Light-Induced Fern-Spore Germination under Reduced Water Potential

The phytochrome-mediated spore germination of the fern Dryopteris filix-mas was studied under reduced water content. Spores were sown on solutions containing polyethylene glycol (PEG), sorbitol, or mannitol as osmotically active substances, yielding water potential values (ψ) between ? 500 and ? 3500 kPa for most of the experiments. Our main results can be summarized as follows:

• 1 The germination of the spores is completely blocked by ψ ≤ ?1200 kPa (≥ 300 g/l PEG).
• 2 Reduced water content of the spores, at least up to 300 g/l PEG, does not impair the phototransformation of phytochrome from its inactive form P_r to its active form P_fr.
• 3 The inhibiting effect of the osmoticum is phase-specific: the reduced water content of the spores impairs the coupling of the phytochrome system to the transduction chain. Under these conditions, P_fr is found to be stable for several days.
• 4 In addition, the reduced water potential acts on the terminal response, although differently on different parameters of germination: Whereas formation of the rhizoid is completely blocked by 180 g/1 PEG, chlorophyll formation requires 300 g/1 PEG to become slightly retarded.

     

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     

Correlation between far-red absorbing phytochrome and response in phytochrome-mediated anthocyanin synthesis

Abstract Mustard seedlings were light treated at 24 h after sowing (25°C) to induce phytochrome-mediated anthocyanin synthesis in cotyledons and hypocotylar hook. All light treatments were performed within the range of the reciprocity law. The in situ photoconversion kinetics of phytochrome (P_r→ P_fr) were measured under the same light treatment. It was found that between 0.4 and 1.0 relative P_fr level the amount of anthocyanin extracted from the organs at 52 h after sowing was linearily correlated with the amount of P_fr produced at 24 h in cotyledons and hypocotylar hook. It is concluded that an explanation of the fluence response function for red light mediated anthocyanin synthesis in the mustard seedling does not require the concept of active vs. bulk phytochrome.     

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.     

Light and nitrate interaction in phytochrome-controlled germination ofPaulownia tomentosa seeds

Pulsed light and nitrate exhibit an interactive effect on the germination ofPaulownia tomentosa Steud. seeds that require long periods of light irradiation. Two pulses of red light (R), separated by an adequately long dark interval, substitute for continuous prolonged irradiation. A far-red (FR) pulse given at the beginning of the dark interval inhibits germination, while it has no effect if given at the end. The requirement for certain ratios of the far-red-absorbing form of phytochrome/total phytochrome (P_fr/P_tot) differs when a FR+R-pulse is given as the first or second of two pulses (FR+R or R) separated by a dark interval. An equal decrease of the P_fr/P_tot ratio leads to a more pronounced decrease in germination when the pulse of the same FR+R ratio is given as the second pulse at the end of the dark interval. The length of dark interval between light pulses needed for maximal germination, differed in (i) seeds with a natural requirement for long periods of light irradiation from that in (ii) seeds with their long light requirement imposed by two weeks of imbibition in darkness or by (iii) imbibition in 40% heavy water. However, a single R pulse was sufficient to induce a high percentage of germination if the seeds were supplied with KNO₃ (10 mM) from the onset of imbibition up to the onset of light. This effect decreased with a delayed time of application, and was prevented if FR preceded the KNO₃ application. We dedicate this paper to Professor Hans Mohr on the occasion of his 60th birthday     

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.     

