Ageing of chloroplasts in vitro — III. Comparison of the effects of ageing in vitro and senescence on the red absorption band and primary photochemical reactions of sunflower chloroplasts

A comparison of changes in absorption properties and electron transport activities of chloroplasts ageing in vivo and in vitro is made. Chloroplasts from sunflower leaves senescing in vivo during 7 days in dark do not show a blue shift of the red absorption band; in contrast, the shift becomes apparent within 24 h of in vitro ageing of isolated organelles. Photosynthetic activity by chloroplasts is lost much faster during in vitro than in vivo ageing. During in vitro ageing, the rate of degradation of thylakoid membranes as characterised by the shift in the red absorption band and loss in Hill reaction is further accelerated in chloroplasts isolated from dark-induced senescing leaves, suggesting the influence of the in vivo status of the chloroplasts on their in vitro stability.Abbreviations DCPIP 2,6-dichlorophenol indophenol - PSI Photosystem I - Chl+ Chlorophyll     

Ageing of chloroplasts in vitro — III. Comparison of the effects of ageing in vitro and senescence on the red absorption band and primary photochemical reactions of sunflower chloroplasts

A comparison of changes in absorption properties and electron transport activities of chloroplasts ageing in vivo and in vitro is made. Chloroplasts from sunflower leaves senescing in vivo during 7 days in dark do not show a blue shift of the red absorption band; in contrast, the shift becomes apparent within 24 h of in vitro ageing of isolated organelles. Photosynthetic activity by chloroplasts is lost much faster during in vitro than in vivo ageing. During in vitro ageing, the rate of degradation of thylakoid membranes as characterised by the shift in the red absorption band and loss in Hill reaction is further accelerated in chloroplasts isolated from dark-induced senescing leaves, suggesting the influence of the in vivo status of the chloroplasts on their in vitro stability.Abbreviations DCPIP 2,6-dichlorophenol indophenol - PSI Photosystem I - Chl Chlorophyll     

Aging induced changes in photosynthetic electron transport of detached barley leaves

Primary leaf segments of 11-day-old seedlings of barley (Hordtumvulgare L. cv IB 65) were floated on distilled water in darknessat 25°C to induce senescence. This stress induced agingbrings significant loss in the total content of pigments, proteinsand nucleic acids (DNA, RNA) of the leaves and of chloroplastsisolated from the senescing leaves. Of the three macromolecularcomponents, RNA content of theisolated chloroplasts was foundmost susceptible to stress-induced aging. Loss of DCPIP Hill activity of the isolated chloroplasts couldbe correlated, in a general way, with the loss of pigments,proteins and nucleic acids of the leaves and chloroplasts isolatedfrom them. However, during the stress period, the ability ofdifferent exogenous electron donors like MnCl₂ and diphenylcarbazide(DPC) to feed electrons to Photo System II (PS II) was foundto be different. MnCl₂ supported photoreduction of DCPIP onlyup to the fourth day, whereas DPC sustained its ability to donateelectrons up to the seventh day of incubation of the leavesin darkness. These results suggest a sequential alteration ofthe sites in the electron-transport chain between H₂O and PSII reaction centers of chloroplasts during dark-induced senescence.Kinetin not only prevented the loss of pigments and proteinsduring senescence, but also preserved the integrity of the electron-transportchain. (Received November 15, 1975; )     

The roles of weak light,oxygen and dithiothreitol in the photoreactivation of the oxygen evolving center of tris-washed and 2, 6-dichlorophenol indophenol-treated chloroplasts

The inactivated O₂-evolving center of Tris-washed chloroplasts was reactivated by DCPIP-treatment and photoreactivation in the presence of Mn²⁺, Ca²⁺, DTT and weak light. Many electron donors (Asc and reduced DCPIP, etc.) were found to be suitable substitutes for DTT. By studying the anaerobic inhibition of the reactivation, the electron acceptors O₂, NADP⁺, etc. were also found to be essential factors in photoreactivation. Weak light stimulated the chloroplast electron transport from the above-mentioned electron donors to the electron acceptor and effected the photoreactivation. More than 280 electrons were transported to NADP⁺ in the anaerobic photoreactivation of one unit of an O₂-evolving center with 400 Chl. Electron transport in the reactivation was inhibited by omitting DTT or Mn²⁺ ion, and by adding DCMU. The photoreactivated chloroplasts incorporated about 2 Mn by 400 Chl. Omission of DTT in the reactivation caused chloroplasts in the weak light to bind large amounts of excess Mn.Abbreviations Asc ascorbate - Chl chlorophyll - DCPIP 2, 6-dichlorophenol indophenol - DPC diphenyl carbazide - DTT dithiothreitol - Fd ferredoxin - STN a chloroplast preparation medium, containing 0.4 M sucrose, 0.05 M Tris-Cl and 0.01 M NaCl (pH 7.8 and 8.0) - TMPD tetramethyl-p-phenylenediamine     

