Photoregulation of germination in seed of transgenic lines of tobacco and Arabidopsis which express an introduced cDNA encoding phytochrome A or phytochrome B

Photoinduction and photoinhibition of germination in seed from a homozygous tobacco (Nicotiana tabacum L.) line containing an introduced oat phyA cDNA (encoding phytochrome A) is compared with that of isogenic wild-type (WT) tobacco. Under continuous irradiation by a light source with a low redfar-red (RFR) ratio the transgenic tobacco seed appeared to be less susceptible to photoinhibition of germination compared with WT seed. However, induction of germination following a short pulse by R (666 nm) was not enhanced in the genotype transformed by oat phyA cDNA compared with the WT; neither did germination of the transgenic tobacco seed show an increased sensitivity to saturating pulses of light of longer wavelengths (666–730 nm). In seeds of transgenic Arabidopsis thaliana (L.) Heynh. which contained an introduced phytochrome-B-encoding cDNA, levels of dark germination were enhanced, consistent with mediation of response by phytochrome B-P_fr. The germination behaviour of Arabidopsis genotypes wich contained an introduced cDNA encoding phytochrome A, however, did not significantly differ from that of the WT.Abbreviations ABO seed transformed with Arabidopsis phyB - cDNA; CaMV cauliflower mosaic virus - FR far-red light - P_fr far-red-absorbing form of phytochrome - P_tot total phytochrome - P_fr/P_tot phytochrome photoequilibrium - R red light - RBO seed transformed with rice phyB cDNA - RFR quantum ratio of red and far-red light - WL white light - WL + FR whitelight supplemented with far-red light - WT wild type The authors wish to thank R.D. Vierstra (Department of Horticulture, University of Wisconsin-Madison, USA) for providing the transgenic tobacco line, and M.T. Boylan, D. Wagner and P.H. Quail (U.C. Berkeley/USDA Plant Gene Expression Center, Albany, Calif. USA) for providing the transgenic Arabidopsis lines. The work presented in this paper was funded by grants from the Agricultural and Food Research Council (H.S., A.C.M., G.C.W.).     

Phytochrome-mediated changes in the ATP content of Kalanchoë blossfeldiana seeds

One short red (R) irradiation increases the ATP content of Kalanchoë blossfeldiana Poelln. cv Feuerblüte seeds before onset of germination. Phytochrome control is demonstrated by the full R/far-red light (FR) reversibility of the effect in water imbibed seeds. In seeds imbibed in the presence of gibberellin A₃ (GA₃, one short R exposure already increases the ATP content when given 2h after start of imbibition, showing phytochrome control at the energy-metabolic level when one R pulse cannot yet induce germination. After longer imbibition periods in the presence of GA₃, one short FR irradiation also increases the ATP content of ungerminated Kalanchoë seeds. The time course of the ATP levels after a R or FR germination inducing irradiation shows an initial increase that clearly preceeds germination. A second increase starts about 15 h after irradiation and is most probably the consequence of the germination itself. The results suggest that, in Kalanchoë seeds, the increase in ATP levels, induced by irradiation(s) and preceding germination, is a phytochrome-mediated process, supplying energy, required for germination.     

Spectrophotometric characterization of phytochromes in dimorphic cocklebur seeds in relation to capacity for germination

Phytochrome was measured spectrophotometrically in different tissues of the upper (positively photoblastic) and lower (negatively photoblastic) seeds of the cocklebur (Xanthium pennsylvanicum Wallr.). Axial parts of the seeds, in particular parts of the radicle, contained high levels of phytochrome, while cotyledonary parts contained only low levels. These results were consistent with the distribution of the light-sensitive areas of the seeds that were associated with germination. Phytochrome levels in both types of dimorphic seeds increased gradually with increasing duration of dark imbibition for 4–8 h, then the rates of increase in levels of phytochrome accelerated. In both types of seed, some phytochrome was measurable even before imbibition. In the lower seeds, up to 20% of the phytochrome was occasionally observed as P_fr in samples imbibed in darkness for a short time (up to 12 h). A slight blue shift of the peak of P_T in the difference spectrum of phytochrome was observed in the case of lower seeds imbibed for 0–2 h. These results suggest that, to some extent, the lower axes contain dehydrated P_fr or intermediate(s) in the photoconversion of phytochrome. The dark reactions of P_fr were also examined in excised axes of both types of dimorphic seed after they had been pre-imbibed for 16 h in darkness. Dark destruction of P_fr was observed in both types of seed. In addition, net increases in levels of P_r were observed in the dark controls and in the samples irradiated with red light after the level of P_fr diminished. No ‘inverse’ dark reversion from P_r to P_fr was detected. Thus, after 16 h of imbibition, there were no differences in terms of properties of phytochrome between the two types of seed, and the different responses to light of upper and lower seeds might depend mainly on a difference in the physiological state of the two types of seed rather than the properties of phytochrome.     

