Photosynthetic Fluorescence Induction and Oxygen Production in Corallinacean Algae Measured on Site

Photoinhibition of photosynthesis was measured in two Mediterranean Corallinaceae, Jania rubens and Corallina mediterranea, using pulse-amplitude modulation (PAM) fluorescence and oxygen production on site. Both algae were found to be adapted to low irradiances of solar radiation and easily inhibited by exposure to excessive radiation. Both algae were impaired even in their natural habitat under overhanging rocks which protected them from direct solar radiation, except for a few hours in the early morning. Recovery from photoinhibition of both the photosynthetic quantum yield, defined as F_v′/F_m′, and oxygen production took several hours and was not complete. Judging from both parameters indicated above, Jania seems to be even more sensitive than Corallina, even though the former alga was found in more exposed habitats.     

Photosynthetic performance of the Atlantic brown macroalgae, Cystoseira abies-marina, Dictyota dichotoma and Sargassum vulgare, measured in Gran Canaria on site

Photosynthetic performance was determined in three common Atlantic brown macroalgae, Cystoseira abies-marina, Dictyota dichotoma and Sargassum vulgare, in Gran Canaria, Canary Islands, on site. The photosynthetic quantum yield was measured with both a portable PAM instrument on site and a diving PAM under water in the habitat. In parallel, solar radiation was measured continuously above and under water by means of two three-channel dosimeters, ELDONET (Real Time Computer, M?hrendorf, Germany), in three wavelength ranges, UV-A, UV-B and PAR. The effective photosynthetic quantum yield decreased in all species in response to exposure to 15 min of solar radiation but recovered in the subsequent shade conditions within several hours. A 30-min exposure caused an even more profound photoinhibition from which the algae recovered only partially. Most of the effect was due to visible radiation, however, the UV wavelength range, and especially UV-B, considerably enhanced the decrease in photosynthetic quantum yield. In all species except Sargassum a significant photoinhibition was detected at their growth sites at high solar angles in the water column, measured with the diving PAM.     

Effects of solar radiation and solar radiation deprived of UV-B and total UV on photosynthetic oxygen production and pulse amplitude modulated fluorescence in the brown alga Padina pavonia

Abstract: The effects of solar radiation on photosynthetic oxygen production and pulse amplitude modulated (PAM) fluorescence were measured in the marine brown macroalga Padina pavonia harvested from different depths from the Greek coast near Korinth. In fluence rate-response curves the light compensation point for photosynthetic oxygen production increased and the saturation level decreased with increasing exposure time to solar radiation. Cutting off the UV-B wavelength range (280–315 nm) from solar radiation reduced the inhibition of photosynthesis, and the organisms were less affected when all of the UV radiation was filtered out. Algae collected from 7 m depth were much more prone to photoinhibition than those harvested from rock pools exposed to unfiltered solar radiation. During continuous exposure to solar radiation, rock pool algae showed photoinhibition after longer periods of time than specimens from 7 m or from dark adapted habitats. When subjected to unfiltered solar radiation the ratio of the variable fluorescence to the maximal fluorescence     (F_v = F_m− F_o) rapidly declined with increasing exposure time. However, again algae from 7 m depth were more prone to photoinhibition than rock pool algae. The differences between the two ecological strains were less obvious when UV-B or total UV was removed from solar radiation. Only in the latter case a complete recovery was observed after 2 h while, when exposed to unifiltered sunlight, only the rock pool algae recovered completely within that time.     

Acclimation of Maximal Quantum Yield of Photosynthesis in the Brown Alga Alaria esculenta under High Light and UV Radiation

Abstract: Macroalgae of the upper sublittoral zone of Arctic coastal ecosystems are subjected to darkness or low light for several months during winter and have to withstand large changes in irradiance after the breakup of sea ice in the Arctic spring. Changes in photosynthetic response to high PAR (pho-tosynthetically active radiation) and UV-B radiation (UV-B) in the cold temperate brown alga A/aria esculenta were monitored with a PAM fluorometer to study photoinhibition, recovery and acclimation of maximal quantum yield of photochemistry. Plants collected in the field, as well as specimens raised in the laboratory, were exposed to various radiation conditions including different levels of PAR and UV radiation (UV-A + UV-B). Measurements of variable chlorophyll fluorescence of photosystem II revealed that the photosynthetic apparatus in A. esculenta was able to acclimate to the respective high light and UV treatments within several days. However, two different mechanisms of acclimation seem to be involved. Initially, the rate of recovery of maximal quantum yield increased after only a few exposures to high light or UV. Second, after several exposure cycles, the degree of inhibition was reduced. Data on fluorescence induction kinetics and quenching analysis showed that exposure to the respective UV radiation resulted in an increase of non-photochemical quenching, while effective quantum yield of photochemistry was hardly affected.     