Vieillissement de l'appareil photosynth?tique II. Effet synergique de la lumi?re et du vieillissement in vitro sur les activit?es photochimiques de chloroplastes isol?s d'?pinard

The consequences of chloroplast ageing in vitro were furtherinvestigated, especially on the photochemical activities ofthese organelles. Ageing of chloroplasts in dark was accompanied by decreasesin activities for photohydrolysis and cyclic and non-cyclicsyntheses of ATP, photoreduction of NADP⁺ and O₂ evolution;but there was no decrease in ferricyanide photoreduction. Therates of decrease in these activities were comparable to therate of increase in chloroplast volume. Complete inhibitionswere reached when maximum chloroplast swelling had occurred,i.e. after 5 to 6 hr of incubation at 20?C in a Tris-NaCl (pH8) medium. Ageing in the light resulted in much accelerateddecreases in activities tested; the loss of capacity for light-inducedshrinkage was also accelerated by the light during ageing. Thus,light acts synergetically towards the ageing process. Moreover,light and, to a less extent, dark ageing, resulted in an uncouplingof chloroplast photophosphorylation and associated electronflow measured by ferricyanide photoreduction. The part of theelectron flow which is induced by coupling (+ ADP, Pi, MgCl₂,pH 8) or by uncoupling (+ NH₄C1, pH 7) was found to be verysensitive to light ageing. The NADP⁺ photoreduction loss wasrestored by addition of the ascorbate-DCPIP electron donor system,suggesting that ageing interferes with the integrity of photosystemII. In many respects, these effects of ageing are comparable withthe action of detergents and fatty acids on the structure andphotochemical activities of chloroplasts, especially in thatthey attack the energy transducing mechanism in chloroplasts. (Received May 24, 1969; )     

Differential changes in the electron transport properties of thylakoid membranes during aging of attached and detached leaves, and of isolated chloroplasts

Abstract. Aging of chloroplasts both in vivo and in vitro causes a considerable loss in the 2,6-dichlorophenol indophenol (DCPIP)-Hill reaction with water as electron donor. The loss can be reduced by exogenous electron donors like diphenyl carbazide (DPC). suggestive of aging-induced damage of the oxygen evolving system. Aging also brings about a considerable loss in methylviologen (MV) reduction mediated by Photosystem I (PS I) of chloroplasts with an ascorbate-DCPIP couple as the electron donating system.
The loss in the electron transport ability of the plastids is faster during in vitro compared to in vivo aging of the chloroplasts.
Light protects the photo-electron transport ability of chloroplasts during aging of intact leaves in contrast to its action during aging of the isolated organelles.     

Calcium-calmodulin signalling is involved in light-induced acidification by epidermal leaf cells of pea, Pisum sativum L.

Pathways of signal transduction of red and blue light-dependentacidification by leaf epidermal cells were studied using epidermalstrips of the Argenteum mutant of Pisum sativum. In these preparationsthe contribution of guard cells to the acidification is minimal.The hydroxypyridine nifedipine, a Ca²⁺-channel blocker, partlyinhibited the response to both blue and red light, while thephenylalkylamine, verapamil, a Ca²⁺-channel blocker that hasbeen shown in plant cells also to block K⁺-channels, causednearly complete inhibition. The Ca²⁺-channel activator S(–)BayK 8644 induced acidification when added in the dark and diminishedthe light-induced lowering of the extracellular pH. The Ca²⁺-ionophores,ionomycin and A23187 [GenBank] , also reduced the light response. Furthermore,the light-induced acidification was inhibited by the calmodulinantagonists W-7 and trifluoperazine, but not by W-5. These calmodulininhibitors completely inhibited the red light-induced acidification,but inhibited the response to blue light by only 60–70%.In general, inhibition by compounds affecting Ca-calmodulinsignalling was always stronger on the red light response thanthat on the blue light response (with the exception of verapamilthat blocked both the red and blue light responses equally well).This differential effect on red and blue light-induced responsesindicates a role for Ca²⁺-CaM signalling in both the red andblue light responses, while a second process, independent ofCa²⁺ is activated by blue light. Key words: Signal transduction, light-induced acidification, epidermal cells, pea     

