The Effect of Light Intensity on Total Photophosphorylation and the ATP Level in Scenedesmus

The dependence of in vivo photophosphorylation on light intensity was studied in the unicellular green alga Scenedesmus obtusiusculus. By selective use of the inhibitor DCMU, phosphorylation in (I) the complete system, (II) the pseudocyclic system alone, and (III) the true cyclic system alone, were followed. When the total binding of phosphate was studied, all reaction types became light saturated in about the same manner. The effect of DCMU on the level of ATP varied according to light intensity. As for the specific systems of photophosphorylation, the following ATP data were found: (I) In the complete system the level of ATP decreases with light intensity. (II) Under pseudo-cyclic conditions light first increases and then decreases the ATP level. Under the atmospheric conditions used (i.e. CO₂-free nitrogen) this indicates a regulation between photophosphorylation and glycolysis, for which possible explanations are discussed. (III) In the true cyclic conditions light has little effect on the ATP level. The possibility is indicated that there is a structural difference between the non-cyclic (site 1) and the pseudocyclic (site 2) sites of photophosphorylation on the one hand and the true cyclic site (3) on the other.     

Effect of Hill Reaction Inhibitors on the Photoreduction of Ferricyanide and Cyclic Photophosphorylation by Spinach Chloroplasts

Some derivatives of phenylurea, N-phenylcarbamate, s-triazine and acylanilide inhibited the cyclic photophosphorylation of spinach chloroplasts catalyzed by phenazine methosulfate and accelerated the photosystem I-dependent electron flow estimated as the disproportionation of diphenylcarbazone. Acceleration was slightly stimulated by the simultaneous addition of methylamine. Thus, these Hill reaction inhibitors act as uncouplers of cyclic photophosphorylation as does methylamine. The inhibiting activities of the chemicals on the photoreduction of ferricyanide and on photophosphorylation had a parabolic relation to the partition coefficient in the octanol-water system of the chemicals.     

Antitranspirant Activity of Inhibitors of Cyclic Photophosphorylation

The antitranspirant activity of three inhibitors of cyclic photophosphorylation(2,4-dinitro-phenol, 2-chloromercuri-4,6-dinitrophenol, andsalicylaldoxime) on 6-week-old plants of Datura arborea wasexamined. The test compounds reduced transpiration and the effectpersisted for 12 d. The dry matter production by the plantswas not affected. The reduction in water loss was caused bystomatal closure. Inhibitors of cyclic photophosphorylation,by virtue of their prolonged action associated with a smalleffect on dry matter accumulation, may be useful as antitranspirants.     

The Relation between Photophosphorylation and Delayed Light Emission in Chloroplasts

One millisecond delayed light emission has been studied in isolated coupled lettuce (Lactuca sativa var. romaine) chloroplasts. Delayed light emission was increased upon addition of ferricyanide or 2,3,5,6-tetramethyl-p-phenylene diamine. In the presence of ferricyanide, the magnitude of the signal was increased by the addition of ADP (in the absence of orthophosphate), ATP, DI0-9, or phlorizin. The signal was also increased by the addition of NaCl and by the addition of NH₄Cl in the presence of a high NaCl concentration. The signal of delayed light emission was decreased by the addition of gramicidin, valinomycin, and by the addition of NH₄Cl in the presence of a low NaCl concentration.     

Effects of Respiration Inhibitors and Uncouplers on Dark- and Light-Induced Leaflet Movements of Cassia fasciculata

Respiration inhibitors, in particular KCN and NaN₃, inhibited slightly the dark-induced (scotonasty) as well as the light-induced (photonasty) leaflet movements of Cassia fasciculata: they act only at concentrations higher than 1 millimolar and 0.1 millimolar, respectively. Amytal induced a stronger inhibitory effect on scotonasty. Salicylhydroxamic acid, which inhibits the cyanide-insensitive respiration pathway, was also poorly effective when applied alone. KCN and salicylhydroxamic acid applied together increased the inhibition. Uncouplers of oxidative phosphorylation were very effective: 2,4-dinitrophenol and carbonylcyanide-m-chlorophenylhydrazone inhibited the scotonastic movements at concentrations higher than 10 μm and 1 μm, respectively. Although uncouplers reduced the photonastic movements at higher concentrations, they promoted leaflet opening at other concentrations in an unexpected way.     

The Effect of Light and Inhibitors on Chloroplast and Cytoplasmic RNA Synthesis

Chloroplast RNA is synthesized in dark-grown radish cotyledons at about one-third the rate of that in the light. The synthesis, however, continues for longer in the dark and the percentage of chloroplast RNA can approach that in light-grown tissue. Light stimulates the synthesis and accumulation of both cytoplasmic and chloroplast RNA, but shows a 4-fold greater stimulation of the chloroplast RNA. Chloramphenicol, streptomycin and cycloheximide inhibit the synthesis of chloroplast RNA with little effect on cytoplasmic RNA. 5-Fluorouracil inhibits the synthesis of cytoplasmic more than chloroplast RNA. Synthesis of the 0.56 x 10(6) mol wt chloroplast RNA is inhibited much less than the other ribosomal RNA components by actinomycin D.     