Photoinhibition by visible and ultraviolet radiation in the red macroalga Porphyra umbilicalis grown in the laboratory

Photosynthetic oxygen production and PAM fluorescence measurements were used to follow photoinhibition in the red macroalga Porphyra umbilicalis. Exposure to simulated solar radiation caused inhibition of the effective photosynthetic quantum yield from which the thalli partially recovered in the shade in subsequent hours. There were no significant differences between samples exposed to unfiltered radiation and those exposed to radiation from which increasing portions of UV radiation had been removed indicating that the thalli are well adapted to current levels of solar PAR and UV radiation. This notion was supported by the finding of high concentrations of UV screening pigments which were even enhanced by exposure to increased UV radiation. However, when exposed to (only) UV radiation about 50% higher than that encountered by the organisms in their natural habitat, the photosynthetic yield decreased slowly and did not show any recovery even when the degree of inhibition did not exceed 10%.     

Photoinhibition in common atlantic macroalgae measured on site in Gran Canaria

The photosynthetic quantum yield was analysed in four common atlantic macroalgae, the Rhodophytes Gelidium arbuscula and Halopithys incurvus and the Phaeophytes Halopteris scoparia and Lobophora variegata in Gran Canaria, Canary Islands at their growth site. The fluorescence parameters were measured using a portable pulse amplitude modulated (PAM) fluorometer (PAM 2000) instrument and a diving PAM under water without removing the thalli from their growth sites. Solar radiation was monitored continuously above and under water during the whole experimental period using two three-channel dosimeters (European light dosimeter network; ELDONET) (Real Time Computer, M?hrendorf, Germany). These instruments measure solar radiation in three wavelength ranges, ultraviolet (UV)-A, UV-B and photosynthetic active radiation (PAR). In all four algae the effective photosynthetic quantum yield decreased significantly from the optimal values measured after dark adaptation due to exposure to 15 min solar radiation, but at least partially recovered subsequently in the shade within several hours. Increasing the exposure period to 30 min intensified the photoinhibition. In some algae no recovery was observed after this treatment and in others no significant recovery could be detected. Exposure to unfiltered solar radiation caused a significantly higher photoinhibition than PAR-only radiation or PAR plus UV-A. A substantial inhibition was found in all algae at their growth sites in the water column when the sun was at high angles, as measured with the diving PAM. Received in revised form: 15 May 2000 Electronic Publication     

Effects of UV-B radiation on photosynthesis and growth of terrestrial plants

The photosynthetic apparatus of some plant species appears to be well-protected from direct damage from UV-B radiation. Leaf optical properties of these species apparently minimizes exposure of sensitive targets to UV-B radiation. However, damage by UV-B radiation to Photosystem II and Rubisco has also been reported. Secondary effects of this damage may include reductions in photosynthetic capacity, RuBP regeneration and quantum yield. Furthermore, UV-B radiation may decrease the penetration of PAR, reduce photosynthetic and accessory pigments, impair stomatal function and alter canopy morphology, and thus indirectly retard photosynthetic carbon assimilation. Subsequently, UV-B radiation may limit productivity in many plant species. In addition to variability in sensitivity to UV-B radiation, the effects of UV-B radiation are further confounded by other environmental factors such as CO₂, temperature, light and water or nutrient availability. Therefore, we need a better understanding of the mechanisms of tolerance to UV-B radiation and of the interaction between UV-B and other environmental factors in order to adequately assess the probable consequences of a change in solar radiation.Abbreviations A_max light and CO₂ saturated rate of oxygen evolution - Ci internal CO₂ concentration - F_v/F_m ratio of variable to total fluorescence yield - PAR photosynthetically active radiation (400–700 nm) - PS II Photosystem II - _app apparent quantum yield of photosynthesis - SLW specific leaf weight - UV-B ultraviolet-B radiation between 290–320 nm     

