The effect of a calcium-channel antagonist, nifedipine and agonist, Bay K-8644, on the phytochrome-controlled swelling of etiolated wheat protoplasts

The effect of external Ca²⁺ and Ca²⁺-channel modulators on the phytochrome-controlled swelling of etiolated wheat ( Triticum aestivum L. cv. Arminda) mesophyll protoplasts has been studied. The red light (R)-stimulated swelling of the protoplasts requires Ca²⁺ in the surrounding medium and maximum response was observed in a medium containing I m M CaCI₂. Far-red light (FR) irradiation of protoplasts in the presence or absence of Ca²⁺ does not influence the protoplast volume. The Ca²⁺-channel antagonist nifedipine prevents R-induced protoplast swelling at very low concentrations (0.1 μ M ). The Ca²⁺ -channel agonist Bay K-8644 stimulates the swelling of protoplasts incubated in darkness or irradiated with FR. Action of nifedipine depends on whether it is applied before or after the R pulse. The results are compatible with the hypothesis that phytochrome controls the activity of dihydropyridine-sensitive L-type Ca²⁺ channels.     

Effects of Temperature and Various Red or Far-red Irradiations on Phytochrome- and Gibberellin A3-mediated Germination Control in Partially Hydrated Lettuce Seeds

Dark reversion of phytochrome in partially hydrated lettuceseeds (Lactuca sativa cv. Grand Rapids) is temperature dependent.After initial red irradiation (R) the higher the storage temperature,the higher the dark reversion rate. Following dark moist storage(DMS) at 30 ?C for 15 d none of the seeds receiving initialR germinated, whereas seeds stored at 0 ?C germinated nearlyas well (about 80%) as unstored controls. The half-time fordark reversion at 20 ?C and 30 ?C is 9 d and 3 d respectively.Repeated R treatments given at 5 d intervals during DMS at 20?C and 30 ?C maintained a high germination capacity. With threeor more R treatments the effect of high temperature largelydisappeared. Dark reversion of phytochrome was not observed in partiallyhydrated lettuce seeds receiving continuous red irradiation(cont R) for two or more days. The promotive effect of contR could be reversed at any time with a brief far-red irradiation(FR), indicating that the phytochrome system remained fullyphototransformable. With continuous far-red light (cont FR)the ability of gibberellin A₃ (GA₃) to stimulate germinationdisappeared and response to GA₃ also diminished in cont R followedby FR but at a slower rate indicating the induction of secondarydormancy in these partially hydrated seeds. This induction ofdormancy was retarded by repetitive or cont R but was enhancedby cont FR. The results of this study suggest a role for theaccumulated stable intermediates of phytochrome transformationin partially hydrated seeds with repeated or continuous R treatmentsand different effects of GA₃ and R in the regulation of germination. Key words: Phytochrome, Lactuca sativa, Seed germination, Temperature, Dark reversion of phytochrome, Seed water content     

Regulation of Phytochrome on Swelling of Protoplasts Isolated from Hypocotyl of Etiolated Mung Bean Seedlings

Protoplasts from hypocotyls of etiolated mung bean (Phaseolus raditus L. ) seedlings were maintained at a constant osmotic potential at 20±2℃, and they were found to swell gradually after being pulsed with red light (R) (10.5 W · m-2, 3 min) when CaCl2 was present in the medium. The volume reached maximum during 30--60 min after R-irradiation and decreased swelling afterwards. Farred light (FR) irradiation in presence or absence of Ca2+ did not influence the protoplast volume. The R-effect was photoreversible by subse- quent FR (2.5 W · m-2, 5 min) irradiation, usually seen over two R-FR cycles. Furthermore, swelling response was in positivecorrelation with red light intensity and duration of R pulse, indicating the involvement of phytoehrome. FR became less effective in reversing the effect of R after 10 min in dark between R and FR. Protoplast swelling occurred only when Ca2+ ions (1 mmol/L) then Ca2+ ions (1 mmol/L) is added to the medium 5 rain after R. The effect of Ca2+ could not be replaced by Mg2+, Ba2+, Zn2+, or K+. The time course of water (3H20) uptake into protoplasts after R-irradiation was consistent with the trend of protoplast swelling, indicating the existence of certain relationship between the swelling and water uptake of the protoplasts.     