Photophosphorylation Associated with Photosystem II: II. Effects of Electron Donors, Catalyst Oxidation, and Electron Transport Inhibitors on Photosystem II Cyclic Photophosphorylation

Incubation of KCN-Hg-NH₂OH-inhibited spinach (Spinacia oleracea L.) chloroplasts with p-phenylenediamine for 10 minutes in the dark prior to illumination produced rates of photosystem II cyclic photophosphorylation up to 2-fold greater than the rates obtained without incubation. Partial oxidation of p-phenylenediaine with ferricyanide produced a similar stimulation of ATP synthesis; addition of dithiothreitol suppressed the stimulation observed with incubation. Addition of ferricyanide in amounts sufficient to oxidize completely p-phenylenediamine failed to inhibit completely photosystem II cyclic activity. This is due at least in part to the fact that the ferrocyanide produced by oxidation of p-phenylenediamine is itself a catalyst of photosystem II cyclic photophosphorylation. N,N,N′N′-Tetramethyl-p-phenylenediamine catalyzes photosystem II cyclic photophosphorylation at rates approaching those observed with p-phenylenediamine. The activities of both proton/electron and electron donor catalysts of the photosystem II cycle are inhibited by dibromothyoquinone and antimycin A. These findings are interpreted to indicate that photosystem II cyclic photophosphorylation requires the operation of endogenous membrane-bound electron carriers for optimal coupling of ATP synthesis to electron transport.     

Effect of Changing the Light/Dark Schedule, the Time of Onset of the Light or Dark Period, or the Daylength, on Rhythms of Epidermal Cell Proliferation

Rhythms of labeling and mitotic indices were studied in the hindlimb epidermis of the anuran tadpole Rana pipiens under different light/dark (LD) cycles and daylengths in order to examine the role of the various parameters of the lighting regimen in setting the periods of the rhythms and the timing of the cell proliferation peaks. Altering the time of, or inverting, the 12 h light period on a 24 h day resulted in phase shifting of basically bimodal circadian rhythms with peaks in the light and dark. Thus the cell proliferation rhythms were entrained to the LD cycle. These rhythms also entrained to noncircadian schedules since they lengthened on a 15L : 15D cycle and shortened on a 9L : 9D cycle, although the bimodal characteristic of a peak in the light and a peak in the dark remained. Studies of 18L: 6D and 6L : 18D cycles in which either the time of onset of light or dark was changed relative to the 12L: 12D control indicated that the onset of dark may regulate the timing of the labeling index peaks while the onset of light may determine the time of occurrence of mitotic index peaks. Control of the timing of labeling and mitotic index peaks by different parameters of the LD cycle suggests a mechanism for cell cycle regulation by the environmental lighting schedule. Analysis of the rhythms on all the cycles studied suggested that labeling index rhythms equal the length of, or twice the length of, the dark period. Mitotic index rhythms equal the daylfength or a multiple of the length of the dark period.     

Effects of Light and Dark on the Ultrastructure of Lichen Algae

Quantitative ultrastructural observations have been made onthe algal cells (Trebouxia) in two lichens, Parmelia sulcataand P. laevigata, stored for 48 h in the dark or under a light/darkregime. The response of the alga was found to differ in theselichens. In P. sulcata the dark treatment caused a decreasein starch grains, lipid-rich pyrenoglobuli and peripheral cytoplasmicstorage bodies and an increase in pyrenoid and chloroplast proteinbodies. The algae in P. laevigata contained little starch andno chloroplast protein bodies. However, after dark treatment,starch, cytoplasmic storage bodies and pyrenoid dimensions sometimesdeclined, while pyrenoglobuli numbers increased. Some of theseapparent changes depended upon the units used for calculatingthe cross-sectional areas of structures, e.g. absolute units,percentage of cell wall, protoplast or chloroplast cross-sectionalarea. Chloroplast area increased in the dark (as a % of cellwall area) in both species while mitochondria were larger inthe dark in P. sulcata but not in P. laevigata. Ultrastructuralchanges were not clearly correlated with changes in photosyntheticand respiratory rates. These results directly support the suggestionthat some intra-cellular structures are energy-generating reservesthe dimensions of which can rapidly change. Parmelia sulcata, Parmelia laevigata, lichen algae, light and dark storage, starvation, reserve substances, organdie dimensions     

Stimulation of Photophosphorylation by Ascorbate as a Function of Light Intensity

When isolated, stroma-free thylakoids are illuminated in the presence of ADP and orthophosphate in the absence of any electron acceptor except O2, the addition of ascorbate stimulates electron transport through the formation of the radical monodehydroascorbate and the coupled synthesis of ATP (G. Forti and G. Elli [1995] Plant Physiol 109: 1207-1211). The stimulation is shown here to be higher at low light intensity. These observations are explained in terms of the increase of the electron transport rate by ascorbate, which established a higher value of the steady-state pH gradient, causing activation of ATP synthase, which is known to be dependent on the level of the H+-electrochemical potential difference, and a higher rate of proton flux across the membranes available for utilization by ATP synthesis.     