Effect of temperature on senescing detached rice leaves: I. Photoelectron transport activity of chloroplasts

Detached rice(Oryza sativaL cv Mousouri)leaves were induced to senesce in darkness at 0°C(cold), 27°C(room temperature) and 40°C(heat). 2,6-dichlorophenol indophenol (DCPIP) Hill reaction activity of chloroplasts isolated from senescing leaves under all experimental temperatures with H₂O, Mn²⁺ or diphenyl carbazide (DPC) as electron donor declined during the period of incubation. Since DPC and Mn²⁺ augmented 2,6-dichlorophenol indophenol photoreduction by chloroplasts from senescing leaves, damage and/or change in the conformation of a site between H₂O and DCPIP in photosystem II (PS II) is suggested. Heat caused a faster decline of the Hill activity compared to cold or room temperature. However, cold treatment showed no significant effect on the photoelectron transport from H₂O to DCPIP compared with room temperature.     

Effect of Water Stress on Photochemical Activity of Chloroplasts during Greening of Etiolated Barley Seedlings

Water stress retards accumulation of chlorophyll and chlorophylla/b protein complex during greening of barley seedlings in light.The rate of 2,6-dichlorophenol indophenol (DCPIP) photoreductionin isolated chloroplast which decreases under water stress isenhanced significantly in the presence of electron donors, diphenylcarbazide (DPQ) and Mn²⁺. Under water stress, the decrease ofthe rate of oxygen evolution measured in continuous white lightwas 40–73% and that of oxygen uptake (as a measure ofelectron transport through PS I from reduced DCPIP) was onlyabout 20%. During greening, under water stress, (i) a differentialinhibition of PS II biosynthesis as compared to PS I occurs,(ii) the site of electron donation by DPC seems to be closerto the reaction center ofPS II, (iii) the oxidizing side ofPS II near the oxygen-evolving system is affected maximallyby water stress. (Received March 11, 1980; Accepted November 13, 1980)     

Carbon dioxide and the reduction of indophenol and ferricyanide by chloroplasts

Pea chloroplasts isolated in salt media show decreased rates of 2:6 dichlorophenolindophenol (DCPIP) and ferricyanide reduction when depleted of CO₂ at pH values below 7.5. The greatest effect of CO₂ was on uncoupled systems. The incorporation of 10⁻², 2 × 10⁻² and 4 × 10⁻² m sodium acetate into the reaction mixtures progressively increased the bicarbonate concentration required for half maximal rates of reduction of DCPIP. The reaction was saturated by bicarbonate concentrations of 1 to 4 × 10⁻² m. With both DCPIP and ferricyanide, the addition of bicarbonate to illuminated chloroplast systems depleted of CO₂ gave very rapid increases in the rates of reduction. Bicarbonate also stimulated oxygen uptake by the illuminated chloroplasts when added hydrogen acceptors had been reduced. There was no effect of bicarbonate on ferricyanide reduction at low light intensities, but with DCPIP reduction, the apparent magnitude of the effect was independent of light intensity. This suggests that DCPIP reacts with the chloroplast electron transport chain at a site nearer to a photochemical stage than does ferricyanide. It also suggests that CO₂ has at least 2 sites of action.     

Electron Flow to the Intersystem Chain from Stromal Components and Cyclic Electron Flow in Maize Chloroplasts, as Detected in Intact Leaves by Monitoring Redox Change of P700 and Chlorophyll Fluorescence