Biochemical and spectroscopic analysis of UV effects in the marine flagellate Cryptomonas maculata

The effects of solar and artifical ultraviolet radiation on the marine cryptoflagellate, Cryptomonas maculata, were studied. Even after short exposure to UV the accessory photosynthetic pigment phycoerythrin is bleached; likewise the fluorescence undergoes significant changes both in amplitude and in the maximal peak wavelength. In parallel, the photosynthetic oxygen production decreases rapidly during exposure. Gel electrophoresis and FPLC of membrane proteins show a significant decrease in chromoproteins after 2 h UV, which is confirmed by fluorescence excitation and emission spectra of the FPLC fractions.Abbreviations APS ammonium persulfate - DCMU 3-(3,4dichlorophenyl)1,1-dimethylurea; Emulphogen, polyoxyethylene 10 tridecyl ether - FPLC fast protein liquid chromatography - PMSF phenylmethylsulfonyl fluoride - SDS sodium dodecylsulfate - SDS PAGE sodium dodecylsulfate polyacrylamide gel electrophoresis - TEMED NN NNtetramethylethylene diamine - UV-A wavelength range between 320 nm and 400 nm - UV-B wavelength range between 280 nm and 320 nm Dedicated to the 60th birthday of Professor Dr. W. Wehrmeyer     

Effects of solar UV-B radiation on canopy structure of Ulva communities from southern Spain

Within the sheltered creeks of Cádiz bay, Ulva thalli form extended mat-like canopies. The effect of solar ultraviolet radiation on photosynthetic activity, the composition of photosynthetic and xanthophyll cycle pigments, and the amount of RubisCO, chaperonin 60 (CPN 60), and the induction of DNA damage in Ulva aff. rotundata Bliding from southern Spain was assessed in the field. Samples collected from the natural community were covered by screening filters, generating different radiation conditions. During daily cycles, individual thalli showed photoinhibitory effects of the natural solar radiation. This inhibition was even more pronounced in samples only exposed to photosynthetically active radiation (PAR). Strongly increased heat dissipation in these samples indicated the activity of regulatory mechanisms involved in dynamic photoinhibition. Adverse effects of UV-B radiation on photosynthesis were only observed in combination with high levels of PAR, indicating the synergistic effects of the two wavelength ranges. In samples exposed either to PAR+UV-A or to UV-B+UV-A without PAR, no inhibition of photosynthetic quantum yield was found in the course of the day. At the natural site, the top layer of the mat-like canopies is generally completely bleached. Artificially designed Ulva canopies exhibited fast bleaching of the top layer under the natural solar radiation conditions, while this was not observed in canopies either shielded from UV or from PAR. The bleached first layer of the canopies acts as a selective UV-B filter, and thus prevents subcanopy thalli from exposure to harmful radiation. This was confirmed by the differences in photosynthetic activity, pigment composition, and the concentration of RubisCO in thalli with different positions within the canopy. In addition, the induction of the stress protein CPN 60 under UV exposure and the low accumulation of DNA damage indicate the presence of physiological protection mechanisms against harmful UV-B. A mechanism of UV-B-induced inhibition of photosynthesis under field conditions is proposed.     

Inhibition of photosynthesis by solar radiation in Dunaliella salina: relative efficiencies of UV-B, UV-A and PAR

Inhibition of photosynthesis after exposure to solar radiation was investigated in the marine green alga Dunaliella salina by monitoring photosynthetic optimal quantum yield F_v/F_m and efficiency of oxygen production. Samples were exposed to solar radiation in Ancient Korinth, Greece (37°58′ N, 23°0′ E) in August 1994. Within 30 min, F_v/F_m and efficiency of oxygen production decreased with similar kinetics with increasing exposure time. The inhibition, however, diminished when ultraviolet radiation was progressively excluded by means of colour filter glasses. Samples exposed for 3 h showed complete or partial recovery of photosynthesis, with almost the same rate under all irradition conditions. The fit of the experimental data with an analytical model describing inhibition of photosynthesis as a function of a linear combination of the photon fluence in the UV-B, UV-A and PAR allows one to estimate the relative mean effectiveness for inhibition by the three spectral ranges [about 2 × 10^?4, 4 × 10^?6 and 2 × 10^?7 (μmol photons m^?2)^?1 for UV-B, UV-A and PAR, respectively].     