Partial Reactions of Photosynthesis in Briefly Sonicated Chlamydomonas: II. Photophosphorylation Activities

Briefly sonicated Chlamydomonas reinhardi cells are capable of both cyclic and noncyclic photophosphorylation and, in each case, the maximum rates approach those reported for higher plant chloroplasts. Photophosphorylation coupled to ferricyanide reduction occurs with a P/2e ratio approaching unity.     

Effect of Cold Hardening on the Components of Respiratory Decarboxylation in the Light and in the Dark in Leaves of Winter Rye

In the dark, all decarboxylation reactions are associated with the oxidase reactions of mitochondrial electron transport. In the light, photorespiration is also active in photosynthetic cells. In winter rye (Secale cereale L.), cold hardening resulted in a 2-fold increase in the rate of dark respiratory CO2 release from leaves compared with nonhardened (NH) controls. However, in the light, NH and cold-hardened (CH) leaves had comparable rates of oxidase decarboxylation and total intracellular decarboxylation. Furthermore, whereas CH leaves showed similar rates of total oxidase decarboxylation in the dark and light, NH leaves showed a 2-fold increase in total oxidase activity in the light compared with the dark. Light suppressed oxidase decarboxylation of end products of photosynthesis 2-fold in NH leaves and 3-fold in CH leaves in air. However, in high-CO2, light did not suppress the oxidase decarboxylation of end products. Thus, the decrease in oxidase decarboxylation of end products observed in the light and in air reflected glycolate-cycle-related inhibition of tricarboxylic acid cycle activity. We also showed that the glycolate cycle was involved in the decarboxylation of the end products of photosynthesis in both NH and CH leaves, suggesting a flow of fixed carbon out of the starch pool in the light.     

Light Breaks and Fruit-Body Maturation in Coprinus congregatus: Dark Inhibitory and Dark Recovery Process

Experiments were conducted to better understand the inhibitoryeffect of lightduring the dark-induced process of fruit-bodysporulation of Coprinus congregatus Bull, ex Fr. Light-initiatedfruit-body primordia were subjected to different dark periodsinterrupted by a short blue light break at different times.The sporulating response depended on the duration of the darkperiod following the light break. For any inductive dark periodlonger than 3.5 h, a period of darkness lasting half as longas the inductive night completely inhibited fruit-body maturationwhen given after the light break (dark inhibitory process).Longer dark periods after the light break causedrecovery ofthe maximal sporulating response (dark recovery process). Theeffects of the dark inhibitory and the dark recovery processwere alternately reversible, the sporulating response dependingon the duration of darkness after the last light break. Studyof the time course of sporulation showed that a new dark-inducedprocess of fruit-body sporulation was initiated by the beginningof the dark period after the light break. (Received August 2, 1982; Accepted May 6, 1983)     

Variations of Rat Brain Calmodulin Content in Dark and Light Phases: Effect of Pentylenetetrazol-Induced Kindling

Calmodulin (CaM) through activation of CaM-kinase II may be involved in the molecular mechanisms underlying the epileptogenic processes. Some evidence suggests that kindling responses change across the day-night cycle. In order to test if kindling stimulation modifies CaM content, we measured CaM concentrations in amygdala, hippocampus and hypothalamus obtained from control and kindled rats during light and darkness. Male Wistar rats (250–300 g), were injected i.p. with Pentylenetetrazol (PTZ) (35 mg/kg/24 h). Once chemical kindling was established, rats were sacrificed by decapitation at 10:30 a.m. and 01:30 a.m. The brains were obtained, and the amygdala, hippocampus and hypothalamus dissected. CaM content was measured in the cytosol and membrane fractions by radioimmunoassay. We found a significant increase in CaM content in cytosol and membrane fractions of both control and kindled rats during the dark phase. No significant differences in CaM concentrations were observed between control and experimental rats, whether during the light or the dark phase. The data suggest a well defined photoperiodic variation in CaM concentrations in limbic structures, despite the neuronal excitability produced by kindling. In addition, the observed CaM increases during the dark time may be related to a protective mechanism against enhanced sensitivity to seizures observed during the night.