Involvement of phytochrome and a blue light photoreceptor in UV-B induced flavonoid synthesis in parsley (Petroselinum hortense Hoffm.) cell suspension cultures

Flavonoid synthesis in cell suspension cultures of parsley (Petroselinum hortense Hoffm.) occurs only after irradiation with ultraviolet light (UV), mainly from the UV-B (280–320 nm) spectral range. However, it is also controlled by phytochrome. A P_fr/P_tot ratio of approximately 20% is sufficient for a maximum phytochrome response as induced by pulse irradiation. Continuous red and far red light, as well as blue light, given after UV, are more effective than pulse irradiations. The response to blue light is considerably greater than that to red and far red light. Continuous red and blue light treatments can be substituted for by multiple pulses and can thus probably be ascribed to a multible induction effect. Continuous irradiations with red, far red and blue light also increase the UV-induced flavonoid synthesis if given before UV. The data indicate that besides phytochrome a separate blue light photoreceptor is involved in the regulation of the UV-induced flavonoid synthesis. This blue light receptor seems to require the presence of P_fr in order to be fully effective.Abbreviations HIR high irradiance response - P_fr far red absorhing form of phytochrome - P_tet total phytochrome - UV ultraviolet light     

Developmental Physiology Apparent Dark Decay of Phytochrome Pfr in Fern Spores

Germination of spores of Dryopteris fllix-mas has been induced by two pulses of saturating red light, separated by a dark period of about 8 to 24 h. By chosing different wavelengths, different P_fr/P_tot levels could be established. Thus, by a “null method” the second pulse could be used as a “test pulse”, determining the actual P_fr level remaining from the “start pulse”, and thus providing information about an apparent Pf_r decay. It cannot be decided yet whether this apparent P_fr decay results from dark destruction or dark reversion. The apparent P_fr decay depends, as expected, on the temperature, being accelerated with increasing temperatures. Moreover, the later after sowing that the decay is tested, the faster it proceeds; a tentative interpretion is that newly synthesized P_r undergoes faster decay after phototransformation than that phytochrome pool present in the resting spores. A third factor that influences the apparent P_fr decay is the Pf_r/P_tot level established by the first pulse (start pulse). The lower this level, the slower the decay kinetics. This could be due to phytochrome biosynthesis partly compensating for P_fr destruction, and the relative contribution of this biosynthesis to the total effect increases with lower P_fr levels. Spores of D. paleacea yield virtually the same results. Whatever the real basis of the observed P_fr decay, i.e. destruction, reversion, or a combination of these reactions with biosynthesis, it can be concluded that modification of this P_fr decay by various factors is the basis of the effect of those factors on light-induced germination.     

Effects of long-term storage on phytochrome-mediated germination in lettuce seeds

