PHYSIOLOGICAL CHARACTERIZATION OF A HIGH-PIGMENT MUTANT OF TOMATO

Abstract— A high-pigment (hp) mutant, which shows exaggerated phytochrome responses and three other genotypes of Lycopersicon esculenrum Mill. cv. Ailsa Craig: the aurea (au) mutant deficient in the bulk light-labile phytochrome (PI) pool, the au, hp double mutant, and their isogenic wild type, were used in this study. Measurements of phytochrome destruction in red light (R) revealed that the exaggerated responses of the hp mutant are not caused by a higher absolute phytochrome level or a reduced rate of phytochrome destruction. Fluence-response relationships for anthocyanin synthesis after a blue-light pretreatment were studied to test if the hp mutant conveys hypersensitivity to the far-red light (FR)-absorbing form of phytochrome (Pfr), i.e. the threshold of Pfr required to initiate the response is lower. The response range for the hp mutant and wild type was identical, although the former exhibited a 6-fold larger response. Moreover, the kinetics of anthocyanin accumulation in continuous R were similar in the wild-type and hp-mutant seedlings, despite the latter accumulating 9-fold more anthocyanin. Since the properties of phytochrome are the same, the hp mutation appears to affect the state of responsiveness amplification, i.e. the same amount of Pfr leads to a higher response in the hp mutant. We therefore propose that the hp mutation is associated with an amplification step in the phytochrome transduction chain. Escape experiments showed that the anthocyanin synthesis after different light pretreatments terminated with a R pulse was still 50% FR reversible after 4–6 h darkness, indicating that the Pfr pool regulating this response must be relatively stable. However, fluence-rate response relationships for anthocyanin synthesis and hypocotyl growth induced by a 24-h irradiation with 451, 539, 649, 693, 704 and 729 nm light showed no or a severely reduced response in the au and au, hp mutants, suggesting the importance of PI in these responses. We therefore propose that the capacity for anthocyanin synthesis (state of responsiveness amplification) could be established by PI, while the anthocyanin synthesis is actually photoregulated via a stable Pfr pool. The Hp gene product is proposed to be an inhibitor of the state of responsiveness amplification for responses controlled by this relatively stable Pfr species.     

EXTENSION GROWTH AND ANTHOCYANIN RESPONSES OF PHOTOMORPHOGENIC TOMATO MUTANTS TO CHANGES IN THE PHYTOCHROME: PHOTOEQUILIBRIUM DURING THE DAILY PHOTOPERIOD

Four genotypes of tomato (Lycopersicon esculentum Mill.) in the genetic background ‘Aha Craig’ were used: an aurea (au) mutant, deficient in the bulk light-labile phytochrome pool; a high pigment (hp) mutant, showing exaggerated phytochrome responses at the time of de-etiolation; the au,hp double mutant and the isogenic wild type (WT). A dramatic increase in plant height resulting from an increase in the length of all internodes for each of the genotypes studied was observed upon reduction of the red light: far-red light photon ratio (R:FR) from 6.90 to 0.13 by addition of FR for the whole photoperiod. A concomitant increase in leaf length was also observed. Since au and au,hp mutants, deficient in the bulk light-labile phytochrome pool, respond to this reduction in the R:FR these data demonstrate that the phytochrome pool that mediates this response is present and fully functional. Anthocyanin was detectable in the comparably developed young growing leaves of the WT and hp mutant under the high R:FR, but not in the au and au,hp mutants, suggesting that the potential for anthocyanin synthesis is correlated with the presence of the bulk light-labile phytochrome pool. The kinetics of anthocyanin decrease in the young growing leaves were investigated in the hp mutant and the results suggest a very rapid cessation of flavonoid biosynthesis upon reduction of the R:FR. The functions of different phytochrome types are discussed.     