The role of calcium ions in phytochrome-controlled swelling of etiolated wheat (Triticum aestivum L.) protoplasts

Protoplasts from dark-grown wheat (Triticum aestivum L.) maintained at a constant osmotic potential at 22°C, were found to swell upon red irradiation (R) and the effect was negated by subsequent far-red light (FR), indicating phytochrome involvement. Swelling only occurred when Ca²⁺ ions were present in the surrounding medium, or were added within 10 min after R. Furthermore, Mg²⁺, Ba²⁺ or K⁺ could not replace this requirement for Ca²⁺. The presence of K⁺ did not enhance the Ca²⁺-dependent swelling response. When the Ca²⁺-ionophore A 23187 was added to the medium, protoplasts swelled in the dark to the same extent as after R. Both the Ca²⁺-channelblocker Verapamil and La³⁺ inhibited R-induced swelling. It is proposed that R causes the opening of Ca²⁺-channels in the plasma membrane. Boyle-van't Hoff analyses of protoplast volume after R and FR are consistent with the conclusion that R irradiation causes changes in membrane properties.Abbreviations EDTA ethylenediaminetetraacetic acid - FR far-red light - nov non-osmotic-volume - Pfr FR-absorbing form of phytochrome - Pr R-absorbing form of phytochrome - R red light     

STIMULATION OF NITRATE NET UPTAKE AND REDUCTION BY RED AND BLUE LIGHT AND REVERSION BY FAR-RED LIGHT IN THE GREEN ALGA ULVA RIGIDA1

The steady-state levels of nitrate, nitrite, and ammonium were estimated in the green alga Ulva rigida C. Agardh in darkness after addition of 0.5 mM KNO₃ and irradiation with red (R) and blue (B) light pulses of different duration (5 and 30 min). The net uptake of nitrate was very rapid. Seventy-five percent of the nitrate added was consumed after 60 min in darkness. Although uptake was stable after R or B, efflux of nitrate occurred within 3 h in the dark control and when R or B were followed by far-red (FR) irradiation. The internal nitrate concentration after 3 h in darkness was similar after R and B light pulses; however, the intracellular ammonium was higher after R than after B. The intracellular nitrate and ammonium decreased when FR tight pulses were applied immediately after R or B. Thus, the involvement of phytochrome in the transport of nitrate and ammonium is proposed. Nitrate reductase activity, measured by the in situ method, was increased by both R and B light pulses. The effect was partially reversed by FR light. Nitrate reductase activity was higher after 5 min of R light than after 5 min of B. However, after 30-min light pulses, the relative increase in activity was reversed for R and B. We propose that phytochrome and a blue-light photoreceptor are involved in regulation of nitrogen metabolism. Nitrate uptake and reduction correlates with previously detected light-regulated accumulation of protein in Ulva rigida under the same experimental conditions.     

The involvement of a G-protein in phytochrome-regulated, Ca2+-dependent swelling of etiolated wheat protoplasts

The red light (R)-induced swelling of mesophyll protoplasts, isolated from dark-grown wheat ( Triticum aestivum L. cv. Arminda) leaves, was inhibited by guanosine-5'-0-(2-thiodiphosphate) (GDP-β-S). In darkness or after control irradiation with far-red light (FR), guanosine-5'-O-(3-thiotriphosphate) (GTP-γ-S) induced swelling to the same extent as after R. Both GDP-β-S and GTP-γ-S were introduced into the cytoplasm by means of electroporation. The possibility of R-induced activation of the phosphatidylinositol pathway of transmembrane signalling was investigated. Neomycin, Li⁺ and l-(5-isoquinolinesulfonyl)-2-methylpiperazine (H₇) inhibited the R-induced swelling. Phorbol 12-myristate 13-acetate (PMA) induced swelling after control irradiation with FR. Neomycin and Li⁺ also inhibited GTP-γ-S-induced swelling. These results suggest that a GTP-binding protein is involved in the phytochrome-regulated swelling response. Addition of N⁶, 2'-0-dibutyryladenosine 3':5'-cyclic monophosphate (DB-cAMP) induced swelling to the same extent as R-irradiation. The calmodulin antagonist N-(6-aminohexyl)5-chloro-l-naphthalenesulfonamide (W₇) induced swelling after FR, while R-induced swelling was not affected. The less active analogue N-(6-aminohexyl)-l-naphthalenesulfonamide (W₅) induced no swelling after FR. It is speculated that the protoplast volume is correlated with the cytoplasmic concentration of free Ca²⁺.     

