Decrease In Photochemical Activity Research Articles

Influence of light intensity and its spectral composition on photosynthetic activity of cucumber Cucumis sativus under fusarium wilt

The responses of cucumber chloroplasts of the Kustovoi variety formed under lighting of different intensity (6000 and 11 000 lx) or under LED lighting with a predominance of red light and far red light to infection with fungus Fusarium oxysporum were studied. The amount of chlorophylls and carotenoids in chloroplasts formed at low light increased in 72 h after infection, and at high light a significant increase in pigment catabolism was observed. Under fusarium wilt, the violaxanthin cycle was not involved in the conditions of the studied light range, and the photochemical activity of chloroplasts did not depend much on the level of illumination. The predominance of red light or far red light caused an increase in both chlorophylls and carotenoids content in terms of the dry mass of the leaf compared to their content in plants grown on white light. Infection on white light and red light caused an increase in the total content of chlorophylls and carotenoids, and a decrease in these parameters relative to healthy plants was noted on far red light. Fusarium wilt led to a decrease in photochemical activity and electron transport of photosystem I (by 20 %) in chloroplasts on red light and far red light. A decrease in the functional efficiency of photosystem II was also observed, most likely due to irreversible changes in pigment-protein complexes with two types of LED lighting. Various mechanisms of the response of cucumber chloroplasts to pathogen infection are discussed, depending on the light conditions of the formation of photosynthetic membranes.

Impact of UV-B radiation on the photosystem II activity, pro-/antioxidant balance and expression of light-activated genes in Arabidopsis thaliana hy4 mutants grown under light of different spectral composition

The effect of UV-B irradiation on the photosystem II (PSII) activity, the content of photosynthetic and UV-absorbing pigments (UAPs), activity of antioxidant enzymes such as catalase (CAT) and peroxidase (POD), as well as H2O2 content in 25-day-old wild type (WT) and the cryptochrome 1 (Cry1) mutant hy4 of Arabidopsis thaliana Col-0 plants was studied. In addition, expression of photoreceptor genes Cry1, Cry2 and UVR8, photomorphogenetic gene COP1 and transcription factors genes HY5, HYH, the gene of chlorophyll-binding protein of the PSII CAB1 as well as the flavonoid biosynthesis genes CHS, PAL and thylakoid ascorbate peroxidase gene tAPX was examined. It has been shown that UV-B leads to a decrease in the photochemical activity of PSII (FV/FM) and the PSII performance index (PIABS) of WT plants grown on white (WL) and red (RL) light and also hy4 mutants grown on WL, RL and blue light (BL). In plants grown on BL and WL, the decrease in the PSII photochemical activity was significantly greater in hy4 compared to WT. The PSII of WT plants grown in BL was resistant to UV-B. The UAPs content of hy4 grown on BL and WL was lower than that in WT. The POD and CAT activities of WT grown in BL were significantly higher than in the mutant. In WT and hy4 plants grown in RL, a noticeable difference in these enzymes activity was not found. In both types of plants grown in BL and RL, the expression of photomorphogenetic genes HYH, HY5 markedly increased after UV-B treatment but the expression of the UV-B photoreceptor gene UVR8 was reduced in hy4 grown in BL and RL. It is assumed that reduced resistance of PSII in hy4 plants grown in BL and WL can be associated with low UAPs content as well as lowered POD and CAT activities. In addition, we suggest the lowered expression of UVR8 and COP1 genes caused by Cry1 deficiency leads to a shift of balance of oxidants and antioxidants towards oxidants.

Tolerance of the Photosynthetic Apparatus to Acidification of the Growth Medium as a Possible Determinant of CO2-Tolerance of the Symbiotic Microalga Desmodesmus sp. IPPAS-2014.

The symbiotic unicellular chlorophyte Desmodesmus sp. IPPAS-2014 capable of growth at extremely high CO2 levels prohibitive for most other microalgae is an interesting model for studies of CO2 tolerance mechanisms and a promising organism for CO2 biocapture. We studied the initial (0-60min) phase of acclimation of this microalga to an abrupt decrease in pH of the medium sparged with air/20% CO2 mixture. Acclimation of the culture to these conditions was accompanied by a sharp decrease in photochemical activity of the chloroplast followed by its recovery with a characteristic time of 10-50min. We hypothesize that acidification of the cultivation medium by dissolving CO2 plays a key role in the observed decrease in the photochemical activity. The possible role of photosynthetic apparatus tolerance to abrupt acidification in overall high tolerance of symbiotic microalgae to extremely high CO2 levels is discussed.

