Phytochrome decay in seedlings under continuous incandescent light

Summary Under continuous high intensity incandescent light the decay of phytochrome in Amaranthus seedlings deviates from the predicted first order rate characteristic of the P _fr/P _total ratio maintained. This deviation takes the form of a slower decay than would be predicted and is only observed at high intensities. Experiments are presented to test the hypothesis that this reduced rate of decay is the result of a high level of phytochrome intermediates maintained under high intensity incandescent light. Accumulation of intermediates under these conditions has been demonstrated using a quasi-continuous measuring spectrophotometer. They are weakly absorbing and their concentration increases with light intensity. Although they form P _fr in darkness, it is proposed that they do not decay. The model predicts that in a sample cuvette, where a light intensity gradient exists, there is more probability of a phytochrome molecule being presnet as P _fr at the back of the cuvette: the region of lowest light intensity. Under conditions which favour phytochrome decay, a preferential loss of phytochrome should result at the back of the cuvette and an increasingly higher proportion of the remaining phytochrome will consequently be measured as intermediate as the experiment progresses. The results confirm the hypothesis and in addition, after 60 min incandescent light, demonstrate an accumulation of intermediates which form P _fr with a longer half-life that at the begining of the experiment. Pisum epicotyl hooks show no such intermediate accumulation or preferential decay at the back of the cuvette, which is in agreement with the observed first order phytochrome decay under high intensity incandescent light. A scheme is presented explaining the results on the basis of the decay process.Abbreviations FR far-red light - R red light - P phytochrome - P _fr far-red-absorbing form of P - P _r red-absorbing form of P 321st communication of this Laboratory.     

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)     

Variation in dynamics of phytochrome A in Arabidopsis ecotypes and mutants

Phytochromes are photoreceptors in plants which can exist in two different conformations: the red light‐absorbing form (P_r) and the far‐red light‐absorbing form (P_fr), depending on the light quality. The P_fr form is the physiologically active conformation. To attenuate the P_fr signal for phytochrome A (phyA), at least two different mechanisms exist: destruction of the molecule and dark reversion. Destruction is an active process leading to the degradation of P_fr. Dark reversion is the light‐independent conversion of physiologically active P_fr into inactive P_r. Here, we show that dark reversion is not only an intrinsic property of the phytochrome molecule but is modulated by cellular components. Furthermore, we demonstrate that dark reversion of phyA may be observed in Arabidopsis ecotype RLD but not in other Arabidopsis ecotypes. For the first time, we have identified mutants with altered dark reversion and destruction in a set of previously isolated loss of function PHYA alleles (Xu et al. Plant Cell 1995, 7, 1433–1443). Therefore, the dynamics of the phytochrome molecule itself need to be considered during the characterization of signal transduction mutants.     

Phytochrome-controlled extension growth of Avena sativa L. seedlings

The effects of continuous red and far-red light and of brief light pulses on the growth kinetics of the mesocotyl, coleoptile, and primary leaf of intact oat (Avena sativa L.) seedlings were investigated. Mesocotyl lengthening is strongly inhibited, even by very small amounts of P_fr, the far-red light absorbing form of phytochrome (e.g., by [P_fr]0.1% of total phytochrome, established by a 756-nm light pulse). Coleoptile growth is at first promoted by P_fr, but apparently inhibited later. This inhibition is correlated in time with the rupturing of the coleoptile tip by the primary leaf, the growth of which is also promoted by phytochrome. The growth responses of all three seedling organs are fully reversible by far-red light. The apparent lack of photoreversibility observed by some previous investigators of the mesocotyl inhibition can be explained by an extremely high sensitivity to P_fr. Experiments with different seedling parts failed to demonstrate any further obvious interorgan relationship in the light-mediated growth responses of the mesocotyl and coleoptile. The organspecific growth kinetics, don't appear to be influenced by P_fr destruction. Following an irradiation, the growth responses are quantitatively determined by the level of P_fr established at the onset of darkness rather than by the actual P_fr level present during the growth period.Abbreviation P_fr far-red light absorbing form of phytochrome     