Regulation of swelling of etiolated-wheat-leaf protoplasts by phytochrome and gibberellic acid

The effect of light on the size of intact protoplasts isolated from the primary leaves of etiolated Triticum aestivum was studied. A 2-min red-light irradiation in the presence of 1 mM KCl was sufficient to cause a swelling of protoplasts compared with those maintained in darkness. The effect was photoreversible by far-red light over two light cycles, indicating the involvement of phytochrome. At 4°C, escape from reversibility occurred between 2 and 5 min after the exposure to red light. In exposure-response experiments, 20 s red light at 27 μmol m^-2s^-1 was sufficient to saturate the response. Exogenous gibberellic acid added in darkness in the presence of KCl also induced protoplast swelling. Gibberellins may act as an intermediate in the phytochrome-induced swelling of protoplasts.     

Effects of Repetitive Acid Immersion, Red Light, and Gibberellin A3 Treatments on Phytochrome-mediated Germination Control in Skotodormant Lettuce Seeds

Dry lettuce seeds (Lactuca sativa L. cv. Grand Rapids), whichreceived 5 min far-red light (FR) 0.5 h after the onset of waterimbibition, showed 17% and 50% germination without and withacid immersion treatment (pH 0.1) for 1 h and rinsing with water,respectively. The acid treatment caused only 6% germinationor less in FR-treated seeds held for 10 to 30 d in dark storage.The 10 to 30 d skotodormant seeds did not respond to red light(R) or gibberellin A₃ (GA₃) singly, but showed 84% or higherpercentage germination if 1 h acid immersion was given beforeR or GA₃. The 20 d skotodormant seeds, which received R treatmentat day 10 but remained dormant showed 89% germination with onlyacid treatment. Similar values were obtained with 30 d skotodormantseeds which received one or two R treatments at day 10 or 20,i.e. the only requirement for these R-treated dormant seedswas an acid immersion. This releases the skotodormancy and rendersthe seeds more sensitive to R or GA₃, but the skotodormancywas initiated again if no light or hormone treatments were givenimmediately. The repetitive R or GA₃ treatments, which did notcause skotodormant seeds to germinate, lessened the degree ofskotodormancy. The germination of these skotodormant seeds canonly be induced by the synergistic action of R and GA₃. In thisstudy, GA₃ caused higher germination percentages in R-treatedskotodormant seeds than R stimulated in GA3-treated seeds. Itis suggested that (i) repetitive R or Ga₃ treatments maintaina high endogenous level of the far-red-absorbing form of phytochrome(P_fr) and GA activity, respectively, (ii) the accumulated stableintermediates of phytochrome persist in fully-imbibed skotodormantseeds for up to 20 d, without phytochrome expressing its functionuntil the seeds are acidified and (iii) a model is formulatedto interpret the results of acidification, growth promotersand R effects on germination of light-sensitive lettuce seeds. Key words: Phytochrome, Latuca saliva, seed germination, dark reversion of phytochrome, gibberellin A₃, acidification, skotodormancy     

Phytochrome Regulation of the Response to Exogenous Gibberellins by Epicotyls of Vigna sinensis

The elongation rate of cowpea epicotyls from whole cowpea (Vigna sinensis) seedlings and derooted and debladed plants (explants) increased after the main light period (8-hour duration) was extended with either continuous low intensity tungsten light or brief (5 minutes) far-red (FR) irradiation. This end-of-day FR effect was reversed by red (R) irradiation suggesting the involvement of phytochrome. These results confirm and extend those obtained previously with other species. Localization studies indicate the epicotyl to be the site of the photoreceptor. Treatment of cowpea seedlings with paclobutrazol, a gibberellin (GA) biosynthetic inhibitor, abolished the FR promoted epicotyl elongation, indicating a role for GAs in this process. There was no significant difference in epicotyl elongation rates of R plus FR irradiated explants treated with GA₁ or GA₂₀ and R irradiated explants treated with GA₁. However, R irradiation inhibited subsequent epicotyl elongation of GA₂₀ treated explants. Moreover, the observation, using GC-MS, that GA₁ and GA₂₀ are native GAs in cowpea lends support to the concept that phytochrome may control the conversion of endogenous GA₂₀ to GA₁ in cowpea.     

