Guy Samson

COPPER QUENCHING OF THE VARIABLE FLUORESCENCE IN Dunaliella tertiolecta. NEW EVIDENCE FOR A COPPER INHIBITION EFFECT ON PSII PHOTOCHEMISTRY

Abstract— The copper quenching effect on fluorescence in Dunaliella tertiolecta was studied. 30% of variable fluorescence was quenched in the presence of 70 μ,M CuS04. We confirmed that the copper inhibitory effect on photosystem II (PSII) activity is located on its oxidizing side. Further, we indicate that the complementary area is decreased by copper. Since the quantum yield of PSII photochemistry was lowered and the rate of PSII primary electron acceptor QA remained unaffected, we can conclude that some PSII reaction centers were inactivated by copper.

Use of algal fluorescence for determination of phytotoxicity of heavy metals and pesticides as environmental pollutants

The phytotoxicity of heavy metals and pesticides was studied by using the fluorescence induction from the alga Dunaliella tertiolecta. The complementary area calculated from the variable fluorescence induction was used as a direct parameter to estimate phytotoxicity. The value of this parameter was affected when algae were treated with different concentrations of mercury, copper, atrazine, DCMU, Dutox, and Soilgard. The toxic effect of these pollutants was estimated by monitoring the decrease in the complementary area, which reflects photosystem II photochemistry. Further, the authors have demonstrated the advantage of using the complementary area as a parameter of phytotoxicity over using variable fluorescence yield. The complementary area of algal fluorescence can be used as a simple and sensitive parameter in the estimation of the phytotoxicity of polluted water.

PHYSIOLOGY OF EFFECTS OF TEMPORARY IMMERSION BIOREACTORS ON MICROPROPAGATED PINEAPPLE PLANTLETS

SummaryTemporary immersion bioreactors are an efficient tool for plant mass propagation because they increase multiplication rate and plant quality. Little knowledge is available on the ecosystem and physiological behavior of plantlets when using this new culture technique. In order to evaluate the effects of the conditions on physiological change of pineapple plantlets, a factorial experiment was conducted, where axillary clusters were cultured under two levels of photosynthetic photon flux (PPF): 30 μmol m−2s−1 (low) and 225 μmol m−2s−1 (high), using two culture methods (conventional micropropagation in liquid medium and a temporary immersion bioreactor) during the elongation phase. CO2 concentration in the headspace volume container was measured during a whole cycle of temporary immersion (3h). At the time before the next immersion period, the levels of CO2 increased significantly to 14171 μmol mol−1 at high PPF. The maximal photosynthetic rate as well as the maximum quantum yield of photosystem II were low for plantlets cultivated in the femporary immersion bioreactor at high PPF. However, these plantlets showed large increases in sugar and nitrogen uptake and also increases in dry weight and foliar area. These results indicate that shoot growth did not totally depend on the photosynthesis process. In vitro pineapple plantlets appeared to use more nutrients in the culture medium than those from photosynthesis. In summary, temporary immersion bioreactor-derived plantlets showed remarkable nutrient uptake, indicating a higher photo-mixotrophic metabolism.

Nonphotosynthetic Reduction of the Intersystem Electron Transport Chain of Chloroplasts Following Heat Stress. Steady-state Rate

Abstract The consequence of elevated temperatures in the range of 39–51°C on the steady-state rate of light-induced electron transport through photosystem I (PSI) supported by stromal reductants was studied in intact barley leaves using photoacoustic and chlorophyll fluorescence techniques. Measurable electron flow through PSI in diuron-treated leaves occurred only after exposure to temperatures above 37°C. The steady-state rate of the above diuron-insensitive electron flow with methyl viologen as electron acceptor was estimated to be 3.7 μeq m−2 s−1 or 0.018 μeq μmol chlorophyll−1 s−1 in leaves exposed for 5 min to 45°C.