MODE OF COACTION OF PHYTOCHROME AND BLUE LIGHT PHOTORECEPTOR IN CONTROL OF HYPOCOTYL ELONGATION

Abstract The rate of hypocotyl longitudinal growth in seedlings of Sesamum indicum L. is strongly inhibited by continuous blue light (cBL)† and slightly by continuous far-red light while continuous red light (cRL) or red light pulses are hardly effective from 60 h after sowing onwards. Between 36 and 60 h after sowing the growth rate responds to red light pulses the effect of which is fully reversible by long wavelength far-red light. When seedlings are kept in cBL for 3 days and then treated with red light hypocotyl growth rate responds strongly. However, RL effectiveness decreases with time after transfer from BL to RL. BL → darkness transfer experiments with different levels of P_fr established at the beginning of darkness show that after a BL pretreatment phytochrome (P_fr) alone is capable of fully controlling growth rate. When white light (WL) is given no BL effect is detectable in weak WL. Only high light fluxes maintain a typical BL growth rate. At medium WL fluxes elongation rate returns gradually to the dark rate. The simplest explanation of the data is that light absorbed by a separate BL photoreceptor is necessary to maintain responsivity to P_fr. With increasing age of the seedlings the requirement for BL increases strongly. On the other hand, brief light pulses—given to demonstrate photoreversibility of phytochrome—remain equally effective provided that responsivity to P_fr exists.     

THE DOSE RESPONSE CURVE IN PHYTOCHROME-MEDIATED ANTHOCYANIN SYNTHESIS IN THE MUSTARD SEEDLING

Abstract —The dose response curve for light (phytochrome)-induced anthocyanin synthesis was determined in the mustard seedling. The curve gives the amount of anthocyanin (A) synthesized within 24 h as a function of the amount of P_fr* produced by a brief light pulse. The [P_fr] response curve is composed of two linear parts with very different slopes ( a _1,2) connected by a relatively narrow transient range (curved segment). The [P_fr] response curve extrapolates precisely through zero [P_fr]. The reciprocity law is valid over the whole range investigated (up to 320 s of irradiation). It is concluded that the initial (or primary) reaction of P_fr (P_fr+ X → P_frX) does not involve any significant cooperativity in the case of phytochrome-mediated anthocyanin synthesis. It is speculated that the linear parts of the [P_fr] response curve truly reflect the mode of phytochrome action ( A = a _1,2 [P_fr]; X does not come into play since it is not rate limiting) whereas the curved segment represents a transition of the reaction matrix of P_fr. The large difference between a₁ and a₂ seems to indicate that the physiological effectiveness of a given amount of P_fr (or P_frX) is determined by [P_fr] through a P_fr-induced change in the reaction matrix.     

THE ''HIGH-IRRADIANCE RESPONSE'' IN ANTHOCYANIN FORMATION AS RELATED TO THE PHYTOCHROME LEVEL

Abstract— The involvement of phytochrome in light-mediated anthocyanin synthesis in the mustard seedling ( Sinapis alba L.) under inductive conditions (law of reciprocity valid) was shown previously (Drumm and Mohr, 1974). In the present paper the hypothesis (Hartmann, 1966) is checked that light-mediated anthocyanin synthesis in continuous high-irradiance far-red light ('high-irradiance response') is also due exclusively to phytochrome. The data indicate that the effectiveness of the far-red light is indeed a function of total phytochrome [ P_total ]* and therewith [ P_fr ]*. The data are not consistent with the suggestion (Schneider and Stimson, 1972) that photosynthesis (in particular, photosystem I) is involved in the 'high-irradiance response' of photomorphogenesis.     

Phytochrome Control of Anthocyanin Biosynthesis in Tomato Seedlings: Analysis Using Photomorphogenic Mutants

Anthocyanin biosynthesis has been studied in hypocotyls and whole seedlings of tomato (Lycoperskon esculentum Mill.) wild types (WTs) and photomorphogenic mutants. In white light (WL)/dark (D) cycles the fri¹ mutant, deficient in phytochrome A (phyA), shows an enhancement of anthocyanin accumulation, whereas the tri¹ mutant, deficient in phytochrome Bl (phyBl) has a WT level of anthocyanin. Under pulses of red light (R) or R followed by far-red light (FR) given every 4 h, phyA is responsible for the non-R/FR reversible response, whereas phyBl is partially responsible for the R/FR reversible response. From R and blue light (B) pretreatment studies, B is most effective in increasing phytochrome responsiveness, whereas under R itself it appears to be dependent on the presence of phyBl. Anthocyanin biosynthesis during a 24 h period of monochromatic irradiation at different flu-ence rates of 4 day-old D-grown seedlings has been studied. At 660 nm the fluence rate-response relationships for induction of anthocyanin in the WT are similar, yet complex, showing a low fluence rate response (LFRR) and a fluence rate-dependent high irradiance response (HIR). The high-pigment-1 (hp-1) mutant exhibits a strong amplification of both the LFRR and HIR. The fri¹ mutant lacks the LFRR while retaining a normal HIR. In contrast, a transgenic tomato line overexpressing the oat PHYA3 gene shows a dramatic amplification of the LFRR. The tri¹ mutant, retains the LFRR but lacks the HIR, whereas the fri¹, tri¹ double mutant lacks both components. Only an LFRR is seen at 729 nm in WT; however, an appreciable HIR is observed at 704 nm, which is retained in the tri¹ mutant and is absent in the fri¹ mutant, indicating the labile phyA pool regulates this response component.     

