence-response relationship for the inhibition of mesocotyl growth by R and far-red light closely resemble those for the reduction of the IAA supply from the coleoptile. The relationship between the concentration of IAA (1–10 M) supplied to the cut surface of the mesocotyl of seedlings with their coleoptile removed and the growth increment of the mesocotyl, measured after 4 h, is linear. The hypothesis that R inhibits mesocotyl growth mainly by reducing the IAA supply from the coleoptile is supported. However, mesocotyl growth in seedlings from which the coleoptiles have been removed is also inhibited by R (about 25% inhibition in 8 h). This inhibition is not related to changes in the IAA level, and not relieved by applied IAA. In intact seedlings, this effect may also participate in the inhibition of mesocotyl growth by R. Inhibition of cell division by R, whose mechanism is not known, will also result in reduced mesocotyl elongation especially in the long term (e.g. 24 h).Abbreviations FR far-red light - IAA indole-3-acetic acid - P_fr phytochrome in the far-red-absorbing form - P_r phytochrome in the red-absorbing form - R red light     

Action spectrum for the effect of day-extensions on flowering and apex elongation in green,light-grown wheat (Triticum aestivum L.)

Fluence-rate response curves for wavelengths from 640 nm to 730 nm were constructed for the day-extension promotion of flowering in green, light-grown, wheat (Triticum aestivum L., cv. Alexandria), a long-day plant. The resultant action spectrum had action maxima at 660 nm and 716 nm and resembles spectra for the high-irradiance reaction (HIR) seen in etiolated plants. Because, the HIR is thought to be controlled by type I pytochrome (that which is most abundant in etiolated tissue) our results indicate the involvement of type I phytochrome in the photomorphogenesis of a light-grown, green plant.Abbreviations Pr red-light-absorbing form of phytochrome - Pfr far-red-light-absorbing form of phytochrome - Ptot total phytochrome level (Pr+Pfr) - HIR high-irradiance reaction - SDP short-day plant(s) - LDP long-day plant(s)     

Role of nitrogen in the photoinduction of protoperithecia and carotenoids in Neurospora crassa

Nitrogen, as KNO₃ or NH₄NO₃, can inhibit the photoinduction of protoperithecia in Neurospora crassa when present in the medium at a high concentration but does not inhibit the photoinduction of carotenoids. The point at which the presence of high nitrogen levels is no longer inhibitory is 5 h after illumination.Abbreviations al albino mutant - wc white-collar mutant - WM Westergaard and Mitchell (1947) medium Dedicated to Professor Wilhelm Nultsch in honour of his 60th birthday     

Allozyme variation among the spontaneous species of Sorghum section Sorghum (Poaceae)

Summary A survey of allozyme variation among the spontaneous taxa of Sorghum section Sorghum was undertaken. Eight plants each of 90 accessions representing the diploid S. bicolor (ssp. arundinaceum and drummondii) and the tetraploids S. almum and S. halepense were analyzed for 17 enzyme systems encoded by 30 loci. Low levels of variation were found within and among accessions, although there was more variation than is typical of inbreeding species. We found an average of 3.2 alleles per locus in ssp. arundinaceum, with a mean expected heterozygosity for the accessions of 0.034 and total panmictic heterozygosity of 0.154. An analysis of the apportionment of genetic variation among accessions of ssp. arundinaceum indicated that 26% of the variation occurs within accessions and 74% among accessions. Cultivated sorghum contains far less allozymic variation than ssp. arundinaceum, its presumed progenitor. This is consistent with the prediction that cultivated sorghum experienced a loss of genetic variation during domestication. For the most part, cultivated sorghum contains a subset of the allozymes found in ssp. arundinaceum. Principal component analysis revealed continuous variation among the accessions and geographic regions, with accessions failing to segregate into discrete clusters. However, accessions of race virgatum of ssp. arundinaceum occupied one end of the continuum and were, in that sense, distinguished from the other accessions. Similarly, most accessions of S. halepense and S. almum occupied the central portion of the continuum. The allozymic data presented here are consistent with the hypothesized origin of S. halepense via autopolyploidy or segmental allopolyploidy.     

Interaction of light and temperature on seed germination of Rumex obtusifolius L.

Germination of Rumex obtusifolius L. seeds (nutlets) is low in darkness at 25° C. Germination is stimulated by exposure to 10 min red light (R) and also by a 10-min elevation of temperature to 35° C. A 10-min exposure to far-red light (FR) can reverse the effect of both R (indicating phytochrome control) and 35° C treatment. Fluence-response curves for this reversal of the effect of R and 35° C treatments are quantitatively identical. Treatment for 10 min with light of wavelenght 680, 700, 710 and 730 nm, after R and 35° C treatment, demonstrates that germination induced by 35° C treatment results from increased sensitivity to a pre-existing, active, far-red-absorbing form of phytochrome (Pfr) in the seeds.Abbreviations FR far-red light - P phytochrome - Pr red-absorbing form of P - Pfr far-red-absorbing form of P - R red light     

Interaction of light and temperature on the germination of Rumex obtusifolius L.

Seeds (nutlets) of Rumex obtusifolius L. fail to germinate in darkness at 25° C, but are stimulated by short exposure to red light (R) the effectiveness of which can be negated by a subsequent short exposure to far red light (F) indicating phytochrome control. Short periods of elevated temperature treatment (e.g. 5 min at 35° C) can induce complete germination in darkness. Although short F cannot revert the effect of 35° C treatment, cycling the phytochrome pool by exposure to short R before short F results in reversion of at least 50% of the population. Prolonged or intermittent F can also revert the germination induced by 35° C treatment. The effect of elevated temperature treatment is interpreted on the basis of two possible models; (i) that it increases the sensitivity of the seeds to a low level of pre-existing active form of phytochrome (Pfr) (ii) that it induces the appearance of Pfr in the dark. In both cases it is envisaged that elevated temperature treatment and Pfr control germination at a common point in the series of reactions that lead to germination.Abbreviations D Dark - F far red light - P phytochrome - Pr red absorbing form of P - Pfr far red absorbing form of P - R red light