RETARDATION OF ULTRAVIOLET LIGHT ACCELERATED LEAF SENESCENCE BY A CYTOKININ: N6 BENZYLADENINE

O ne clear evidence of u.v. injury is the manifestation of chlorosis after leaf irradiation, and such chlorosis can be photorepaired by blue light[1]. We found that such u.v.-accelerated chlorosis could be prevented in the dark by applying N⁶ benzyladenine (BA), a synthetic cytokinin, either before or after u.v. irradiation. Cytokinins are known to retard natural senescence of detached leaves kept in the dark [2]. The present finding suggests that u.v. damage may accelerate the natural senescence process of detached leaves kept in the dark. N. glutinosa leaves were used in this experiment, because chlorosis develops quickly (within one week) when the mature leaves are detached and incubated in the dark. The plants were grown in a glasshouse without white-wash. The light intensity at noon often reached 9000 ft-c., and the temperature ranged from 19–35°C. The leaves shown in Fig. 1 were from mature plants, i.e. plants with the terminal growing point developed into a flower bud. The floral buds were cut off several days before the leaves were detached for exposure to u.v. light.     

FLASH PHOTOLYSIS OF SEVERAL AROMATICS BY ULTRAVIOLET LIGHT AND VISIBLE LIGHT IN THE PRESENCE OF EOSIN Y*

Abstract— A flash photolysis investigation was made of the photo-oxidation of aqueous aniline, resorcinol, βnaphthol, p-sulfanilic acid, and p-bromophenol induced by ultraviolet and visible light irradiation in the presence of eosin Y. The transient spectra show that u.v. irradiation generates the hydrated electron (except in p-bromophenol) and the radical products of one-electron oxidation. The initial products of the eosin-sensitized oxidations are the dye semi-quinone and aromatic radicals which coincide with the u.v. photolysis products in at least several cases. The investigation of the reaction kinetics by rapid spectrophotometry with analog computer analysis shows that the aromatics quench the triplet state of eosin and also react with it in a slower electron-transfer process, in competition with ‘dye-dye’ quenching and electron-transfer reactions. The u.v. and dye-sensitized oxidations are discussed in terms of their energetics.     

INDUCTION OF FREE RADICALS IN DNA BY PROFLAVINE AND VISIBLE LIGHT: INFLUENCE OF OXYGEN AND IONIC STRENGTH

Abstract —The photosensitization of native DNA is observed as an induction of free radicals in the DNA moiety of proflavine-DNA complexes. The intensity of the electron paramagnetic resonance spectra (at 77 K) is a measure of the number of free radicals present in frozen solutions of DNA-proflavine complexes after irradiation with visible light (Λ > 320 nm). In the absence of O₂, the photosensitization is significant but very low; it increases slightly with increasing NaCl ionic strength; it appears to be due to intercalated dye molecules and the qualitative analysis of the spectra obtained shows that mainly thymidine is involved. The reaction measured after saturation with O₂ is the same as the reaction in air but is quantitatively higher; the free radicals observed are peroxides. This induction of free radicals appears to be due to the intercalated dye molecules, each molecule acting independently. The important observation is a very sharp and large (around a hundred-fold) increase in the photosensitizing efficiency of the bound dye molecules occurring in NaCl between μ, # 0–25 and μ= 0–5 and in MgCl₂ between μ# 0–01 and μ=0–1.     

THE EFFECT OF LEVULINIC ACID ON THE LIGHT INDUCED DEVELOPMENT OF PHOTOSYSTEM I AND II ACTIVITIES IN GREENING MAIZE LEAVES*

Photosystem I and Photosystem II activities were measured in chloroplasts isolated after 0–20 h illumination from etiolated maize leaves in which chlorophyll synthesis was specifically inhibited by levulinic acid. In control leaves not treated with levulinic acid, Photosystem I activity/chlorophyll developed rapidly during the first 2h in light, then fell off, and reached a constant level after 6h of illumination. In levulinic acid treated leaves, in which chlorophyll accumulation was inhibited up to 60%, a similar initial rise in Photosystem I activity was observed. However, the decrease in activity was much slower and continued for at least 20 h. The development of Photosystem I activity calculated on a leaf fresh weight basis was similar for control leaves or leaves treated with levulinic acid. This indicates that development of Photosystem I activity may not be related to chlorophyll accumulation during greening. Photosystem II activity/chlorophyll in leaves treated with or without levulinic acid increased similarly during the first 6h and then remained constant. Activity of Photosystem II per leaf fresh weight increased linearly, after the first h, for 20 h in the control leaves; in levulinic acid treated leaves this development was reduced by about 60%. Thus, development of Photosystem II activity can be related to chlorophyll accumulation. SDS gel electrophoresis of plastid membranes from control leaves illuminated for 12 h showed the presence of chlorophyll-protein complex I as well as Chl-protein 11; in the case of levulinic acid treated leaves only Chl-protein complex I was detectable, while Chl-protein complex II was markedly reduced.     

