Effect of Temperature on Photosynthetic Activities of Senescing Detached Wheat Leaves

Changes in various components of photosynthetic activity duringthe dark induced senescence of detached wheat leaves, maintainedat 25°C (control) and 35°C (mildly elevated temperaturetreatment), were examined. Senescence-associated decline measuredup to 96 h, in photosynthetic activity was appreciably hastenedat 35°C, than at 25°C as evident by the relative higherlosses of chlorophyll, photosystem (PS) II and PS I catalyzedphotochemical activities and ribulose-1,5-bisphosphate (RuBP)carboxylase activity. In addition, a comparatively higher risein light scattering profile of isolated chloroplasts was notedat 35°C than at 25°C. Senescence-induced degradationof chlorophyll was faster at 35°C than at 25°C; on theother hand, the degradation of carotenoids was faster at 25°Cthan at 35°C. Furthermore, the ratio of carotenoids to chlorophyllincreased with senescence up to 96 hours, higher ratio beingobtained at 35°C than at 25°C. Both PS II and PS I activitiesshowed a transient rise in the beginning phase of dark incubation,whereas loss in chlorophyll was continuous throughout the periodof senescence. The initial rise observed in photochemical activitieswas attributable to the uncoupling of electron transport fromphotophosphorylation. Elevated temperature treatment resultedin greater inactivation of RuBP carboxylase than control. Itappears that during senescence the loss in chlorophyll and RuBPcarboxylase activity are triggered simultaneously. (Received June 7, 1985; Accepted October 30, 1985)     

Reversible inhibition of photochemistry of photosystem II by Ca2+ removal from intact cells of Anacystis nidulans

Depletion of Ca²⁺ from Anacystis nidulans produces an inhibition of O₂ evolution that is accompanied both at 39°C and 77 K by a loss of chlorophyll fluorescence of variable yield. This indicates that Ca²⁺-depletion causes disruption of normal photosystem II function, manifested by the disappearance of photoreduction of Q. Delayed light emission in the ms time range is also eliminated in Ca²⁺-depleted cells, which confirms that Ca²⁺ removal prevents charge separation and recombination in reaction centers of photosystem II. Readdition of Ca²⁺ to depleted cells restores fully the fluorescence of variable yield and delayed light emission, as well as O₂ evolution. Thus, Ca²⁺ may be a required component for photosystem II in A. nidulans.     

Control of differentiation in streptomycetes: involvement of extrachromosomal deoxyribonucleic acid and glucose repression in aerial mycelia development.

When Streptomyces alboniger spores were grown in Hickey-Tresner broth containing 5 muM ethidium bromide, a high frequency of permanently cured aerial mycelia-negative (am-) colonies was recovered. The appearance an am- colonies was time dependent: a very low frequency (0.3%) at zero time, a maximum (9 to 21%) after 2 to 5 days of growth, and a decline again to low frequencies later in the growth cycle. On agar, cured am- colonies of S. alboniger still produced puromycin. The development of aerial mycelia in S. alboniger, S. scabies, and S. coelicolor was also sensitive to glucose repression. Colonies grown on Hickey-Tresner agar containing 2% glucose remained phenotypically am- throughout the observation period. Adenine (2.5 mM or greater), and to a lesser extent adenosine and guanosine, specifically reversed the repression. The accumulation of undissociated organic acids appears to be involved in glucose repression of aerial mycelia formation. However, this does not appear to be the case with puromycin production in S. alboniger; glucose repression was observed over the pH range 5.0 to 7.5.     

Temperature Dependent Alterations in the Pattern of Photochemical and Non-Photochemical Quenching and Associated Changes in the Photosystem II Conditions of the Leaves

The alterations in the PSII activity of leaves, subsequent toa mild or severe heat stress were characterized by monitoringthe Chl a fluorescence and thermoluminescence emission fromintact leaves. The Chl a fluorescence measurements were carriedout in leaves adapted to either ‘state I’ or ‘stateII’ since under these two conditions the photosyntheticapparatus is known to have distinctly different structure-functionrelationships. The pattern of Chl a fluorescence induction instate II-adapted leaves was different from that of state I-adaptedleaves due to the alterations in the extent of photochemical(q_Q) and non-photochemical (q_E) quenching during the time courseof induction. The pattern of changes in q_Q and q_E values wasalso altered by heat treatment depending on the severity ofheat stress; severe heat stress (47°C) suppressing theseparameters drastically. Mild heat treatment (42°C) did notaffect the ability of leaves to undergo state I to state IItransition whereas the severe heat stress totally abolishedsuch transition. The fluorescence and thermoluminescence characteristicsof the leaves that have been exposed to the severe heat stresssuggest that a large number of affected PSII units retain afunctional water-oxidizing complex at the donor side. (Received June 14, 1994; Accepted July 19, 1995)     