Action spectra for light-induced germination in dormant lettuce seeds

Fluence response curves for red light-induced germination of thermodormant (TD) seeds of Lactuca sativa L. show two regions that differ in their light sensitivity. In the region of high sensitivity, the germination responses differ between seed batches and can be altered by dark storage or far red irradiation. Induction of germination in far red dormant (FRD) seeds requires far higher fluences. Action spectra for induction to 60% germination were determined for these various response types. Spectra for the regions of low sensitivity response are similar for TD and FRD seeds. In comparison, the action spectrum for the highly sensitive response in TD seeds is significantly shifted to longer wavelengths. Analogous differences exist in the action spectra for far red reversal of the red induced germination responses. Germination induction in the low sensitivity region shows repeated red-far red reversibility. Far red reversal of red induction in the high sensitivity region does not saturate even at the highest far red fluences available and requires increased red fluences for subsequent reinduction. A model quantitatively accounting for these observations is presented. It is pointed out that action spectra of processes involving photoreversible pigments with partly overlapping absorption spectra in general are not identical with the absorption spectra of the partners. They should depend upon the degree of phototransformation required to elicit a given physiological response. In the case of induction of lettuce seed germination the observed action spectra can be interpreted as reflecting different requirements for P _fr of the various response types. Our results do not necessitate the assumption of spectroscopically different forms of phytochrome in these seeds.Abbreviations TD thermodormant - FRD far red dormant - P phytochrome - P _r red absorbing form of P - P _fr far red absorbing form of P     

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])     

Phytochrome in seeds of Amaranthus caudatus

Summary Dry seeds of Amaranthus caudatus show little or no photoreversible absorption changes, attributable to phytochrome. During imbibition phytochrome appears in two phases, one immediately after sowing and the second after about 8 hr. Experiments at different temperatures and under continuous illumination with red, far-red and blue light suggest that there are two pools of phytochrome. The first phase in the appearance of phytochrome could be due to the change in optical properties of the sample on hydration or to rehydration of inactive phytochrome, or both. The second phase probably represents phytochrome synthesis. It is absent at 0° and precedes the water uptake associated with germination by some 10 hr. This second pool of phytochrome does not accumulate in red and blue illuminated seeds indicating that the rate of P _fr decay is more rapid than the rate of phytochrome synthesis. The difference spectra of phytochrome in both 2 hr imbibed seeds and 72 hr old seedlings show peaks of absorption at 663 and 735 nm. The presence of P _fr in dark imbibed seeds and the process of inverse reversion of P _r to P _fr in darkness have been demonstrated. The results are discussed in relation to previous hypotheses for the mechanism of photocontrol of Amaranthus seed germination.     

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     

Photoperiodism and rhythmic response to light

Abstract. Seedlings of Pharhitis nil show a circadian rhythm in the capacity to flower in response to the timing of a second red light pulse given at various times after a first saturating exposure to red when this is given together with a benzyladeninc spray. There are also changes in the photon irradiance required for half maximum response to the second red pulse. The photochemical properties of phytochrome in the photoperiodically sensitive cotyledons were also shown to change rhythmically. Oscillations in both p_r→ P_fr and P_fr→ P_r photoconversion characteristics persisted over at least two circadian cycles with a periodicity of about 12 h. There were, however, no significant oscillations in either P_fr peak absorbance or in Δ(ΔA). The changes in sensitivity for the photoconversion of P_r→ P_fr did not parallel the much larger changes in sensitivity of the flowering response to red light. The amplitude of the P_r→ P_fr rhythm was at least as great as that for P_r→ P_fr, but the flowering response to far-red light was not rhythmic, nor was there any large change in sensitivity. The changes in photoconversion properties may reflect a basic biochemical oscillation which affects both photoreceptor properties and sensitivity to photoreceptor input. There was also a marked rhythm in the P_fr/P ratio that would be established by a saturating pulse of red light and this too may have affected the flowering response to such a pulse. Far-red light inhibited flowering when given at any time during the inductive night. After 14 h in darkness, P_fr could still be measured in the cotyledons and it was concluded that far-red light inhibited flowering by removing P_fr As red light also inhibited flowering at this time, there may be two pools of phytochrome with different kinetic properties.     

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.).     