The effects of long-term seed storage on the physiological properties of phytochrome-mediated germination including water uptake, the temperature and light flunnce dependencies of germination and dark germination were studied. The fluenceresponse relationships of the brief irradiation with monochromatic red (660 nm, 7.5 W m⁻²) and far-red (750 nm, 6.6 W m⁻²) light at various times after sowing were also studied. The samples used consisted of three lots of seeds ofLactuca sativa L. cv. MSU-16, which had been harvested in 1976, 1979 and 1985 and stored dry for 9, 6 and 0 years, respectively, in darkness at 23±2 C until the experiments were carried out in July–August, 1985. Seeds with the longer storage periods showed the higher ability to germinate in both continuous darkness and continuous white fluorescent light at 20–30 C. In the seeds stored for 6 or 9 years, red light irradiation for 20 sec given at 15 min or more after sowing at 25 C induced as high a percent germination (85–95%) as those under continuous white fluorescent light. In the freshly harvested seeds, however, germination under continuous white fluorescent light (46%) was considerably lower than the germination induced by the red pulse (97%). Germination of the seeds decreased when the intervals between sowing and a far-red irradiation for 20 sec increased up to 100 min (or 30 min in the freshly harvested seeds). The far-red pulse given later than 100 min (or 6 hr in the freshly harvested seeds) after sowing resulted in an increased germination up to the dark-germination levels with increasing intervals between sowing and the pulse irradiation. Before or at 3 min after sowing, the seeds stored for 6 or 9 years were responsive to the far-red pulse although they were not or hardly responsive to the red pulse, while the freshly harvested seeds were responsive to both the far-red and the red pulses. These data indicate that normal functions of phytochrome completely survived in the dry seeds during storage at 25 C for as long as 6 or 9 years and that these functions are restored into full operation by means of imbibition. The differences in the dependence of germination on the time and fluence of a single pulse of red or far-red light seems to be related to the smaller water content throughout the imbibition in the seeds with the longer storage periods. The greater ability to germinate in the dark indicates the greater amounts of P_FR or the greater responsivity to P_FR, in the seeds with the longer storage periods.     

Light effects upon dry lettuce seeds

Germination of certain dry seeds (achenes) of Lactuca sativa L. cv. Grand Rapids was increased to ca. 75% after irradiation with 665 nm red light (R; 1x10³ J m^-2); this response was eliminated by far-red light (FR) following the R. The response of dry seeds required an order of magnitude more light than that of wetted seeds, and was not maximal until 48 h after irradiation. Other seeds, which could not be stimulated by R in dry state, showed a partial response after 10 min hydration. Irradiation of dry seeds (or seeds wetted 1 h) with FR (1x10³ J m^-2) reduced dark germination from 26% to 2%. Seeds dehydrated in an oven (60°C, 90 min) showed a decrease in germination if irradiated with R (1x10⁵ J m^-2) before wetting. The results show that phytochrome is present in dry lettuce seeds (and functional in some seed lots) prior to wetting; and that in other seed lots the molecule becomes functional within minutes after wetting the seeds. Transformation of the FR absorbing from of phytochrome (P_FR) to the inactive from (P_R) occurs at lower seed moisture content than the reverse reaction. It appears that dormancy in seeds ripened in sunlight might be assured during seed drying and maturation by the more effective transformation of P_FR to P_R than vice versa as phytochrome is dehydrated.Abbreviations FR far-red - R red - CAL seeds from California - NC seeds from North Carolina (see text)     

Photocontrol of germination in Amaranthus caudatus

Summary Germination of Amaranthus caudatus is inhibited by light, far-red being the most effective part of the spectrum. At temperatures of 25° and below there is a low final germination percentage under continuous far-red whereas above 25° there is only a delaying effect. In the presence of a saturating concentration of gibberellic acid (GA₃) at 25° seeds germinate under continuous far-red although they are delayed. At 25° seeds exposed to 48 hr far-red fail to germinate when transferred to darkness. This induced dormancy can be broken by a single short exposure to red light given at any time after the far-red illumination. This effect of short red can be reversed by a subsequent short period of far-red indicating that the seeds are phytochrome controlled. Although most seeds have escaped from the reversing effect of short far-red after an intervening dark period of 5 hours, germination is greatly reduced by continuous far-red at this time. Results of exposing seeds to varying periods of far-red before and after dark imbibition are interpreted in terms of a continual production of phytochrome in its active P _fr form and a requirement for P _fr action over a long period of time. Effects of intermittent and continuous low intensity far-red on the inhibition of germination provides further evidence for a low energy photoreaction involving phytochrome. Effects on Germination Index of continuous illumination with various light sources maintaining different P _fr /P _total ratios have been investigated. The results suggest that the proportion of phytochrome in the P _fr form is the most important factor in the regulation of germination. A scheme for the phytochrome control of germination in Amaranthus caudatus is presented and possible explanations for the dependence on P _fr /P _total ratio are discussed.Holder of a Science Research Council Studentship.     