The functional pool size of electrons in the intersystem chainof the chloroplasts of maize was estimated to be about 25 perP700 by the redox change in P700 with single- and multiple-turnoverlights under far-red light in intact leaves. This is about twicethe pool size observed in C₃ plants. Furthermore, the stromalpool size of electrons that can be donated to P700⁺ after actinicillumination was larger in maize leaves than in leaves of C₃plants, giving a maximum value of 225 electrons per P700. Maizeleaves showed an increase in the yield of modulated Chl fluorescenceafter turning off of actinic light, which confirms the donationof electrons in the dark to the intersystem chain from the stromaldonors that accumulated during actinic illumination. We proposethat the mesophyll chloroplasts are responsible for a high levelof electron-donating activity to the intersystem chain fromstromal donors such as triose phosphates and malate with NADPHas an intermediate. The level of P700⁺ under strong far-redlight was decreased after actinic illumination, suggesting theoperation of an actinic light-triggered cyclic electron flowin chloroplasts of the bundle sheath cells. (Received August 14, 1992; Accepted October 13, 1992)     

Reappraisal of the Role of Sodium in the Light-Dependent Active Transport of Pyruvate into Mesophyll Chloroplasts of C4 Plants

The mechanism of light-dependent active transport of pyruvatein C₄ mesophyll chloroplasts has not been clarified, particularlyin Na⁺-type C₄ species, in which the pyruvate uptake into mesophyllchloroplasts is enhanced by illumination or by making a Na⁺gradient (Na⁺-jump) across the envelope in the dark. We re-investigatedhere the effect of Na⁺ on the active transport of pyruvate inmesophyll chloroplasts of Panicum miliaceum, a Na⁺-type C₄ species,by comparing the rate of pyruvate uptake at various externalpHs under four conditions; in the light and dark together with/withoutNa⁺-jump: (1) At neutral pH, the rate of pyruvate uptake inthe dark was enhanced by Na⁺-jump but scarcely by illumination.(2) While the enhancement effect by Na⁺-jump was independentof external pH, that by illumination increased greatly at pHover 7.4, and the effects of light and Na⁺ at the alkaline pHwere synergistic. (3) The light-enhanced pyruvate uptake wasrelated to stromal alkalization induced by illumination. Infact, pyruvate uptake was induced by H⁺-jump in the medium frompH 8.0 to 6.7. (4) Stromal pH was lowered by the addition ofK⁺-pyruvate and more by Na⁺-pyruvate into the medium at pH 7.8in the light. (5) However, the pH and ATP levels in the stromawere not affected by Na⁺-jump. Thus, we discussed possibility that besides pyruvate/Na⁺ cotransportat neutral pH in the medium, pyruvate/H⁺ cotransport enhancedby the presence of Na+ operates in mesophyll chloroplasts ofNa⁺-type C4 species at alkaline medium. ¹Present address: Biological Resources Division, Japan InternationalResearch Center for Agricultural Sciences (JIRCAS), Ministryof Agriculture, Forestry and Fisheries, 2-1 Ohwashi, Tsukuba,305 Japan     

The retention of photosynthetic activity by senescing chloroplasts of oat leaves

The retention of photosystems I and II and or RuDP carboxylase activity in chloroplasts isolated from the first leaves of Victory oat (Avena sativa L.) seedlings was followed as the chloroplasts senesced in darkness. Both photosystems (PS) I and II retained their full activity after 3 days at 1°C, while even after 7 days at 1°C around 80% of the activity was still present. After 3 days at 25°C, PS I lost only 20% and PS II 50% of the initial activity. Acid pH increased the rate of decay of both systems, PS II falling almost to zero after 3 days at pH 3.5 (at 25°C). The preparations were almost bacteria-free, and addition of antibiotics not only did not improve their stability, but accelerated the rates of loss of photosynthetic activity. This is held to indicate that the enzymes are undergoing some turnover even in isolated chloroplasts. If the leaves were allowed to senesce in the dark first and the chloroplasts then isolated, their photosynthetic activities had greatly decreased, showing that senescence is more rapid in situ than in isolation. Under these conditions PS I decayed more rapidly than PS II. Ribulosediphosphate carboxylase, as measured by CO₂ fixation, declined more rapidly than the photosystems, though the addition of kinetin and indole-3-acetic acid somewhat decreased the rate of loss, at least for the first 24 h. When the intact (detached) leaves were held in the dark, the rate of oxygen evolution declined rapidly, but in monochromatic blue light (450 nm) at 25°C about 30% of the initial rate was retained after 72 h.Abbreviations BSA bovine serum albumin, chl, chlorophyll - DCPIP dichlorophenol-indophenol - EDTA ethylenediaminetetraacetic acid - IAA indole-3-acetic acid - PS photosystem - PVP soluble polyvinylpyrrolidine - RuDP Ribulose-1,5-diphosphate - TES N-tris-(hydroxymethyl)-methyl-2-amino-ethane sulfonic acid     