Photocontrol of petiole elongation in light-grown strawberry plants

Summary The mode of phytochrome control of elongation growth was studied in fully-green strawberry (Fragaria x Ananassa Duch.) plants. Petiole growth showed two distinct types of response to light. In one, the end-of-day response, petioles were lengthened by low-intensity far-red irradiation for 1 h immediately following the 8 h photoperiod. The response was little or no greater with prolonged exposure and less when the start of far-red was delayed. It was already evident in the first leaf to emerge after treatment began. With the development of successive leaves a second, photoperiodic, type of response appeared, in which petioles lengthened following only prolonged exposure to red, far-red, mixtures of the two, or tungsten lighting, all at low levels of intensity. As with the inhibition of flowering in previous experiments, irradiation with red light during the second half of the otherwise long dark period gave the greatest response.Abbreviations and Symbols FR far-red light - HIR high irradiance response - R red light - P_r phytochrome in the red light absorbing form - P_fr phytochrome in the far-red light absorbing form - SDP short-day plant - LDP long-day plant - PAR photosynthetically active radiation     

Kinetics of the dichroic reorientation of phytochrome during photoconversion inMougeotia

In the green algaMougeotia, the dichroic orientation of the red-absorbing form of phytochrome (P_r) is parallel of the cell surface, whereas the far-red-absorbing form (P_fr) is oriented normal to it. The time course of the change from parallel to normal was investigated by double-flash irradiation with polarized red and far-red light. The results obtained by two different methods indicate that most of the phytochrome intermediates existing in the first 5 ms after the inducing red flash are still oriented parallel to the cell surface, similar to P_r. At increasing intervals between the red and the far-red flashes, more and more phytochrome molecules turn their transition moments to the P_fr orientation. This reaction is finished after approximately 30 ms. We conclude that the change in dichroic orientation of the phytochrome molecules inMougeotia occurs during the last relaxation steps of the intermediates on the way from P_r to P_fr. It cannot be decided yet, whether the first surface-normal phytochrome species is an intermediate or P_fr itself.Abbreviations P_r red-absorbing form of phytochrome - P_fr far-red-absorbing form of phytochrome A preliminary report of this work was presented at the European Symposium on Photomorphogenesis, University of Reading, UK (Kraml et al. 1982)     

Regulation of enzyme levels by phytochrome in mustard cotyledons: Multiple mechanisms?

Phytochrome controls the appearance of many enzymes in the mustard (Sinapis alba L.) cotyledons. The problem has been whether the effect of phytochrome on the appearance of enzymes in this organ is due to a common initial action of P_fr, e.g. due to the liberation of a second messenger. We have compared the modulation by light (phytochrome) of the appearance of phenylalanine ammonia lyase (PAL)⁺ and ribulosebisphosphate carboxylase (Carboxylase)⁺. PAL becomes detectable in the mustard cotyledons at 27 h after sowing while Carboxylase starts to appear only at 42 h after sowing (starting points, 25° C). The starting points cannot be shifted by light. As a major result, in the case of PAL the inductive effect of continuous red light (given from the time of sowing) remains fully reversible by 756 nm-light up to the starting point (27 h after sowing) while with Carboxylase full reversibility in continuous red light is lost at approximately 15 h after sowing. While the induction of Carboxylase is already saturated at a very low level of P_fr (e.g. continuous 756 nm-light saturates the response) and does not depend on irradiance (e.g. continuous 675 mW m^-2 red light and 67.5 mW m^-2 red light lead to the same time course), PAL induction is a graded response over a wide range of P_fr doses and depends strongly on the fluence rate (high irradiance response, HIR). It is concluded that PAL induction and Carboxylase induction are not only separated in time but differ in every regard except that both responses are mediated by phytochrome.The present data support the previous conclusion that the specification of the temporal and spatial pattern of development is independent of phytochrome even though the realization of the pattern of development can only occur in the presence of phytochrome (P_fr). It seems that there is no feedback from pattern realization to pattern specification.Abbreviations P_fr the far-red absorbing, physiologically active form of phytochrome - P_r the red absorbing physiologically inactive form of phytochrome - P_total [P_r]+[P_fr] - PAL phenylalanine ammonia-lyase (EC 4.3.1.5) - Carboxylase ribulosebisphosphate carboxylase (EC 4.1.1.39)     