Light requirement,phytochrome and photoperiodic induction of flowering of Pharbits nil Chois

The low chlorophyll content of cotyledons of Pharbitis nil grown for 24 h in far-red light (FR) or at 18° C in white light from fluorescent lamps (WL) allows spectrophotometric measurement of phytochrome in these tissues. The (A) measurements utilize measuring beams at 730/802 nm and an actinic irradiation in excess of 90 s. The constancy of the relationship between phytochrome content and sample thickness confirms that, under these conditions of measurement, a true maximum phytochrome signal was obtained. These techniques have been used to follow changes in the form and amount of phytochrome during an inductive dark period for flowering. Following exposure to 24h WL at 18° C with a terminal 10 min red (R), P_fr was lost rapidly in darkness and approached zero in less than 1 h; during this period there was no change in the total phytochrome signal. Following exposure to 24 h FR with a terminal 10 min R, P_fr approached zero in 3 h, and the total phytochrome signal decreased by about half. The relevance of these changes to photoperiodic time measurement is discussed.Abbreviations BCJ irradiation from photographic ruby-red lamps - FR far-red light - P_fr far-red-absorbing form of phytochrome - P_r red-absorbing form of phytochrome - P total phytochrome content - R red light - WL white light from fluorescent lamps     

The involvement of phytochrome in controlling chlorophyll and 5-aminolevulinate formation in a gymnosperm seedling (Pinus sylvestris)

Chlorophyll a (Chl a) accumulation in the cotyledons of Scots pine seedlings (Pinus sylvestris L.) is much higher in the light than in darkness where it ceases 6 days after germination. When these darkgrown seedlings are treated with continuous white light (3,500 lx) a 3 h lag phase appears before Chl a accumulation is resumed. The lag phase can be eliminated by pretreating the seedlings with 7 h of weak red light (0.14 Wm^-2) or with 14 red light pulses separated by relatively short dark periods (<100 min). The effect of 15s red light pulses can be fully reversed by 1 min far-red light pulses. This reversibility is lost within 2 min. In addition, the amount of Chl a formed within 27 h of continuous red light is considerably reduced by the simultaneous application of far-red (RG 9) light. It is concluded that phytochrome (P_fr) is required not only for the elimination of the lagphase but also to maintain a high rate of Chl a accumulation in continuous light. Since accumulation of 5-aminolevulinate (ALA) responds in the same manner as Chl a accumulation to a red light pretreatment it is further concluded that ALA formation is the point where phytochrome regulates Chl biosynthesis in continuous light. No correlation has been found between ALA and Chl a formation in darkness. This indicates that in a darkgrown pine seedling ALA formation is not rate limiting for Chl a accumulation.Abbreviations Chl chlorophyll(ide) - PChl protochlorophyll(ide) - ALA 5-aminolevulinate - P_r the red absorbing form of phytochrome - P_fr the far-red absorbing form of phytochrome - P_tot total phytochrome ([P_r]+[P_fr])     

Photocontrol of seed germination in Impatiens wallerana Hook. f.

General characteristics of light sensitivity of Impatients wallerana seeds were investigated. Germination was absolutely dependent on light, irrespective of temperature. High percentages of germination were obtained by exposure to long periods of illumination or, alternatively, to several repeated short irradiations with red light. In this case, responsiveness to light was not altered by increasing either the initial incubation period in darkness or the dark intervals between short exposures. Effects of red light were reversed by far-red light, thus demonstrating the involvement of phytochrome. Evidence was presented for an interactive effect, of unknown physiological nature between red and far-red light on the germination of the seeds.Abbreviations P_r phytochrome, red light absorbing form - Pfr phytochrome far-red absorbing form     

The roles of inorganic nitrogen salts in maintaining phytochrome- and gibberellin A3-mediated germination control in skotodormant lettuce seeds