Changes of Photochemical Activities and Photosynthetic Electron Transport of Chloroplasts in the Course of Ageing

Chloroplasts were isolated from Spinacia olerecea L. and Doblichos lablab L. Chloroplasts suspension was stored in refrigerator at 5–8 ℃. Photochemical activities and chlorophyll content of chloroplasts at different times of storage were deter- mined. The results can be summarized as follows: 1. In the course of chloroplasts ageing, the lost of K3Fe(CN)6 photo reduction activity was more than that of DCPIP photoreduction activity. 2. The activity of K3Fe(CN)6 photoreduction during storage began to decrease markedly after 12 hours, but activity of DCPIP photoreduction began to decrease markedly after 24 hours. 3. The DCPIP photoreduction activity of aged chloroplasts was stimulated by the addition of 1.5-diphenylcarbazide. 4. Destruction of oxidized side of PSⅡ was earlier and higher than that of the other side (from the active center of PSⅡ to the reduced side of PSⅠ). 5. During chloroplasts ageing, the decrease of chlorophyll content was less than the rate of decrease of photochemical activities.     

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)     

Inactivation of Ascorbate Peroxidase in Spinach Chloroplasts on Dark Addition of Hydrogen Peroxide: Its Protection by Ascorbate

Dark addition of hydrogen peroxide to intact spinach chloroplastsresulted in the inactivation of ascorbate peroxidase accompaniedby a decrease in ascorbate contents. This was also the casein reconstituted chloroplasts containing ascorbate, NADP⁺, NAD⁺and ferredoxin. The addition of hydrogen peroxide during light,however, showed little effect on ascorbate contents and ascorbateperoxidase activity in either the intact or reconstituted chloroplasts.In contrast to ascorbate peroxidase, the enzymes participatingin the regeneration of ascorbate in chloroplasts (monodehydroascorbatereductase, dehydroascorbate reductase and glutathione reductase)were not affected by the dark addition of hydrogen peroxide.Ascorbate contents increased again by illumination of the chloroplastsafter the dark addition of hydrogen peroxide. These resultsshow that the inactivation of the hydrogen peroxide scavengingsystem on dark addition of hydrogen peroxide [Anderson et al.(1983) Biochim. Biophys. Acta 724: 69, Asada and Badger (1984)Plant & Cell Physiol. 25: 1169] is caused by the loss ofascorbate peroxidase activity. Ascorbate peroxidase activitywas rapidly lost in ascorbate-depleted medium, and protectedby its electron donors, ascorbate, isoascorbate, guaiacol andpyrogallol, but not by GSH, NAD(P)H and ferredoxin. (Received June 14, 1984; Accepted August 15, 1984)     

Photoreduction and Photophosphorylation with Tris-Washed Chloroplasts

The artificial electron donor compounds p-phenylenediamine (PD), N, N, N′, N′-tetramethyl-p-phenylenediamine (TMPD), and 2,6-dichlorophenol-indophenol (DCPIP) restored the Hill reaction and photophosphorylation in chloroplasts that had been inhibited by washing with 0.8 m tris (hydroxymethyl) aminomethane (tris) buffer, pH 8.0. The tris-wash treatment inhibited the electron transport chain between water and photosystem II and electron donation occurred between the site of inhibition and photosystem II. Photoreduction of nicotinamide adenine dinucleotide phosphate (NADP) supported by 33 μm PD plus 330 μm ascorbate was largely inhibited by 1 μm 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) while that supported by 33 μm TMPD or DCPIP plus ascorbate was relatively insensitive to DCMU. Experiments with the tris-washed chloroplasts indicated that electron donors preferentially donate electrons to photosystem II but in the presence of DCMU the donors (with the exception of PD at low concentrations) could also supply electrons after the DCMU block. The PD-supported photoreduction of NADP showed the relative inefficiency in far-red light characteristic of chloroplast reactions requiring photosystem II. With phosphorylating systems involving electron donors at low concentrations (33 μm donor plus 330 μm ascorbate) photophosphorylation, which occurred with P/e₂ ratios approaching unity, was completely inhibited by DCMU but with higher concentrations of the donor systems, photophosphorylation was only partially inhibited.     