PHOTOMORPHOGENIC RESPONSES TO UV RADIATION III: A COMPARATIVE STUDY OF UVB EFFECTS ON ANTHOCYANIN and FLAVONOID ACCUMULATION IN WILD-TYPE and aurea MUTANT OF TOMATO (Lycopersicon esculentum MILL.)

The UV-mediated induction of anthocyanin and UV-absorbing compounds was characterized in etiolated hypocotyls of wild-type and aurea (au) mutant tomato seedlings. Ultraviolet radiation induced significant increases of anthocyanin and UV-absorbing compounds in hypocotyls of die au mutant and of its isogenic wild-type, but the differences in the time courses of UV-induced pigment accumulation indicate mat different photoregulatory mechanisms are involved for each of these two groups of pigments. It appears mat prolonged presence of adequate levels of UVB (290–320nm) energy and consequently the action of a specific UVB photoreceptor are indispensable for the photoinduction of anthocyanin accumulation in UV-irradiated hypocotyl of the au mutant that is missing the labile phytochrome pool. The large difference found between the wild-type and the au mutant strongly indicate the involvement of labile phytochrome as the primary functional photoreceptor for the photoinduction of anthocyanin accumulation in wild-type tomato hypocotyls. The UVB photoreceptor could at least partly replace the action of labile phytochrome (as far as anthocyanin accumulation is concerned) when the functional phytochrome pool is missing as in the au mutant. The general picture of UV-mediated induction of total UV-absorbing compounds shows only a macroscopic difference between wild-type and die au mutant of tomato: the higher initial level (in darkness) of these compounds in die wild-type in contrast to the au mutant. Although there is UV-induced accumulation of UV-absorbing compounds in bom genotypes, the levels in the au mutant never reach mat of the wild-type under the same UV exposure. A UVB photosensor may play a more important role in the photoinduction of UV-absorbing compounds. Indeed, in the absence of labile phytochrome, i.e. in the au mutant, a UVB-absorbing photoreceptor alone is able to establish high responsiveness for the UV-induced flavonoid accumulation.     

PHYTOCHROME and THE DEVELOPMENT OF PHOTOPHOSPHORYLATION CAPACITY—AN APPRAISAL OF THE EXPERIMENTAL APPROACH*

Abstract— Development of the capacity for photophosphorylation (= total capacity for light-driven ATP formation) in the mustard ( Sinapis alba L.) cotyledons is strongly influenced by a red light pulse pretreatment which operates through phytochrome. The present report deals with several objections raised against the in situ assay of the rate of photophosphorylation. Experimental evidence is given in support of the assumption that the linear increase of the ATP content of the cotyledons as measured over 1.5 min after the onset of saturating white light (370 Wm^-2) in fact represents the maximum rate of photophosphorylation ('capacity'). Moreover, it is confirmed that control by phytochrome of the development of the photophosphorylation capacity and of the capacity for chlorophyll synthesis are unrelated phenomena. The failure of development of the capacity for photophosphorylation in isolated cotyledons from dark-grown seedlings cannot be attributed to deficiencies of chlorophyll synthesis.
It is concluded that the photophosphorylation response is particularly useful to study the mechanism of phytochrome (P_fr) action in case of a response which involves a threshold reaction and an interorgan (hook→cotyledon) cooperation.     

MODE OF COACTION BETWEEN UV-A AND LIGHT ABSORBED BY PHYTOCHROME IN CONTROL OF APPEARANCE OF RIBULOSE-1.5-BISPHOSPHATE CARBOXYLASE IN THE SHOOT OF MILO (Sorghum vulgare Pers.)