Skotodormant seeds of Lactuca sativa Grand Rapids imbibed in darkness for 10 days (10-day DS) germinated poorly upon terminal treatment with red light (R) or gibberellin A₃ (GA₃). Inorganic nitrogen salts in the imbibition solutions reduced seed skotodormancy. Ten-day DS seeds, imbibed in 25 mm salt solutions followed by terminal R, germinated 99% if imbibed in NH₄NO₃, 70% if imbibed in KNO₃ or NH₄Cl, and 55% if imbibed in NaNO₃. Seeds imbibed in higher salt concentrations germinated fully upon terminal R treatment. Seeds imbibed in 25 mm NH₄Cl or in 50 mm NH₄NO₃ germinated completely upon GA₃ treatment. Osmotic effects of imbibition media accounted for only part of the effect, since seeds imbibed in 50 mm CaCl₂ or NaCl germinated poorly following R or GA₃ treatment. Seeds imbibed in 500 mm polyethylene glycol (PEG) 1000 or mannitol solutions for 10 days still exhibited skotodormancy. Treatments of R or GA₃ did not stimulate germination in seeds imbibed in mannitol, but germination was complete if seeds were given 1-h acid immersion plus a water rinse before the terminal R or GA₃ treatment. Seeds imbibed in 50–500 mm PEG during 10-day DS germinated significantly better in response to terminal R. Terminal GA₃ significantly improved germination only in seeds imbibed at 500 mm PEG. P_fr appeared to function in mannitol-imbibed seed only after an acid treatment. Seed exposure to inorganic nitrogen salts during the 10-day DS maintained seed sensitivity to terminal R or GA₃ treatment. The depth of seed skotodormancy was related to the availability of inorganic nitrogen and also involved the levels of P_fr or endogenous GA₃.Abbreviations FR far red - DS dark storage - R red - GA₃ gibberellin A₃ - PEG polyethylene glycol - SHAM salicylhydroxamic acid - ANOVA analysis of variance - GLM general linear model - LSD least squares difference - P_fr far-red absorbing form of phytochrome     

The role of phytochrome in photoperiodic time measurement and its relation to rhythmic timekeeping in the control of flowering in Chenopodium rubrum

Summary To follow changes in the status of phytochrome in green tissue and to relate these changes to the photoperiodic control of flowering, we have used a null response technique involving 1.5-min irradiations with mixtures of different ratios of R and FR radiation.Following a main photoperiod of light from fluorescent lamps that was terminated with 5 min of R light, the proportion of P_fr in Chenopodium rubrum cotyledons was high and did not change until the 3rd hour in darkness; at this time, P_fr disappeared rapidly. When the dark period began with a 5-min irradiation with BCJ or FR light to set the proportion of P_fr low P_fr gradually reappeared during the first 3 h of darkness and then disappeared again.The timing of disappearance of P_fr is consistent with the involvement of phytochrome in photoperiodic time measurement. Reappearance of P_fr after an initial FR irradiation explains why FR irradiations sometimes fail to influence photoperiodic time measurement or only slightly hasten time measurement. A R light interruption to convert P_r to P_fr delayed, the timer by 3 h but only for interruptions after and not before the time of P_fr disappearance. Such 5-min R-light interruptions did not influence the operation of the rhythmic timekeeping mechanism. Continuous or intermittent-5 min every 1.5 h-irradiations of up to 6 h in duration were required to rephase the rhythm controlling flowering. A skeleton photoperiod of 6 h that was began and terminated by 5 or 15 min of light failed to rephase the rhythm.The shape of the curves for the rhythmic response of C. rubrum to the length of the dark period are sometimes suggestive of clocks operating on the principle of a tension-relaxation mechanism. Such a model allows for separate timing action of a rhythm and of P_fr disappearance over the early hours of darkness. Separate timing action does not, however, preclude an interaction between the rhythm and phytochrome in controlling flowering.Abbreviations FR far-red - P_fr far-red-absorbing form of phytochrome - P_r red-absorbing form of phytochrome - R red - BCJ photographic ruby-red irradiation A grant in aid of research from the National Research Council of Canada to B. G. Cumming is gratefully acknowledged.     