Nitrogen fixation andnif gene organization in branched heterocystous cyanobacteria: Variation in the presence ofxisA

Five branched heterocystous cyanobacteria (Scytonematopsis sp.,Scytonema sp.,Tolypothrix ceylonica, Mastigocladus sp. andFischerella sp.) were examined for their pattern of induction of nitrogenase activity andnif gene organization. All the forms showed the onset of nitrogenase activity after 12 h which could be correlated with the appearance of proheterocysts. The highest activity was exhibited byT. ceylonica. Hybridization studies revealed the presence of thenifD gene but the absence of thexisA gene inMastigocladus sp. andScytonematopsis sp. Interestingly,Tolypothrix sp. andScytonema sp. DNA samples hybridized withxisA. Hence no uniformity seems to exist regarding the presence ofxisA and the relatednif gene organization in branched heterocystous cyanobacteria. This investigation throws light on the primitive character and phylogenetic relatedness of branched forms to the coccoid/colonial forms. It also provides evidence for the proposition that stigonematacean cyanobacteria may not represent the most advanced cyanobacterial forms; rather they may link the coccoid and filamentous forms.     

The role of CP 47 in the evolution of oxygen and the binding of the extrinsic 33-kDa protein to the core complex of Photosystem II as determined by limited proteolysis

In order to identify the domain within Photosystem II complexes that functions in the evolution of oxygen, we performed limited proteolysis with lysylendopeptidase of the core complex of Photosystem II which had been depleted of the extrinsic 33-kDa protein (Mn-stabilizing protein). The cleavage sites were estimated from the amino-terminal sequences of the degradation fragments, their apparent molecular masses and amino-acid compositions. Under certain conditions, the D2 protein was cleaved at Lys13; and a chlorophyll a-binding protein, CP 47, was cleaved at Lys227 and Lys389. Another chlorophyll a-binding protein, CP 43, was degraded more rapidly than CP 47. The oxygen-evolving activity and the capacity for rebinding of the 33-kDa protein to the core complex of Photosystem II decreased in parallel, with kinetics very similar to those of the cleavage of CP 47 at Lys389. These observations strongly suggest that the hydrophilic domain around Lys389 of CP 47, which are located on the lumenal side, is important in the binding of the 33-kDa protein and in maintaining the oxygen-evolving activity of the Photosystem II complex.Abbreviations CP 47 and CP 43- intrinsic chlorophyll a-binding proteins with apparent molecular masses of 47 and 43 kDa, respectively - PBQ- phenyl-p-benzoquinone - TLCK- N--p-tosyl-L-lysine chloromethyl ketone     

Inhibition of the Chloroplast Photochemical Reactions by Treatment of Wheat Seedlings with Low Concentrations of Cadmium: Analysis of Electron Transport Activities and Changes in Fluorescence Yield

The effect of treatment of wheat plants with Cd²⁺ ions on thephotochemical activity of the primary leaves was examined. Threeday-old etiolated seedlings were treated with Cd²⁺ ions for24 h in dark, and after this treatment the plants were grownin the light until the primary leaves were fully developed.Cd²⁺ ions (30–120 µM) induced a significant decreasein activities of both photosystem II and photosystem I. Theextent of the decrease in PS II activity was much greater thanthat in the PS I activity. Analysis of changes in the fluorescenceyield of chlorophyll also indicated that Cd²⁺ ions drasticallyaffect the photochemistry of photosystem II. Cd²⁺ ions induceddecrease in the rates of photoreduction of 2,6-dichlorophenolindophenol even in the presence of the exogenous electron donor,hydroxylamine, both in Tris-treated and untreated chloroplasts.This result suggests that the site of inhibition is near thesite of donation of electrons by hydroxylamine. Treatment withCd²⁺ ions impairs the electron transport system on the reducingside of PS II. The decrease in the fluorescence yield of Chi is less than that in the evolution of O₂ mediated by oxidizedphenylenediamine. This difference may be a result of inhibitionon the reducing side of PS II. In addition to inhibition onthe reducing side, Cd²⁺ ions may affect the oxidizing side ofPS II. A comparative study of the rates of evolution of O₂ withp-benzoquinone and dichloro-p-benzoquinone as electron acceptorswas performed since the halogenated benzoquinones have beenshown to accept electrons from both active and inactive centersof photosystem II while some of the benzoquinones accept electronsonly from active centers. The results suggest that Cd²⁺ ionsinduced a marginal increase in the number of inactive reactioncenters in PS II. Analysis of light-saturation-kinetics of theevolution of O₂ catalysed by PS II indicates a reduction inthe size of the antennae as well as in the concentration ofthe active (-type) reaction centers of PS II. Thus, the Cd²⁺-inducedeffects on the photochemistry of PS II involve changes on thereducing side of PS II as well as possible changes in the sizesof the populations of active and inactive centers. Thus, short-termexposure to Cd²⁺ ions during establishment of seedlings hasa severely detrimental effect on photochemical activities inchloroplasts. (Received October 17, 1990; Accepted July 3, 1991)     