Photoprotective mechanisms in photosystem II of Ephedra monosperma during development of frost tolerance

Dissipation of light energy absorbed by photosystem II (PSII) in assimilating shoots of an ever� green shrub Ephedra monosperma was investigated during its transition from the vegetative to frosttolerant state under natural conditions of Central Yakutia. The dynamics of modulated chlorophyll fluorescence and carotenoid content was analyzed during seasonal decrease in ambient temperature. The seasonal cooling was accompanied by a stepwise decrease in photochemical activity of PSII (Fv/Fm = (Fm - F0)/Fm). The decrease in Fv/Fm occurred from the beginning of September to the end of October, when the temperature was lowered from 10 to -8°C. During winter period the residual activity of PSII was retained at about 30% of the summer values. The seasonal decrease in temperature was accompanied by a significant stimulation of pHindepen� dent dissipative processes in reaction centers and antenna of PSII. The increase in energy losses was paral� leled by a proportional increase in zeaxanthin content on the background of decreasing content of violaxan� thin and β�carotene as possible zeaxanthin precursors. At the same time, inhibition of lightinduced non� photochemical quenching in the PSII antenna was observed. The results suggest that principal photoprotec� tive mechanisms during seasonal lowering of temperature are: (1) inactivation of PSII and dissipation of exci� tation energy in PSII reaction centers and (2) zeaxanthinmediated ener gy dissipation in the antenna com� plexes. The first mechanism seems to prevail at early stages of seasonal cooling, whereas both mechanisms are recruited from the onset of sustained freezing temperatures.

Bicarbonate stabilizes isolated D1/D2/cytochrome b559 complex of photosystem 2 against thermoinactivation.

It has been shown that thermoinactivation of the isolated D1/D2/cytochrome b(559) complex (RC) of photosystem 2 (PS-2) from pea under anaerobic conditions at 35°C in 20 mM Tris-HCl buffer (pH 7.2) depleted of HCO(3)(-), with 35 mM NaCl and 0.05% n-dodecyl-β-maltoside, results in a decrease in photochemical activity measured by photoreduction of the PS-2 primary electron acceptor, pheophytin (by 50% after 3 min of heating), which is accompanied by aggregation of the D1 and D2 proteins. Bicarbonate, formate, and acetate anions added to the sample under these conditions differently influence the maintenance of photochemical activity: a 50% loss of photochemical activity occurs in 11.5 min of heating in the presence of bicarbonate and in 4 and 4.6 min in the presence of formate and acetate, respectively. The addition of bicarbonate completely prevents aggregation of the D1 and D2 proteins as opposed to formate and acetate (their presence has no effect on the aggregation during thermoinactivation). Since the isolated RCs have neither inorganic Mn/Ca-containing core of the water-oxidizing complex nor nonheme Fe(2+), it is supposed that bicarbonate specifically interacts with the hydrophilic domains of the D1 and D2 proteins, which prevents their structural modification that is a signal for aggregation of these proteins and the loss of photochemical activity.

Ultrastructural and metabolic modifications at the plant–pathogen interface in Mesembryanthemum crystallinum leaves infected by Botrytis cinerea

Botrytis cinerea infection-induced ultrastructural and metabolic changes in Mesembryanthemum crystallinum performing C3 and CAM photosynthesis were investigated and compared with the defence response. Regardless of the plant photosynthetic carbon metabolism, infection induced hypersensitive response-like necroses that successfully inhibited the fungal growth. To gain an insight into the co-regulation of photosynthesis, the antioxidant defence and carbohydrate metabolism, we studied the infection-induced ultrastructural changes in chloroplasts and metabolic modifications related to the photochemical activity and prooxidant/antioxidant homeostasis. Different patterns of B. cinerea-induced ascorbate-, glutathione- and tocopherol-related changes were found in C3 and CAM leaves indicating that specific mechanisms were involved in regulation of the antioxidant defence. A decrease in photochemical activity visualized by chlorophyll fluorescence imaging was found only in CAM plants. This was accompanied by the ultrastructural dismantling of chloroplasts, increased number of plastoglobules and diminished number of starch grains in chloroplasts. However, in infected C3 plants, showing similar ultrastructural disturbances in the chloroplasts, the photochemical activity and the number of starch grains remained unaffected whereas decreased surface of starch grains were found. We propose that the specific co-regulation of the response to pathogen and photochemical activity, sink/source relations and antioxidant mechanisms found in C3 and CAM plants were possibly related to the different photosynthetic carbon metabolisms.