Abstract— In shoots of milo ( Sorghum vulgare Pers.) appearance of ribulosebisphosphate carboxylase (RuBPCase) and of translatable mRNA for its small subunit is stimulated strongly by red light (R, operating through phytochrome) and UV-A light (UV-A). Ultraviolet-A is more effective than R.
The mode of coaction between phytochrome and light absorbed by the blue/UV-A light photoreceptor ('cryptochrome') was analyzed in detail in case of enzyme appearance. Fluence rate dependencies, lagphases and the time course of the response are compatible with the view that UV-A intensifies a process which is occurring in R alone albeit at a lower rate.
With both light qualities the light effect is fully reversible by far-red light up to 1 h. This means that during this period only phytochrome (P_fr) controls the terminal response, i.e. the actual appearance of RuBPCase. During this 1 h period after the onset of light UV-A or R have no effect on the level of translatable mRNA for the small subunit of RuBPCase indicating that it requires more than 1 h for the light signal to affect gene expression.
When R and UV-A are given longer onset of escape from full reversibility is observed at the same time for both light qualities in the case of RuBPCase appearance. The extent of the reversible response is greater after UV-A pretreatment than after a R pretreatment.
It is argued that the data are consistent with the concept that phytochrome (P_fr) controls the terminal photoresponse, in the present case appearance of RuBPCase, while light absorbed via cryptochrome leads to an increase in responsiveness of the RuBPCase producing machinery towards P_fr.     

APPEARANCE OF PHOTOPHOSPHORYLATION CAPACITY: THRESHOLD vs GRADED CONTROL BY PHYTOCHROME

Abstract— It is shown that in attached mustard cotyledons graded control of chlorophyll synthesis by physiologically active phytochrome (P_fr) and threshold control by P_fr of the 'potential capacity' to photophosphorylate are totally different phytochrome actions even though both controls are essential for the build-up of the same functional complex, the machinery for photophosphorylation. The essential findings are as follows: The action of P_fr (made by a 1 min red light pulse) on the capacity and efficiency of photophosphorylation is rapid—detectable after 15 min and completed after 30 min—whereas the action of P_fr on chlorophyll formation is slower—only detectable 45 min after the original red light pulse (R). Detailed escape studies (loss of full reversibility of the inductive effect of a R pulse by far-red) show that the effect of a R pulse on chlorophyll synthesis remains fully reversible for 45 min whereas the action of P_fr on the capacity for photophosphorylation is very fast (occurring within 2 min). Control of capacity for photophosphorylation is a threshold response (whereby the threshold value is approximately 1.25% P_fr based on total phytochrome at 36 h = 100%) whereas control by P_fr of chlorophyll synthesis is graded. Control of capacity for photophosphorylation by P_fr only operates if the hypocotyl hook is connected to the cotyledons for at least 2 min after the inductive R pulse, i.e. until full escape from reversibility has occurred, whereas chlorophyll formation in the cotyledons is not affected by the separation of hook and cotyledons.     

Effects of Moderately Low Temperature (20°C) on Phytochrome Responses During Preirradiation: Anthocyanin Synthesis in Sorghum bicolor at High- and Low-Pfr/Ptot Ratios

Abstract— Effects on phytochrome-mediated anthocyanin synthesis of moderately low temperature (MLT) given during the preirradiation culture period were studied with seedlings of broom sorghum ( Sorghum bicolor Moench, cvs. Acme Broomcorn and Sekishokuzairai-Fukuyama). Seedlings were grown in the dark at 20°C (MLT) and 24°C (control). The MLT treatment strikingly enhanced the action induced by a red light (R) pulse above ca 200 μmol m⁻² and suppressed the action induced by an R pulse below ca 30 μmol m⁻² and by a far-red light (FR) pulse alone. We refer to these MLT-affected distinct responses as "high-Pfr/Ptot response" and "low-Pfr/Ptot response," as they have features different from the high-irradiance and very-low-fluence responses, respectively. The destruction rate of spectroscopically detectable phytochrome (phyA) and the time course of escape of anthocyanin synthesis from FR reversibility did not match, and hence the possibility of phyA being involved in high-Pfr/Ptot response was rejected, although it might be involved in low-Pfr/Ptot response. Possible mechanisms for the two distinct phytochrome responses are discussed.     