Dark transformations of phytochrome in cotyledons of Pharbitis nil

Pharbitis seedlings grown in total darkness or continuous far-redirradiation were exposed to 30 min of red irradiation followedby a dark period, and in vivo phytochrome in their cotyledonswas photometrically assayed at various times. Loss of photo-reversibilityof P_fr after the exposure to red light occurred without darkreversion to P_r in cotyledons of both seedlings. P_fr decay inthe former cotyledons was mostly prevented in the first 30 minunder red light illumination, while that in the latter occurredwithout such a lag phase. P_fr was no longer photometricallydetectable by the eighth hr after irradiation at both 18?C and25?C. No evidence has yet been obtained to show a correlation betweenphotometrically detectable phytochrome in vivo and the red far-redreversible responses of flowering. (Received August 6, 1974; )     

Mechanism of phytochrome action in the control of biosynthesis of anthocyanin in Brassica oleracea

-The synthesis of anthocyanin in red-cabbage is very sensitive to control by light, R/FR reversibility being effected by exposures of 5 min duration, The demonstration of this control does not depend upon a preceding irradiation of high-intensity light but depends upon the duration of incubation in darkness subsequent to irradiation. R/FR reversibility is well shown in seedlings kept in darkness for 48 hr after exposure but after 120 hr this reversibility is no longer evident. This is due to the fact that a further synthesis of anthocyanin occurs in unexposed seedlings and in FR and R/FR treated material in the period from 48 to 120 hr but does not occur in the R treatment after 48 hr. Reagents such as n-propanol which are believed to increase membrane permeability, greatly increase anthocyanin synthesis in dark grown material. n-PrOH also reverses the effect of 5 min FR irradiation but, by contrast with R light, does not promote PAL activity. It is concluded that the limitation to synthesis in material unexposed to light is substrate availability at the site of flavonoid biosynthesis, rather than the level of PAL activity. The evidence presented supports the hypothesis that R/FR reversible phytochrome action involves the control of the passage of substrate through a membrane to the site of anthocyanin biosynthesis.     

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.     

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

For dark-grown seedlings of Pharbitis nil capacity to flower in response to a single inductive dark period was established by 24 h white, far-red (FR) or ruby-red (BCJ) light and by a skeleton photoperiod of 10 min red (R)-24 h dark-10 min R. FR alone was ineffective without a brief terminal (R) irradiation, confirming that the form of phytochrome immediately prior to darkness is a crucial factor for flowering in Pharbitis. The magnitude of the flowering response was significantly greater after 24 h FR or white light (WL) (at 18° C and 27° C) than after two brief skeleton R irradiations, but the increased flowering response was not attributable to photosynthetic CO₂ uptake because this could not be detected in seedlings exposed to 24 h WL at 18° C. Photophosphorylation could have contributed to the increased flowering response as photosystem I fluorescence was detectable in plants exposed to FR, BCJ, or WL, but there were large differences between flowering response and photosystem I capacity as indicated by fluorescence. We conclude that phytochrome plays a major role in photoresponses regulating flowering. There was no simple correlation between developmental changes, such as cotyledon expansion and chlorophyll formation during the 24-h irradiation period, and the capacity to flower in response to a following inductive dark period. Changes in plastid ultrastructure were considerable in light from fluorescent lamps and there was complete breakdown of the prolamellar body with or without lamellar stacking at 27 or 18° C, respectively, but plastid reorganization was minimal in FR-irradiated seedlings.Abbreviations BCJ irradiation from photographic ruby-red lamps - FR far-red light - P_fr far-red-absorbing from of phytochrome - P total phytochrome content - R red light - WL white light from fluorescent lamps     

Photoregulation of Anthocyanin Synthesis in Apple Fruit under UV-B and Red Light

The involvement of photomorphogenic photoreceptors in anthocyaninsynthesis was investigated in apple fruits under UV light from280 to 320 nm (UV-B) and red light (R). Short-term R treatmentwas ineffective in the induction of anthocyanin synthesis butthe involvement of phytochrome was indicated by the resultsof long-term irradiation (18 h) with R. The inductive effectof 18 h UV-B on anthocyanin synthesis was stimulated synergisticallyby subsequent irradiation with R for 15 min, and the R, far-redlight (FR) photorevesibility of this effect indicated the involvementof phytochrome in this synergism. The effect of UV-B on anthocyaninsynthesis was not influenced by subsequent irradiation withFR, suggesting that the effect of UV-B was independent of phytochrome,and that a specific photoreceptor for UV-B was involved. WhenR was given simultaneously with UV-B (18 h), anthocyanin wassynthesized at a much higher rate than it was after sequentialirradiation with UV-B and R. Photosynthesis was shown to beinvolved inthis synergistic increase in the synthesis of anthocyanin,although the involvement of phytochrome in the expression ofthis response, at least in part, was suggested by a reductionin the rate of anthocyanin synthesis by FR. (Received March 14, 1988; Accepted September 28, 1988)     