Redshift of the purple membrane absorption band and the deprotonation of tyrosine residues at high pH: Origin of the parallel photocycles of trans-bacteriorhodopsin

At high pH (> 8) the 570 nm absorption band of all-trans bacteriorhodopsin (bR) in purple membrane undergoes a small (1.5 nm) shift to longer wavelengths, which causes a maximal increase in absorption at 615 nm. The pK of the shift is 9.0 in the presence of 167 mM KCl, and its intrinsic pK is ~8.3. The red shift of the trans-bR absorption spectrum correlates with the appearance of the fast component in the light-induced L to M transition, and absorption increases at 238 and 297 nm which are apparently caused by the deprotonation of a tyrosine residue and red shift of the absorption of tryptophan residues. This suggests that the deprotonation of a tyrosine residue with an exceptionally low pK (pK_a ≈ 8.3) is responsible for the absorption shift of the chromophore band and fast M formation. The pH and salt dependent equilibrium between the two forms of bR, “neutral” and “alkaline,” bR ↔ bR_a, results in two parallel photocycles of trans-bR at high pH, differing in the rate of the L to M transition. In the pH range 10-11.8 deprotonation of two more tyrosine residues is observed with pK's ~ 10.3 and 11.3 (in 167 mM KCL). Two simple models discussing the role of the pH induced tyrosine deprotonation in the photocycle and proton pumping are presented.

It is suggested that the shifts of the absorption bands at high pH are due to the appearance of a negatively charged group inside the protein (tyrosinate) which causes electrochromic shifts of the chromophore and protein absorption bands due to the interaction with the dipole moments in the ground and excited states of bR (Stark effect). This effect gives evidence for a significant change in the dipole moment of the chromophore of bR upon excitation.

Under illumination alkaline bR forms, besides the usual photocycle intermediates, a long-lived species with absorption maximum at 500 nm (P500). P500 slowly converts into bR_a in the dark. Upon illumination P500 is transformed into an intermediate having an absorption maximum at 380 nm (P380). P380 can be reconverted to P500 by blue light illumination or by incubation in the dark.

     

Photoorientation of chloroplasts in protonemal cells of the fernAdiantum as analyzed by use of a video-tracking system

Photoorientation of chloroplasts mediated by phytochrome and blue light-absorbing pigment in protonemal cells of the fernAdiantum was studied by use of inhibitors of the cytoskeleton and was analyzed with a video-tracking system. The photoorientation responses were inhibited by cytochalasin B and by N-ethylmaleimide (NEM) but not by colchicine, suggesting that the photomovement depends on the actomyosin system. In the dark, chloroplasts moved randomly, being independent of one another. After induction of photoorientation by polarized red light, most chloroplasts that had been located at the margin of cells moved almost perpendicularly to the cell axis toward the site of photoorientation. This type of movement was hardly ever observed in the dark. Under polarized blue light, such specific movements were less evident but were still observed in the case of a few chloroplasts. After photoorientation was complete, chloroplasts still moved in random directions but their mobility was lower than that in the dark, indicating the presence of some anchoring mechanism. When EGTA was applied, photoorientation was inhibited but this inhibition was overcome by the addition of CaCl₂. Video-tracking of chloroplasts in the dark revealed that the mobility of chloroplasts was higher in medium with EGTA than in medium with EGTA plus CaCl₂ and that many of the chloroplasts moved jerkily in the medium with EGTA. This change in the nature of movements was also seen under polarized light, resulting in the disturbance of photoorientation. These results indicate that the inhibition of photoorientation at low concentrations of Ca²⁺ ions may be due to change in the nature of chloroplast movement.     

Action Spectrum for Photoactivation of the Water-splitting System in Plastids of Intermittently Illuminated Wheat Leaves

The chloroplasts from wheat leaves developed under intermittent illumination (1 ms light + 12 min dark) were able to photoreduce DPIP with DPC as electron donor but unable to photoreduce DPIP with water as electron donor. On exposure of these leaves to continuous light, the Hill activity with water as electron donor was rapidly induced. The photoactivation was sensitive to the treatment with DCMU prior to exposure to continuous light. The action spectrum for the photoactivation showed a sharp band at 680 nm with a distinct shoulder at 650 nm, and was similar to the absorption spectrum of photosytem-2 particles. These data suggest that the electron transfer driven by photosystem 2 is essential for the activation of the water-splitting system in the chloroplasts of intermittently illuminated leaves.