Effect of solar ultraviolet-B radiation during springtime ozone depletion on photosynthesis and biomass production of Antarctic vascular plants

We assessed the influence of springtime solar UV-B radiation that was naturally enhanced during several days due to ozone depletion on biomass production and photosynthesis of vascular plants along the Antarctic Peninsula. Naturally growing plants of Colobanthus quitensis (Kunth) Bartl. and Deschampsia antarctica Desv. were potted and grown under filters that absorbed or transmitted most solar UV-B. Plants exposed to solar UV-B from mid-October to early January produced 11% to 22% less total, as well as above ground biomass, and 24% to 31% less total leaf area. These growth reductions did not appear to be associated with reductions in photosynthesis per se: Although rates of photosynthetic O(2) evolution were reduced on a chlorophyll and a dry-mass basis, on a leaf area basis they were not affected by UV-B exposure. Leaves on plants exposed to UV-B were denser, probably thicker, and had higher concentrations of photosynthetic and UV-B absorbing pigments. We suspect that the development of thicker leaves containing more photosynthetic and screening pigments allowed these plants to maintain their photosynthetic rates per unit leaf area. Exposure to UV-B led to reductions in quantum yield of photosystem II, based on fluorescence measurements of adaxial leaf surfaces, and we suspect that UV-B impaired photosynthesis in the upper mesophyll of leaves. Because the ratio of variable to maximal fluorescence, as well as the initial slope of the photosynthetic light response, were unaffected by UV-B exposure, we suggest that impairments in photosynthesis in the upper mesophyll were associated with light-independent enzymatic, rather than photosystem II, limitations.     

Exposure to solar radiation increases damage to both host tissues and algal symbionts of corals during thermal stress

Elevated seawater temperatures have long been accepted as the principal stressor causing the loss of symbiotic algae in corals and other invertebrates with algal symbionts (i.e., bleaching). A secondary factor associated with coral bleaching is solar irradiance, both its visible (PAR: 400–700 nm) and ultraviolet (UVR: 290–400 nm) portions of the spectrum. Here we examined the synergistic role of solar radiation on thermally induced stress and subsequent bleaching in a common Caribbean coral, Montastraea faveolata. Active fluorescent measurements show that steady-state quantum yields of photosystem II (PSII) fluorescence in the zooxanthellae are markedly depressed when exposed to high solar radiation and elevated temperatures, and the concentration of D1 protein is significantly lower in high light when compared to low light treatments under the same thermal stress. Both photosynthetic pigments and mycosporine-like amino acids (MAAs) are also depressed after experimental exposure to high solar radiation and thermal stress. Host DNA damage is exacerbated under high light conditions and is correlated with the expression of the cell cycle gene p 53, a cellular gatekeeper that modulates the fate of damaged cells between DNA repair processes and apoptotic pathways. These markers of cellular stress in the host and zooxanthellae have in common their response to the enhanced production of reactive oxygen species during exposure to high irradiances of solar radiation and elevated temperatures. Taking these results and previously published data into consideration, we conclude that thermal stress during exposure to high irradiances of solar radiation, or irradiances higher than the current photoacclimatization state, causes damage to both photochemistry and carbon fixation at the same time in zooxanthellae, while DNA damage, apoptosis, or necrosis are occurring in the host tissues of symbiotic cnidarians.Abbreviations _PSII Functional absorption cross-section for PSII - F_o, F_m Minimum and maximum yields of chlorophyll a fluorescence measured after dark acclimation (relative units) - F_v Variable fluorescence after dark acclimation (=F_m–F_o), dimensionless - F_v/F_m Maximum quantum yield of photochemistry in PSII measured after dark acclimation, dimensionless - F, F_m Steady-state and maximum yields of chlorophyll a fluorescence measured under ambient light (relative units) - F/F_m Quantum yield of photochemistry in PSII measured at steady state under ambient light Communicated by R.C. Carpenter     