PHOTOINACTIVATION OF A Chlamydomonas MUTANT(NL–11) IN THE PRESENCE OF METHIONINE: ROLES OF H2O2 and O-2

Abstract— A mutant of Chlamydomonas reinhardtii (NL–11) isolated from a wild type (137c+) was inactivated in the light in the presence of methionine at concentrations where the wild type was not inactivated. The inactivation was suppressed by either catalase or superoxide dismutase (SOD). Light-induced H₂O₂ formation and nitroblue tetrazolium (NBT) reduction inNL–11 were greater than those in the wild type. Methionine stimulated both the H₂O₂ formation and the NBT reduction inNL–11 as well as the wild type. The light-induced NBT reduction inNL–11 in the presence of methionine was partially suppressed by externally added SOD suggesting the participation of O^-₂. These results suggest that the hypersensitivity ofNL–11 to methionine in the light is due to stimulated formation of H₂O₂ and O^-₂.     

STIMULATION OF THE SHIBATA SHIFT BY PHOTOCHROME IN THE COTYLEDONS OF THE MUSTARD SEEDLING SINAPIS ALBA L.

Abstract— In the cotyledons of the mustard seedling Sinapis alba L. the duration of the Shibata shift can be greatly shortened by a pretreatment with light pulses prior to the protochlorophyllide– chloro-phyllide a photoconversion. It was shown that the light pulses act through photochrome (P _fr ). Since reversibility of a red light pulse induction by a far-red light pulse is rapidly lost (within 2 min) it is concluded that at least the initial action of P_fr occurs rapidly in this response. On the other hand, the effect of a red light pulse on the rate of protochlorophyll regeneration in the mustard seedling cotyledons is fully reversible by a far-red light pulse for more than 5 min. It is concluded that control of protochlorophyll regeneration and control of the Shibata shift by phytochrome cannot be consequences of the same initial action of P_fr Apparently P_fr controls both phenomena independently.     

INDEPENDENT EFFECTS OF PHYTOCHROME AND NITRATE ON NITRATE REDUCTASE AND NITRITE REDUCTASE ACTIVITIES IN MAIZE

Abstract— Involvement of phytochrome in the regulation of nitrate reductase (NR) and nitrite reductase (NIR) activities in excised, etiolated leaves of Zea mays (L.) variety 'Ganga-5' is demonstrated using low energy and high irradiance responses of phytochrome action. Photoreversibility by far-red light of red light stimulated increases in NR and NIR activities was lost by 2 h. Red light given to the leaves, when induction by NO^-₃, was saturated, further increased both enzyme activities. Even if red light was given 4–8 h before NO^-₃, it still increased both NR and NIR activities.     

KINETIC STUDIES TO INTERPRET 'HIGH-ENERGY PHENOMENA' OF PHOTOMORPHOGENESISL ON THE BASIS OF PHYTOCHROME

Abstract— Kinetic studies with the mustard seedling ( Sinapis alba L.) support the hypothesis that the so-called 'high energy reaction' of photomorphogenesis can be understood solely on the basis of phytochrome. Light-induced anthocyanin synthesis (a typical 'positive' photoresponse⁽¹⁾ and light dependent inhibition of hypocotyl lengthening (a typical 'negative' photoresponse⁽¹⁾) have been investigated. In order to explain the experimental data we have to assume that there are two different types of phytochrome 730 which differ greatly as far as their resistance to irreversible destruction is concerned. The existence of these two different types of phytochrome 730 has already been postulated on the basis of spectrophotometric measurements in vivo .⁽²⁾     

DEPENDENCE OF Pfr/Ptot-RATIOS ON LIGHT QUALITY and LIGHT QUANTITY

Abstract— Not only the spectral distribution of the light source determines the relative proportion of phytochrome in the P_fr(P_r) form, the P_fr/P_tot-ratio also depends strongly on the fluence rate of the irradiation. This dependence has been observed in the cotyledons of etiolated mustard seedlings for blue light of fluence rates below 20 Wm_-2. It has also been observed for white light and seems to be a characteristic of the phytochrome system resulting from the involvement of phytochrome thermal reactions as well as P_r P_fr photoconversions. The fluence rate dependence of P_fr/P_tot-ratios can be used to analyze the characteristic transformations of the phytochrome system. Phototransformations together with a fast thermal transformation (τ_½⋍ 3min) are consistent with the results obtained for blue and white light.     