Induction versus modulation in phytochrome-regulated biochemical processes

The time course of appearance of competence towards phytochrome (P_fr) was studied in cotyledons of mustard (Sinapis alba L.) with regard to the light-mediated formation of anthocyanin (aglycone cyanidin) and NADP-dependent plastidal glyceraldehyde-3-phosphate dehydrogenase (GPD, EC 1.2.1.13). The experiments were performed to answer the following question: Does phytochrome act to turn responses on (induction), or — as an alternative — does phytochrome cause an amplification of processes already occurring in absolute darkness albeit at low rates once competence is reached (modulation)? The data show that in the case of GPD, phytochrome causes an amplification of the rate of synthesis once the competence point is reached at approximately 36 h after sowing at 25° C. In the case of anthocyanin, it was found that two distinct points of competence exist (26 h and 39 h after sowing, 25° C). In the case of ‘early anthocyanin’ (competence point at 26 h), synthesis does not occur in darkness without P_fr, while in the case of ‘late anthocyanin’ (competence point at 39 h), phytochrome causes an amplification of a process occurring in complete darkness albeit at a very low rate. It is concluded that in phytochrome-mediated photomorphogenesis, modulation as well as induction of biosynthetic processes plays a role.     

Action of light on accumulation of carotenoids and chlorophylls in the milo shoot (Sorghum vulgare Pers.)

We have performed a comprehensive study on the mechanism of regulation of carotenogenesis by light in the shoot of Sorghum vulgare. Our work shows that carotenoid accumulation is simultaneously controlled by phytochrome (P_fr) and by the availability of chlorophyll. Throughout plastidogenesis light dependent chlorophyll and carotenoid accumulation are interdependent processes: Accumulation of chlorophyll in natural light requires the presence of carotenoids; likewise, accumulation of considerable amount of carotenoids depends on the availability of chlorophyll. However, in both cases the efficiency of the biosynthetic pathway, the potential biosynthetic rates (capacities) are determined by phytochrome. A push and pull model of carotenogenesis advanced previously (Frosch and Mohr 1980, Planta 148, 279) to explain carotenogenesis in the mustard (Sinapis alba) seedling also applies to the monocotyledonous milo (Sorghum vulgare) seedling. Therefore, we suggest that the model applies to carotenogenesis in higher plants in general.Abbreviations Chl chlorophyll(s) - PChl protochlorophyll(ide) - HIR High irradiance response (of phytochrome) - P_fr far-red absorbing, physiologically active form of phytochrome - P red absorbing physiologically inactive form of phytochrome - P_tot total phytochrome - i.e. [P_r]+[P_fr] =[P_fr]+[P_tot], wavelength dependent photoequilibrium of the phytochrome system - RL red light - FR far-red light     

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     

Control of chlorophyll synthesis by phytochrome

Summary The rate of chlorophyllide esterification in mustard cotyledons can be increased by a pretreatment with 5 min red light applied 24 h prior to the protochlorophyll(ide)chlorophyll(ide) photoconversion at 60 h after sowing. Simultaneously the red light pulse pretreatment leads to a decrease of the total amount of chlorophyll(ide) a in darkness. It has been proven that phytochrome (P_fr) is the photoeffector for both. Since the amounts of esterified chlorophyllide are determined by the ratio [chlorophyll a]/[chlorophyllide a+chlorophyll a] it is assumed that P_fr increases the rate of esterification indirectly via stimulating the decrease of chlorophyll(ide) a. The regulation of chlorophyll synthesis by P_fr does not seem to involve a control of esterification. The duration of the chlorophyllide esterification differs from the duration of the Shibata shift although both are greatly shortened by the red light pulse pretreatment. The effect of 5 min red light on the duration of the esterification is fully reversible by 5 min far-red light while the reversibility with respect to the Shibata shift is lost within 2 min [Jabben, M. and H. Mohr, Photochem. Photobiol. 22, 55–58 (1975)]. We conclude that the control of the chlorophyllide esterification and the control of the Shibata shift cannot be traced back to the same initial action of P_fr.Abbreviations Chl chlorophyll - Chlide chlorophyllide - Chl(ide) sum of Chl and Chlide - PChl protochlorophyll - PChlide protochlorophyllide - PChl(ide) sum of PChl and PChlide - P_fr far-red absorbing form of the phytochrome system