Phytochrome behaves as a dimer in vivo

Abstract It is well established that phytochrome exists as a dimer in vitro. A comparison of the relative photoequilibrium concentrations of P_rP_r, P_rP_fr and P_frP_fr, with the relative sizes of the P_fr-pools which undergo dark reversion in the intact plant, leads to the hypothesis that phytochrome also exists as a dimer in vivo, This hypothesis is in accordance with kinetic properties of the phytochrome system under continuous irradiation. Additional support for this view is provided by the observation that P_fr-destruction after a red light flash, which should favour the formation of P_rP_fr dimers, is paralleled by a decay of P_r, even if the presence of P_r cycled through P_fr can be excluded. Preliminary observations could indicate an interaction of the subunits of a phytochrome dimer during the process of phototransformation.     

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     

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.     

Phytochrome-mediated effects of near-ultraviolet radiation in the induction of flowering in etiolated Lemna paucicostata T-101, a short-day plant

Induction of flowering of etiolated Lemna paucicostata Hegelm. T-101, a short-day plant, was inhibited by far-red (FR) or blue light (BL) applied at the beginning of a 72-h inductive dark period which was followed by two short days. In either case the inhibition was reversed by a subsequent exposure of the plants to near-ultraviolet radiation (NUV), with a peak of effectiveness near 380 nm. Inhibition by BL or FR and its reversion by NUV are repeatable, i.e., NUV is acting in these photoresponses like red light although with much lower effectiveness. Thus, it is considered that NUV acts through phytochrome and no specific BL and NUV photoreceptor is involved in photocontrol of floral induction on this plant.Abbreviations BL blue light - FR far-red light - NUV near ultraviolet radiation - P red-absorbing form of phytochrome - P_fr far-red absorbing form of phytochrome - R red light     

Control by light of hypocotyl growth in de-etiolated mustard seedlings

After sowing, mustard (Sinapis alba L.) seedlings were grown for 48 h in white light (25°C). These fully de-etiolated, green seedlings were used as experimental material between 48 and 72 (84) h after sowing. The question researched was to what extent control by light of hypocotyl elongation is due to phytochrome in these seedlings. It was found that the light effect on hypocotyl growth is very probably exerted through phytochrome only. In particular, we found no indication for the involvement of a specific blue light photoreceptor pigment.Abbreviations HIR high irradiance reaction - P_fr far-red absorbing, physiologically active form of phytochrome - P_r red absorbing, physiologically inactive form of phytochrome - Pot total phytochrome, i.e. [P_r]+[P_fr] - [P_fr]/[P_tot] - red red light - fr far-red light - wl white light - bl blue light - di dichromatic irradiation - l hypocotyl length     

Germination in spores of Dryopteris filix-mas: Regulation of rhizoid elongation as a second phytochrome-mediated response

Spore germination in Dryopteris filix-mas occurs via a cascade of cellular responses, and chlorophyll formation, mitosis or rhizoid elongation are commonly used as parameters to determine spore germination. Detailed investigations of these parameters led to the hypothesis that they are regulated by different, independent phytochrome-mediated responses. This concept could be confirmed, as is described in this paper which demonstrates that perception of light via phytochrome occurs within two different phases separated in time. Presence of the far-red absorbing phytochrome form, P_fr, for 36 h, induces chlorophyll formation and the first unequal cell division, by which a rhizoid initial and a protonemal initial are formed (first phytochrome-mediated response). However, rhizoid elongation requires a second period of P_fr, presence (second phytochrome-mediated response). There is a clear temporal distinction between the first and the second phytochrome-mediated response with respect to the coupling of P_fr to the transduction chain; P_fr is unable to induce rhizoid growth until 60 h after the start of the first red irradiation. The effectivity of P_fr for inducing the second response shows an optimum at ca 96 h after the beginning of the presence of P_fr; thereafter, it declines slowly. The fluence-response relationship and the presence of red/far-red reversibility demonstrate that rhizoid elongation is a low-fluence response mediated by phytochrome and is independent of the first phytochrome response.     