Effect of short-term exposure to NaCl on photochemical activity and antioxidant enzymes in Bruguiera parviflora, a non-secretor mangrove

Effect of short-term (6 days) exposure to high salinity (500 mM NaCl) was studied in Bruguiera parviflora, a tree mangrove. NaCl treatment decreased photochemical activity, but had no effect on growth. Thylakoid protein profile and spectral characteristic were not changed. There was no significant effect on chlorophylls and carotenoids content, total proteins and total free amino acids. However, there was an increase in free proline. The activity of antioxidant enzymes like catalase, ascorbate peroxidase was enhanced, but no significant change in guaiacol peroxidase was observed. Salinity did not cause any alteration in malondialdehyde formation indicating intactness of membrane integrity upon high salinity. We conclude that the effect of high NaCl stress is not revealed in morphology of the plants, but in the metabolic changes as increase in proline and antioxidant enzyme activity. These effects are the adaptive mechanisms that operates under high salt stress in this mangrove; however, the decrease in photochemical activity may be due to onset of senescence which helps plant in remobilization of photosynthate to new leaves after adaptation.

Photoacoustic signal generation in leaves: are O 2-consuming processes involved?

The photoacoustic signal from a green leaf is the vectorial sum of photothermal (heat emitted by non-radiative deexcitation of molecules) and photobaric (photosynthetic O 2 exchanges) contributions. With sugar maple leaf, we have observed a decline of the O 2 signal in the course of time, once the leaf attained a steady-state photosynthesis. This decline of the O 2 signal was not due to a decrease in photochemical activity as was evident by a stable fluorescence signal and constant energy storage. The rate and the extent of decline were larger at low modulation frequency and low modulated light intensity. The decline continued even in the presence of background far-red light (λ>715 nm). The above observations lead us to conclude that the photoacoustic O 2 signal from leaves is influenced by O 2 -consuming processes. Data are discussed in relation to a possible occurrence of an equilibrium between oxygen-evolving and oxygen-consuming processes to account for the O 2 signal in the plant cell. Moreover, the oxygen-uptake component may have to be included in the vectorial model of photoacoustic signal generation in leaves.

Hydrogen peroxide formation from the photooxidation of formaldehyde and its presence in rainwater

Abstract The photooxidation of formaldehyde with sunlamps (Emax = 3100 A) produces hydrogen peroxide at varying concentrations depending upon the amount of water vapor present. It is postulated that the variable production of H2O2 is a result of condensation on the reactor surface. Rainwater samples were also analyzed for H2O2. Summer rain in the Triangle area of North Carolina contained as much as 200 ppb (w/w) of H2O2. Rainwater collected in December contained much less H2O2, usually 2–8 ppb. The lower concentration found in the colder conditions is attributed to a decrease in photochemical activity.

Effect of Digitonin Concentration on Electron Transport, Phosphorylation, and Proton Uptake by Subchloroplast Particles

The relative activity of photosystem I subchloroplast particles was dependent on the digitonin concentration used during incubation. At low digitonin concentrations (2 mg digitonin per mg chlorophyll), the particles were greatly enriched in photosystem I activities and showed high rates of cyclic phosphorylation and appreciable light-induced proton uptake. Increasing the concentration of digitonin increased the yield of photosystem I particles but decreased their specific activity of electron transport and of cyclic phosphorylation. The decrease in activity was not related to the degree of enrichment of the pigments associated with photosystem I nor to the degree of separation of the photosystems. Release of plastocyanin was dependent on the digitonin concentration. As the digitonin concentration was increased, the plastocyanin content of the photosystem I particles decreased. The decrease in photochemical activity with increasing digitonin concentration was attributed to the loss of plastocyanin during isolation.