Effect of light and temperature on the germination of lettuce seeds

A short period (15–30 min) at 30° C promotes germination of seeds of Lactuca sativa L. cv. Repolhuda in darkness. Far-red light reverses this stimulation, and the escape curves for phytochrome and high-temperature action are quite similar, indicating that the two factors act at a common point in the chain of events leading to germination. It is suggested that high temperature acts by decreasing the threshold of the active, far-red absorbing, form of phytochrome (Pfr) needed to promote germination.Abbreviations FR far-red light - Pfr far-red-absorbing form of phytochrome - R red light     

Light Actions in the Germination of Cocklebur Seeds: I. DIFFERENCES IN THE LIGHT RESPONSES OF THE UPPER AND LOWER SEEDS

Germination responses to light were studied in the upper andlower seeds of cocklebur (Xanthium pennsylvanicum Wallr.). Thelower seed was dark-germinating and negatively photoblastic;the upper one had a red-light (R) requirement and was positivelyphotoblastic. Germination of the lower seeds was inhibited bya prolonged single irradiation with R, blue (B) or far-red (FR)light applied during imbibition. The maximal inhibitory effectof a single irradiation occurred 9 h and 13 h after the startof soaking at 33 °C and 23 °C, respectively. However,the inhibitory effect of R differed from that of B and FR, byonly delaying germination. A single exposure to B or FR lightcould be replaced by intermittent B or FR irradiation, and theireffects were repeatedly reversible by the following R irradiation.If the upper seeds were not exposed to R during imbibition,they failed to germinate even at 33 °C which was optimalfor germination, and the promotive effect of R increased withdelay of its application time. The photoperceptive locus incocklebur seeds was the axial tissue for all B, R and FR. Lightreceived by the cotyledonary tissue had little effect. Germinationdimorphism in response to light is discussed with respect tothe phytochrome content and the ageing of axial tissues. Key words: Blue light, Dimorphism, Far red light, Germination, Red light, Xanthium seed     

Phytochrome in cucumber seeds

Summary Phytochrome was found by direct spectrophotometry to be present in whole dry seeds of cucumber. This pigment is spectroscopically different from the pigment found in etiolated plants. It shows the phenomenon of inverse reversion; in darkness, the red-absorbing form (P_r) reverts slowly to the far-red-absorbing form (P_fr). This may explain why 75% of the dry-seed phytochrome is in the P_fr form.After imbibition, total phytochrome in the seeds starts to increase. The newly-formed pigment is all in the P_r form and has properties similar to those of classical phytochrome of etiolated plants. The relationship of this newly-formed phytochrome with control of germination is presently not known. The dry-seed phytochrome remains unchanged during imbibition and appears to retain its capacity for inverse reversion. This may explain the requirement for continuous or intermittent far-red irradiation in the suppression of germination of cucumber seeds. A similar form of phytochrome may be responsible for control of germination in other seeds which are similarly affected by far-red radiation.Partially supported by National Science Foundation grant GB-7526.279th Communication.     

Regulation of hormone metabolism in Arabidopsis seeds: phytochrome regulation of abscisic acid metabolism and abscisic acid regulation of gibberellin metabolism

In a wide range of plant species, seed germination is regulated antagonistically by two plant hormones, abscisic acid (ABA) and gibberellin (GA). In the present study, we have revealed that ABA metabolism (both biosynthesis and inactivation) was phytochrome-regulated in an opposite fashion to GA metabolism during photoreversible seed germination in Arabidopsis. Endogenous ABA levels were decreased by irradiation with a red (R) light pulse in dark-imbibed seeds pre-treated with a far-red (FR) light pulse, and the reduction in ABA levels in response to R light was inhibited in a phytochrome B (PHYB)-deficient mutant. Expression of an ABA biosynthesis gene, AtNCED6, and the inactivation gene, CYP707A2, was regulated in a photoreversible manner, suggesting a key role for the genes in PHYB-mediated regulation of ABA metabolism. Abscisic acid-deficient mutants such as nced6-1, aba2-2 and aao3-4 exhibited an enhanced ability to germinate relative to wild type when imbibed in the dark after irradiation with an FR light pulse. In addition, the ability to synthesize GA was improved in the aba2-2 mutant compared with wild type during dark-imbibition after an FR light pulse. Activation of GA biosynthesis in the aba2-2 mutant was also observed during seed development. These data indicate that ABA is involved in the suppression of GA biosynthesis in both imbibed and developing seeds. Spatial expression patterns of the AtABA2 and AAO3 genes, responsible for last two steps of ABA biosynthesis, were distinct from that of the GA biosynthesis gene, AtGA3ox2, in both imbibed and developing seeds, suggesting that biosynthesis of ABA and GA in seeds occurs in different cell types.     