Elementary processes of photoperception by phytochrome A for high-irradiance response of hypocotyl elongation in Arabidopsis

Elementary processes of photoperception by phytochrome A (PhyA) for the high-irradiance response (HIR) of hypocotyl elongation in Arabidopsis were examined using a newly designed irradiator with LED. The effect of continuous irradiation with far-red (FR) light could be replaced by intermittent irradiation with FR light pulses if given at intervals of 3 min or less for 24 h. In this response, the Bunsen-Roscoe law of reciprocity held in each FR light pulse. Therefore, we determined the action spectrum for the response by intermittent irradiation using phyB and phyAphyB double mutants. The resultant action spectrum correlated well with the absorption spectrum of PhyA in far-red-absorbing phytochrome (Pfr). Intermittent irradiation with 550 to 667 nm of light alone had no significant effect on the response. In contrast, intermittent irradiation with red light immediately after each FR light pulse completely reversed the effect of FR light in each cycle. The results indicate that neither red-absorbing phytochrome synthesized in darkness nor photoconverted Pfr are physiologically active, and that a short-lived signal is induced during photoconversion from Pfr to red-absorbing phytochrome. The mode of photoperception by PhyA for HIR is essentially different from that by PhyA for very-low-fluence responses and phytochrome B for low-fluence responses.     

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     

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

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

Phytochrome-mediated bud development in Pisum sativum

Summary Light-grown dwarf peas were disbudded except for a single lateral bud, then transferred to darkness at 24° C. During the dark period the seedlings were irradiated daily for 5 or 7 min with R or FR. The buds exposed to R developed into shoots faster than those irradiated with FR. The R effect was FR reversible, and the FR effect was R reversible. The P_fr form of phytochrome thus promoted shoot growth including cell division, DNA and RNA synthesis.Abbreviations R red - FR far-red - P phytochrome     

Phytochrome-Mediated Photoorientation of Chloroplasts in Protonemal Cells of the Fern Adiantum Can be Induced by Brief Irradiation with Red Light

Measuring the ratio of the number of photooriented chloroplaststo the total number of chloroplasts, we found that photoorientationof chloroplasts in protonemata of the fern Adiantum capillus-veneriscould be induced by brief irradiation with polarized red light.After irradiation with red light (R) of 3 or 10 min, orientationalmovement was detected as early as 10 min after the irradiation;it continued during the subsequent dark period for 30–60min, after which chloroplasts gradually dispersed again. WhenR-treated protonemata were irradiated briefly with a second10-min pulse of R, 60 min after the onset of the first irradiation,the orientational response of chloroplasts was again observed.Typical red/far-red photoreversibility was apparent in the response,indicating the involvement of phytochrome. By contrast, irradiationwith polarized blue light for 10 min was ineffective, whileirradiation with blue light (B) at the same fluence for a longerperiod of time clearly induced the photoorientation of chloroplasts.It is likely that longterm irradiation is necessary for theresponse mediated by a blue-light receptor. When protonemata were irradiated with far-red light (FR) immediatelyafter R or after a subsequent dark period of 10 min, the magnitudeof the orientational response was smaller and chloroplasts dispersedmore quickly than those exposed to R alone. When FR was appliedat 50 min, when the response to R had reached the maximum level,chloroplasts again dispersed rapidly to their dark positions.These results indicate that P_FR not only induces the photoorientationmovement of chloroplasts but also fixes the chloroplasts atthe sites to which they have moved as a result of photoorientation. (Received June 2, 1993; Accepted January 11, 1994)     

Evidence for phytochrome involvement in light-mediated stomatal movement in Phaseolus vulgaris L.

Observations made with primary leaves of Phaseolus vulgaris L. demonstrated that phytochrome modulates light-induced stomatal movement. Removal of the far-red-absorbing form of the pigment (Pfr) with far-red (FR) radiation decreased the time required by the stomata to reach maximal opening following a dark-to-light transition; this effect of FR was fully reversible with red. Removal of Pfr with FR also decreased the time required to reach maximal closure following a light-to-dark transition, and the rate of closure was dependent on the final irradiation treatment before darkness. No evidence was found for phytochrome involvement in determining stomatal aperture under constant conditions of either darkness of light.Abbreviations and symbols Chl chlorophyll - D darkness - FR far-red - phytochrome photostationary state - Pfr, Pr FR- and R-absorbing forms of phytochrome, respectively - R red