SOME RELATIONSHIPS BETWEEN ULTRAVIOLET LIGHT AND HEME-PROTEIN-INDUCED PEROXIDATIVE LIPID BREAKDOWN IN LIPOSOMES, AS REFLECTED BY FLUORESCENCE CHANGES: THE EFFECT OF NEGATIVE SURFACE CHARGE

Abstract— The water soluble, photolabile nitrene precursor,azidonaphthalene–2,7-disulfonic acid (ANDS) was encapsulated in small unilamellar, isoelectric (egg PC) or negatively charged (egg PC + dihexadecylphosphate) liposomes. The individual and combined effects of heme-proteins and UV irradiation on the fluorescence of these vesicles under aerobic conditions were studied. Consistent with the catalytic action of heme-proteins on lipid peroxidation and peroxide decomposition, addition of cytochrome c (positively charged) or catalase (negatively charged) to the vesicles elicited immediate formation of a fluorescence band at 470 nm, characteristic of Schiff bases that form from aldehyde byproducts of decomposing hydroperoxides. Ultraviolet irradiation of liposomes for 5 min caused no significant changes in the fluorescence spectrum, in spite of the radiolysis of ANDS inside the vesicles with consequent formation of nitrene radicals. When isoelectric vesicles were irradiated with UV light in the presence of cytochrome c or catalase, Schiff base formation was further increased by2–3 fold, which effect was not observed in the absence of internal ANDS, or in the presence of negative surface charge on the vesicles. These findings suggest that (a) UV irradiation, by itself, cannot trigger lipid decomposition even when it is assisted by photoproduced nitrene radicals, (b) there is a ternary synergism between UV light, heme-proteins and nitrene radicals in promoting peroxidative lipid breakdown, and (c) negative surface charge inhibits the above synergism, which effect is unlikely to be due to electrostatic interaction between the vesicles and the protein or the ANDS.     

TEMPORAL CHANGES IN SURFACE ULTRAVIOLET RADIATION: A STUDY OF THE ROBERTSON-BERGER METER AND DOBSON DATA RECORDS

Abstract— Ultraviolet radiation data sets obtained by Robertson-Berger meters located at Bismarck, ND and Tallahassee, FL show variations over the time period 1974–1985 which we interpret in terms of clouds and ozone. Cloudiness is a major source of variance in the irradiance measurements. When this variance is minimized, the monthly mean Robertson-Berger meter record contains trends which are in good agreement with irradiance calculations based on the Dobson ozone measurements in spring, summer and early autumn. Despite the agreement among trends, predictions based on the ozone data explain 40% or less of the variance in the monthly mean radiation values over the 11-year period. The radiation measurements contain negative trends in winter which are contrary to expectations based on the behavior of ozone alone. These trends remain when we minimize the effects of cloudiness. Based on the information available in this study, it is not possible to determine whether the wintertime trends have an instrumental or environmental origin.     

HEAT- AND LIGHT-INDUCED CHLOROPHYLL a FLUORESCENCE CHANGES IN POTATO LEAVES CONTAINING HIGH OR LOW LEVELS OF THE CAROTENOID ZEAXANTHIN: INDICATIONS OF A REGULATORY EFFECT OF ZEAXANTHIN ON THYLAKOID MEMBRANE FLUIDITY

Potato leaf discs were infiltrated in darkness with a buffer of pH 5 containing 100 M ascorbate, resulting in a massive conversion of the carotenoid violaxanthin to zeaxanthin. In vivo measurements of modulated chlorophyll a fluorescence indicated that this treatment (1) caused a marked upward shift of the threshold temperature at which photosystem II denatures and (2) noticeably inhibited the rate of dark reoxidation of the reduced plastoquinone (at low temperature). These changes were not induced in leaves infiltrated with a buffer of pH 5 containing no ascorbate or with 100 mM ascorbate at pH >7.2. The above-mentioned effects were also observed during heat acclimation (34°C for several days) of potato plants and suggested that zeaxanthin interacts with the lipid phase of the thylakoid membranes. Based on those results and the previous data obtained with model systems, it is suggested that the xanthophyll cycle could be a regulatory mechanism adjusting thylakoid membrane fluidity, the significance of which for the photoprotection of the photosynthetic apparatus is discussed.     