Photocontrol of stem elongation in light-grown plants of Fuchsia hybrida

Stems of the caulescent long-day plant, Fuchsia hybrida cv Lord Byron, showed 2 types of response to light. In one, internode length was increased by far-red irradiation given at the end of an 8 h photoperiod: the response was no greater with prolonged exposure and was less when the start of far-red was delayed. The effect of far-red was reversible by a subsequent exposure to red light. Internode length was inversely proportional to the P_fr/P ratio established before entry to darkness and there was no evidence for loss of P_fr during a 16 h dark period. The inhibitory effect of P_fr acted at a relatively late stage of internode growth. With the development of successive internodes a second response appeared in which stems lengthened following prolonged daily exposures to red or far-red light, or mixtures of the two, or to brief breaks with red or white light. In these later internodes, a short exposure to far-red near the middle of the night was not reversible by red because red alone promoted elongation at this time. Internode length increased with increase in the daily duration of light and, when light was given throughout an otherwise dark period of 16 h, with increase in illuminance to a saturation value of 200 lx from tungsten lamps. Elongation increased as a linear function of decrease in photostationary state of phytochrome down to P_fr/P0.3; however, internodes were shorter in far-red light than in 25% red/red+far-red. It was concluded that stem length is a net response to two modes of phytochrome action. An inductive effect of P_fr inhibits a late stage in internode expansion, and a phytochrome reaction which operates only in light (and may involve pigment cycling) promotes an early stage of internode development. Stem elongation is thus a function both of the daily duration of light and its red/red+far-red content. The outgrowth of axillary buds was controlled by the first type of phytochrome action only.Abbreviations and symbols FR far red light - R red light - P phytochrome - P_fr phytochrome in the far-red light absorbing form - SD 8 h short days - LDP long-day plant - SDP short-day plant     

The effect of a plant-derived smoke extract on the germination of light-sensitive lettuce seed

A simple and rapid bioassay using seeds of Lactuca sativa L. Grand Rapids has been developed for the detection of germination-enhancing compound(s) in plant-derived smoke extracts. This light-sensitive species germinates within 24 h in the dark at 20 or 25°C and shows responsiveness to smoke-derived extracts over a wide concentration range. For some seed lots where the P _fr level is high and germination in the dark is unacceptably high, a brief (10 min) exposure to far-red light, one hour after the start of imbibition in the dark, is necessary to clearly demonstrate biological activity in the smoke extracts.     

Phytochrome-mediated induction of phenylalanine ammonia-lyase in the cotyledons of tomato (Lycopersicon esculentum Mill.) plants

Phenylalanine ammonia-lyase (PAL; EC 4.3.1.5.) induction in cotyledons from 96-h dark-grown Lycopersicon esculentum Mill. was studied in response to continuous light and hourly light pulses (blue, red, far red). The increases of PAL promoted by blue and red pulses are reversed completely by immediately following 758 nm irradiations. The response to continuous red light could be substituted for by hourly 6-min red light pulses. The effect of continuous red treatments is mainly due to a multiple induction effect of phytochrome. In contrast to red light, hourly light pulses with far red and blue, light can only partially substitute for continuous irradiation. The continuous blue response could be due to a combination of a multiple induction response and of a high irradiance response of phytochrome. The continuous far red response, could represent a high irradiance response of phytochrome. Dichromatic irradiations indicate that phytochrome is the photoreceptor controlling the light response (PAL) in tomato seedlings.Abbreviations Norflurazon NF-4-chloro-5-(methylamino)-2-(,,,-trifluoro-m-tolyl)-3 (2H) pyridazinone - PAL phenylalanine ammonia-lyase - phytochrome photoequilibrium P_fr/P_tot - P_fr far-red absorbing form of phytochrome - P_r red absorbing form of phytochrome - P_tot total phytochrome: P_r+P_fr