Opposite effects of exogenous sucrose on growth, photosynthesis and carbon metabolism of in vitro plantlets of tomato (L. esculentum Mill.) grown under two levels of irradiances and CO2 concentration

Summary The long-term effects of exogenous sucrose (3 percnt;) on growth, photosynthesis and carbon metabolism of in vitro tomato plantlets were investigated under two sets of growth conditions that respectively favor source- or sink-limitations of photosynthesis: 1) low photosynthetic photon flux (PPF) (50 μmol m −2 · s −1 ) and low CO 2 concentration (400 μmol mol −1 ) and 2) high PPF (500 μmol m −2 · s −1 and high CO 2 concentration (4000 μmol mol −1 ). The supply of sucrose under source-limitation conditions increased the growth, the maximal photosynthetic rate, the chl content, the maximal quantum yield of Photosystem II estimated by the Fv/Fm chl fluorescence ratio as well as the soluble sugars (hexoses, sucrose) and starch contents in roots, young and mature leaves when compared to those of photo-autotrophic plantlets. Also, sucrose feeding under these conditions strongly increased the activity of sucrose synthase (SS) (EC 2.4.1.13) in roots and young leaves whereas the activities of sucrose phosphate synthase (SPS) (EC 2.4.1.14), acid invertase (INV) (EC 3.2.1.26) and ADP-glucose pyrophosphorylase (ADPGppase) (EC 2.7.7.27) were highly stimulated in roots and mature leaves. Contrary to these observations, the supply of sucrose to plantlets developed under high PPF and CO 2 concentration decreased growth and led to a somewhat lower maximal photosynthetic rate relative to photo-autotrophic plantlets. These negative responses to exogenous sucrose were accompanied by stronger accumulations of hexose and starch, larger stimulation of INV in mature leaves developed under conditions of sink limitation than those from source limitation conditions. Moreover, under high PPF and high CO 2 concentration, exogenous sucrose led to a marked repression of the SPS activity and caused much lower stimulations of ADPGppase in mature leaves than those observed at low PPF and low CO 2 concentration. We therefore conclude that under our experimental conditions, the interactive effects of exogenous sucrose and environmental conditions on growth and photosynthesis could be rationalized by the source-sink equilibrium of the in vitro tomato plantlets.

Heterogeneity of Photosystem I reaction centers in barley leaves as related to the donation from stromal reductants

The light-response curves of P700 oxidation and time-resolved kinetics of P700+ dark re-reduction were studied in barley leaves using absorbance changes at 820 nm. Leaves were exposed to 45 °C and treated with either diuron or diuron plus methyl viologen (MV) to prevent linear electron flow from PS II to PSI and ferredoxin-dependent cyclic electron flow around PSI. Under those conditions, P700+ could accept electrons solely from soluble stromal reductants. P700 was oxidized under weak far-red light in leaves treated with diuron plus MV, while identical illumination was nearly ineffective in diuron-treated leaves in the absence of MV. When heat-exposed leaves were briefly illuminated with strong far-red light, which completely oxidized P700, the kinetics of P700+ dark reduction was fitted by a single exponential term with half-time of about 40 ms. However, two first-order kinetic components of electron flow to P700+ (fast and slow) were found after prolonged leaf irradiation. The light-induced modulation of the kinetics of P700+ dark reduction was reversed following dark adaptation. The fast component (half time of 80–90 ms) was 1.5 larger than the slow one (half time of about 1 s). No kinetic competition occurred between two pathways of electron donation to P700+ from stromal reductants. This suggests the presence of two different populations of PSI.

Nonphotosynthetic Reduction of the Intersystem Electron Transport Chain of Chloroplasts Following Heat Stress. The Pool Size of Stromal Reductants

Abstract The properties of a negative transient signal (negative peak) observed during the first seconds of the induction of the photoacoustic (PA) signal in dark-adapted barley leaves treated with methyl viologen (MV) and diuron and then exposed to high temperatures have been examined. Under those conditions no electron donation from photosystem II (PSII) occurred, and electron flow through PSI could be supported only by soluble reductants located in the chloroplast stroma. The negative peak was observed only if the PA signal had been monitored at low, and not high, frequencies. The peak obviously originated from the oxygen consumption by PSI. The size of the peak increased as the temperature of preheating was raised from 39 to 45°C. The size of the peak decreased exponentially with a half-time of 3.7 s during illumination under low light. This decrease was found to be much faster under strong light. The recovery of the peak during dark acclimation required several minutes. It is concluded that the negative peak reflects the oxygen consumption supported by stromal reductants, their pool being rapidly exhausted under light in the presence of MV. The maximal size of the pool was calculated as 140 eq:P700 in dark-adapted leaves.