CHANGES IN SUPEROXIDE DISMUTASE, CATALASE AND GLUCOSE-6-PHOSPHATE DEHYDROGENASE ACTIVITIES OF RABBIT ALVEOLAR MACROPHAGES: INDUCED BY POSTNATAL MATURATION AND/OR IN VITRO HYPEROXIA

Abstract— The total superoxide dismutase activity of rabbit alveolar macrophages (AM) increased twofold during the first postnatal week. This increase in superoxide dismutase activity was primarily mitochondrial and paralleled the increase in the number of mitochondria in these cells which has been previously reported. The superoxide dismutase activity of AM in culture for 18 h was significantly increased by hyperoxia; catalase activity in hyperoxic conditions was slightly adversely affected; glucose-6-phosphate dehydrogenase activity was increased but not significantly over control values. Hyperoxic cultures beyond 42 h decreased the total number of viable cells and the superoxide dismutase activity expressed per viable cell; the disappearance of catalase and glucose-6-phosphate dehydrogenase activities in this period, however, was more rapid in control alveolar macrophages than those under hyperoxia.     

ANALYSIS OF MICROSECOND FLUORESCENCE YIELD AND DELAYED LIGHT EMISSION CHANGES AFTER A SINGLE FLASH IN PEA CHLOROPLASTS: EFFECTS OF MONO- AND DIVALENT CATIONS

Abstract. New results are presented on the effects of mono- and divalent cations on concurrent changes in the microsecond yields and kinetics of chlorophyll a fluorescence and delayed light emission, and the light saturation curve for the latter at 100 μs, following a 10 ns flash at 337 nm. (1) The fluorescence yield increases exponentially from 3 to 30 μs (lifetime, τ, 6.4 ± 0.6/μs), and decays biphasically between 50 and 800μs. (2) The delayed light emission decays biphasically with two exponential phases: fast phase, T= 7–10μs, and slow phase, T= 33–40μs. (3) The light saturation curve for 100μs delayed light emission is satisfactorily represented by a one-hit Poisson saturation curve. (4) Addition of 5 mM NaCl to salt-depleted chloroplasts decreases (by as much as 40%) the yields of μs fluorescence and delayed light emission, and the subsequent addition of 5mM MgCl₂ increases the yields (≤2 × over samples with only NaCl). (5) The fluorescence yield rise and delayed light emission decay kinetics are independent of low concentrations of cations. The lifetime of the fast phase of fluorescence decay changes from ?90μs to ?160μs, when Na⁺ or Na⁺+ Mg²⁺ are added. Based on a detailed analysis presented in this paper, the following conclusions regarding the effects of low concentrations (few mM) of mono-and divalent cations in sucrose-washed chloroplasts at room temperature are made: (a) Na⁺ decreases (?6%) and Mg²⁺ increases (? 20% compared with the Na⁺ sample) the sensitization of photosystem II photochemistry: this effect is small, but significant. (b) Na⁺ increases and Mg²⁺ decreases the efficiency for radiationless transitions in singlet excited Chl a in the antenna and closed reaction center of PS II; this includes non-radiative energy transfer to PS I, intramolecular intersystem crossing and internal conversion. The ratio of the sum of the rate constants for radiationless transitions to that for fluorescence increases by ? 2-fold upon the addition of Na⁺, and is completely reversed by the addition of Mg²⁺. (c) The rate constant for the re-oxidation of Q^- decreases (about 50%) in the presence of Na⁺ or Na⁺+ Mg²⁺. These conclusions imply that cations produce multiple changes in the primary photoprocesses of PS II at physiological temperatures. It is proposed that these changes are mutually independent and can co-exist.     

CONTROL OF PHOTOSYNTHETIC REDUCTANT: THE ROLE OF LIGHT AND TEMPERATURE ON SUSTAINED HYDROGEN PHOTOEVOLUTION BY Chlamydomonas sp. IN AN ANOXIC,CARBON DIOXIDE-CONTAINING ATMOSPHERE*

–Sustained hydrogen photoevolution from Chlamy domonas reinhardtii and C. Moewusii was measured under an anoxic, CO₂-containing atmosphere. It has been discovered that light intensity and temperature influence the partitioning of reductant between the hydrogen photoevolution pathway and the Calvin cycle. Under low incident light intensity (1-3 W m^-2) or low temperature (approx. 0°C), the flow of photosynthetic reductant to the Calvin cycle was reduced, and reductant was partitioned to the hydrogen pathway as evidenced by sustained H₂ photoevolution. Under saturating light (25 W m^-2) and moderate temperature (20±5°C), the Calvin cycle became the absolute sink for reductant with the exception of a burst of H₂ occurring at light on. This burst of H₂ corresponded to the expression of about 450 electrons for each photosynthetic electron transport chain. These results suggest that the hydrogen pathway and the Calvin cycle compete for reductant under anoxic conditions and that partioning between the two pathways can, to a certain extent, be controlled by the appropriate choice of experimental conditions.