Mercury Induces Alteration of Energy Transfer in Phycobilisome by Selectively Affecting the Pigment Protein, Phycocyanin, in the Cyanobacterium, Spirulina platensis

Mercury, at a low concentration (3 µM) caused an enhancementin the intensity of room temperature fluorescence emitted byphycocyanin and induced a blue shift in the emission peak ofSpirulina cells indicating the alterations in the energy transferwithin the phycobilisomes. In vitro the isolated intact Spirulinaphycobilisomes from control cells exhibited only a reductionin fluorescence yield with low concentration of HgCl₂ withoutbeing accompanied by changes in the emission features, whereasthe isolated phycobilisomes from mercury treated cells exhibitedthe alterations in the spectral characteristics at the levelof phycocyanin. When isolated phycocyanin and allophycocyaninwere exposed to very low concentrations of Hg²* ions, C-phycocyaninexhibited a large decrease in the absorbance in the longer wavelength(615–620 nm) region, but not allophycocyanin. In addition,mercury also caused a monotonous decrease in the C-phycocyaninemission intensity at 646 nm accompanied by a blue shift to642 nm. These results on isolated C-phycocyanin suggest thatselective bleaching of beta-84 chromophore of phycocyanin isinduced by mercury. The differential effect of mercury towardsC-phycocyanin and allophycocyanin could possibly be due to thedifference in the protein conformation of phycocyanin and allophycocyanin. (Received July 11, 1990; Accepted December 17, 1990)     

Time-dependent alterations in the antenna pigment-protein complex by mercury ions in the cyanobacterium Spirulina platensis

We have shown that mercury affects energy transfer in Spirulina platensis. It inhibits energy transfer from phycocyanin to chlorophyll a by specifically bleaching the -84 chromophore of the chromo protein, phycocyanin (PC), in the cyanobacterium. This effect is observed during short-term exposure of cells to Hg²⁺ ions. Upon long-term (12 h) exposure, mercury at low concentrations (1–2.5 m) causes the gradual degradation of the polypeptide (22 kDa) of the PC of phycobilisomes in this cyanobacterium. The effect of mercury on this polypeptide is significant compared with the other phycobiliproteins.     

Further characterization of a photosystem ii particle isolated from spinach chloroplasts by triton treatment delayed light emission

Delayed light emission from the Triton-fractionated Photosystem II subchloroplast fragments (TSF-IIa) was measured between 0.5 and 10 ms after the termination of illumination. The delayed light emission was diminished by Photosystem II inhibitors, DCMU and o-phenanthroline, which act between the reduced primary acceptor and the plastoquinone pool.Secondary electron donors to Photosystem II, diphenylcarbazide, phenylenediamine, Mn²⁺, and ascorbate inhibited delayed light emission. Secondary electron acceptors such as ferricyanide, dichlorophenol indophenol, and dimethyl benzoquinone enhanced delayed light emission. The addition of secondary electron acceptors to TSF-IIa particles containing Mn²⁺ restored delayed light emission to almost the control level. The plastoquinone antagonist, 2,5-dibromo-3-methyl-6-isopropyl p-benzoquinone, increased delayed light emission at low concentrations but decreased it at higher concentrations. Silicomolybdate enhanced the delayed light emission of TSF-IIa particles markedly, and reversed the inhibition by DCMU. Silicomolybdate showed a similar stimulatory effect on the delayed-light intensity in broken spinach chloroplasts at shorter times after the termination of illumination. Carbonyl cyanide m-chloro (or p-trifluoromethoxy) phenylhydrazones inhibited the delayed light emission, but NH₄Cl had no effect.