THE EFFECT OF PROLONGED LIGHT EXPOSURE ON THE EFFECTIVENESS OF PHYTOCHROME IN ANTHOCYANIN SYNTHESIS IN TOMATO SEEDLINGS*

Abstract— The hypocotyl of the tomato ( Lycopersicon esculentum ) seedling synthesizes large amounts of anthocyanin if exposed to prolonged light. Single light pulses are totally ineffective. The involvement of phytochrome can be shown by light pulse treatments following a prolonged light exposure. It is predominantly the action of blue/UV light which leads to a high responsiveness of anthocyanin synthesis towards phytochrome. Moreover, the data suggest a phytochrome-independent action of blue/UV light, in particular of UV-B, on anthocyanin synthesis.     

AN ANALYSIS OF PHYTOCHROME-MEDIATED THRESHOLD CONTROL OF HYPOCOTYL GROWTH IN MUSTARD (Sinapis alba L.) SEEDLINGS†

Hypocotyl elongation in mustard (Sinapis alba L.) seedlings is known to be controlled by phytochrome (P_fr) through a threshold response. This phytochrome-mediated threshold response was studied in detail with the following results: (i) The P_fr threshold value required to suppress hypocotyl growth is much lower (0.03% P_rr, based on total phytochrome in the hypocotyl at 36 h after sowing = 100%) than those threshold valued observed previously in threshold control by hook phytochrome of appearance of 'potential capacity for photophosphorylation' and lipoxygenase appearance in the mustard cotyledons (1.25% P_tr, based on total phytochrome in the hypocotyl at 36 h after sowing = 100%). This probably explains why hypocotyl elongation is so extremely sensitive to light, (ii) The P_fr threshold value controlling hypocotyl growth is a system constant, independent of total phytochrome content, developmental age and actual growth rate, (iii) Threshold control of hypocotyl elongation is unaffected by the removal of the cotyledons and half of the hook. However, removal of the whole hook totally eliminates any light control over the residual hypocotyl growth, (iv) After termination of the threshold control, the hypocotyl growth rate immediately returns to precisely that found in untreated dark control even though the partial growth rates of the different parts of the hypocotyl are quite different, relative to their dark controls. Obviously, the organ grows as an integrated unit.
It is concluded that the all-or-none threshold control over hypocotyl growth is exerted from the plumular hook. It appears that the hook can send off phytochrome all-or-none signals in both directions, to the cotyledons and to the hypocotyl.     

PHYTOCHROME INTERMEDIATES IN VIVO– III KINETIC ANALYSIS OF INTERMEDIATE REACTIONS AT LOW TEMPERATURE

Abstract— Kinetic experiments have provided evidence for a series of light and dark reactions of phytochrome intermediates at low temperature in Pisum epicotyl tissue. A photoequilibrium exists between P_r and P₆₉₈, and between P_fr and P₆₅₀. A dark reversion of P₆₉₈P_r and P₆₅₀p_fr at –70°C has been demonstrated. When cooled to 70°C under incandescent light, most of the phytochrome in the tissue is driven into photochemically unreactive intermediates. About 2% of the phytochrome remains as weakly absorbing intermediates that form P_r and P_fr in darkness. A scheme is presented for phytochrome phototransformation in vivo.     

PHOTOREVERSIBLE Ca2+-DEPENDENT AGGREGATION OF PURIFIED PHYTOCHROME FROM ETIOLATED PEA AND RYE SEEDLINGS

The aggregation of phytochrome purified from etiolated pea ( Pisum satirum cv. Alaska) and rye ( Secale cereale cv. Cougar) tissues was investigated by centrifugation and turbidimetry. Purified pea phytochrome (A₆₆₉/A₂₈₀= 0.88), if irradiated with red light, became precipitable in the presence of CaCl₂. The precipitation upon red-light irradiation was optimal at a Ca^2- or Mg²⁺ concentration of 10–20 m M , was greater at increased phytochrome concentration or lower pH values, and was inhibited by 0.1 M KG. The precipitated phytochrome slowly became soluble after far-red light exposure.
Turbidity of pea phytochrome solutions after red-light irradiation also increased rapidly in the presence of either Ca²⁺ or Mg²⁺. Far-red light exposure after the red light cancelled the turbidity increase. Rye phytochrome showed less turbidity increase than pea phytochrome and occurred only in the presence of Ca²⁺. Partially degraded pea phytochrome produced by endogenous proteases in the extract did not show the turbidity increase. Undegraded pea phytochrome also associated with microsomal fractions under conditions similar to those described above, but the partially degraded phytochrome did not.