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)     

In-vitro binding of phytochrome to a particulate fraction: A function of light dose and steady-state P fr level

Summary The binding of phytochrome to a particulate fraction in extracts from hypocotyl hooks of etiolated Cucurbita pepo L. seedlings has been examined as a function of the light dose and P _fr level established in vitro. As the steady-state level of P _fr transiently established in the 500×g supernatant is increased, so the level of P _r subsequently pelletable at 20 000×g increases up to a saturation level. Increasing both the time and irradiance parameters of the light dose while holding the steady-state P _fr level constant, results similarly in increasing P _r pelletability. This agrees with results obtained previously with in-vivo irradiations of maize coleoptiles. Thus, like the in-vivo response, phytochrome binding in vitro appears to be a function of the total number of molecules converted to the P _fr form during the irradiation period.Abbreviations P _fr far-red-absorbing form of phytochrome - P _r red-absorbing form of phytochrome     

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.     

Phytochrome Control of Longitudinal Growth and Phytochrome Synthesis in Maize Seedlings

The active, far-red light absorbing, form of phytochrome was found to inhibit growth and phytochrome levels in the mesocotyl and coleoptile of 4- to 5.5-day-old seedlings of Zea mays L. Short, low-irradiance red or far-red light treatments were used to produce different proportions of active phytochrome at the end of highdirradiance white-light periods, which left different levels of total phytochrome in the plants. After light treatments which left relatively high levels of spectrophotometrically assayable phytochrome in the seedlings, apparent phytochrome synthesis in the subsequent dark period was low regardless of the proportions of each form of the pigment present at the beginning of the dark period. In light treatments producing relatively low levels of assayable phytochrome, levels of apparent phytochrome synthesis in both red and far-red treatments and differences between apparent synthesis in red and far-red treatments were maximal. No simple correlation was found between growth and apparent phytochrome synthesis. However, growth and total phytochrome levels were positively correlated in both organs. Using a subtractive method of correlation, in which only phytochrome effects were plotted, strong linear relationships between phytochrome levels or longitudinal growth and P_fr levels were found in those light treatments leaving greater than 8% of dark control levels of phytochrome in the tissues. Using this technique non-linear, inverse relationships between P_fr and apparent phytochrome synthesis was found, indicating that modes of phytochrome control over phytochrome synthesis and growth differ. Our results are consistent with the view that in vivo assays of “bulk’ phytochrome reflect levels and states of the physiologically active phytochrome fraction under our experimental conditions in maize.     

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     

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.     

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.     

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     

Coaction of three factors controlling chlorophyll and anthocyanin synthesis

In a three-factor analysis the rate of chlorophyll a (Chl) accumulation in excised mustard cotyledons was studied as a function of kinetin, light (operating through phytochrome, P _fr) and an excision factor. It was found that the three factors operate additively provided that the P _fr level is high enough. When the P _fr level is below approximately 1 per cent (<0.01) the effectiveness of the excision factor decreases while the effect of kinetin remains additive. The observed additivity is explained by a model where the three factors operate independently through a common intermediate (presumably 5-aminolevulinate) in the biosynthetic chain leading to Chl. With regard to the coaction of the excision factor and phytochrome it is concluded that the production of the excision factor requires the operation of phytochrome (even though saturated at a low P _fr level) while the action of the excision factor is independent of phytochrome. This conclusion was confirmed by experiments in which the rate of light-mediated anthocyanin synthesis was measured in excised mustard cotyledons. The effect of excision in the case of anthocyanin formation differs kinetically from the effect of excision on Chl formation.Abbreviations Chl chlorophyll(ide) a - P _fr far-red absorbing form of phytochrome - P _fr/P _tot ratio at photoequilibrium - RL red light - FR far-red light - GL green light - RG9 light long wavelength far-red light - WL white light     

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