Contributions of the free oxidized and QB-bound plastoquinone molecules to the thermal phase of chlorophyll-a fluorescence

Variable chlorophyll a (Chl a) fluorescence is composed of a photochemical and a thermal phases of similar amplitudes. The photochemical phase can be induced by a saturating single turnover flash (STF) and reflects the reduction of the Photosystem II (PS II) QA primary electron acceptor. The thermal phase requires multiple turnover flash (MTF) and is somehow related to the reduction of the plastoquinone (PQ) molecules. This article aimed to determine the relative contributions of the QB-bound and the free oxidized PQ molecules to the thermal phase of Chl a fluorescence. We thus measured the interactive effects of exogenous PQ (PQex), of an inhibitor (DCMU) acting at the QB site of PS II and of an artificial quencher, 2-methyl-1,4-naphtoquinone, on Chl a fluorescence levels induced by STF (FF) and MTF (FM) in spinach thylakoids. We observed that: (1) the incorporation of PQex in thylakoids stimulated photosynthetic electron transport but barely affected FF and FM in the absence of DCMU; (2) DCMU significantly increased the amplitude of FF but slightly quenched FM; (3) 2-methyl-1,4-naphtoquinone quenched FM to a larger-extent than FF; (4) DCMU increased the quenching effects of PQex on FF and FM and also, of methyl-1,4-naphtoquinone on FF. These results indicate that: (1) the QB-bound and the free PQ molecules contribute to about 56% and 25%, respectively, to the thermal phase Chl a fluorescence in dark-adapted thylakoids; and (2) the thermal phase of Chl a fluorescence is more susceptible than the photochemical phase to the non-photochemical quenching effect of oxidized quinones.

The effect of milk alpha-casein on the antioxidant activity of tea polyphenols.

In this study, we report how the antioxidant capacities of major tea polyphenols are affected by their interactions with milk alpha-casein (milk protein) using three complimentary oxidation methods: ABTS(+) radical cation scavenging, cyclic voltammetry and lipid peroxidation inhibition. We found that using the ABTS(+) assays, the antioxidant activity of all polyphenols was lowered by 11-27% in the presence of caseins. Using cyclic voltammetry, the overall current measured at the electrode was decreased by the presence of the protein, from 21% to 61%. The peak potentials were also shifted to higher values varying from 13 to 41 mV. However, using lipid peroxidation method, we noticed of the antioxidant activity of all the polyphenols changed (from 6% up to 75%) after the addition of alpha-casein. The results show using this method the larger gallate esters containing polyphenols epicatechingallate (ECG) and (epigallocatechingallate (EGCG) were less affected by the presence of casein than smaller polyphenols catechins (C), epicatechin (EC) and epicgallocatechine (EGC). Alpha-casein caused a small effect on the chain breaking antioxidant capacity of theaflavins as well. Therefore, casein has different effects on the overall antioxidant capacities of tea compounds depending on the methods used. We aim to understand those results with the types of protein-polyphenol interactions that take place in various settings and their effects on the antioxidant capacities of those compounds.

Inhibitory effects of mercury on photosystem II photochemistry in Dunaliella tertiolecta under in vivo conditions

Abstract The inhibitory effect of mercury on photosystem II (PSII) photochemistry was investigated in the green alga Dunaliella tertiolecta. In the presence of 80 μM HgCl2, oxygen evolution is completely inhibited, whereas 50% of the variable fluorescence still remains. The effect of mercury on the fluorescence yield in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) is reversed when the electron donor to PSII is hydroxylamine instead of water. This result confirms that the inhibition site of mercury in the presence of DCMU is associated with the activity of the water-splitting system. Mercury induces a decrease in the complementary area, indicating that the number of functional PSII primary electron acceptors is decreased. Mercury inhibition results in a decrease in the PSII quantum yield in algal cells. However, the reduction rate of the PSII primary electron acceptor (QA) is not affected. Therefore, this effect on PSII induces a complete inactivation of the PSII reaction centres associated with the mercury-inhibited water-splitting system.