Differential responses of Nostoc sphaeroides and Arthrospira platensis to solar ultraviolet radiation exposure

During October to December 2003 we carried out experiments to assess the impact of high solar radiation levels (as normally occurring in a tropical region of Southern China) on the cyanobacteria Nostoc sphaeroides and Arthrospira (Spirulina) platensis. Two types of experiments were done: a) Short-term (i.e., 20 min) oxygen production of samples exposed to two radiation treatments (i.e., PAR+UVR—280–700 nm, and PAR only—400–700 nm, PAB and P treatments, respectively), and b) Long-term (i.e., 12 days) evaluation of photosynthetic quantum yield (Y) of samples exposed to three radiation treatments (i.e., PAB; PA (PAR+UV-A, 320–700 nm) and P treatments, respectively). N. sphaeroides was resistant to UVR, with no significant differences (P>0.05) in oxygen production within 20 min of exposure, but with a slight inhibition of Y within hours. A fast recovery of Y was observed after one day even in samples exposed to full solar radiation. A. platensis, on the other hand, was very sensitive to solar radiation (mainly to UV-B), as determined by oxygen production and Y measurements. A. platensis had a circadian rhythm of photosynthetic inhibition, and during the first six days of exposure to solar radiation, it varied between 80 and 100% at local noon, but cells recovered significantly during afternoon hours. There was a significant decrease in photosynthetic inhibition after the first week of exposure with values less than 50% at local noon in samples receiving full solar radiation. Samples exposed to PA and P treatments recovered much faster (within 2–3 days), and there were no significant differences in Y between the three radiation treatments when irradiance was low (late afternoon to early morning). Long-term acclimation seems to be important in A. platensis to cope with high UVR levels however, it is not attained through the synthesis of UV-absorbing compounds but it seems to be mostly related to adaptive morphological changes.     

Overproduction of Photosynthetic Electrons is Associated with Chilling Injury in Green Leaves

Employing the non-invasive techniques of infra-red gas analysis and pulse amplitude modulated chlorophyll fluorometry, we determined the partitioning of photosynthetic electrons between photosynthetic carbon reduction and other reductive processes resulting in the formation of active oxygen species (AOS) in intact green leaves. This we studied in plant species that are adapted to two different agro-climatic conditions, namely the warm plains (76°36E, 9°32N) and the cool mountains (1 600 m a.s.l.) in the south Indian state of Kerala. Ground frost and low temperature were more harmful to those species adapted to the warm plains than the ones adapted to the cool mountains. Exposure to low temperature decreased leaf photosynthetic carbon assimilation rates and quantum yield of photochemical activity in species naturally adapted to the warm plains. High irradiances further aggravated the harmful effects of low temperature stress possibly by overproducing AOS. This resulted in severe peroxidative damage as inferred by the accumulation of malondialdehyde (MDA) in the leaves.     

The effect of ultraviolet radiation on photosynthesis and ultraviolet-absorbing substances in the endemic Arctic macroalga Devaleraea ramentacea (Rhodophyta)

In field studies conducted at the Kongsfjord (Spitsbergen), the effect of filtered natural radiation conditions (solar without ulraviolet [UV]-A+UV-B, solar without UV-B, solar) on photosynthesis and the metabolism of UV-absorbing mycosporine-like amino acids (MAAs) in the marine red alga Devaleraea ramentacea have been studied. While solar treatment without UV-A+UV-B did not affect photosynthesis during the course of a day, solar without UV-B and the full solar spectrum led to a strong inhibition. However, after offset of the various radiation conditions, all algae fully recovered. Isolates collected from different depths were exposed in the laboratory to artificial fluence rates of photosynthetic active radiation (PAR), PAR+UV-A, and PAR+UV-A+UV-B. The photosynthetic capacity was affected in accordance with the original sampling depth, i.e. shallow-water isolates were more resistant than algae from deeper waters, indicating that D. ramentacea is able to acclimate to changes in irradiance. Seven different UV-absorbing MAAs were detected in this alga, namely mycosporine-glycine, shinorine, porphyra-334, palythine, asterina-330, palythinol, and palythene. The total amount of MAAs continuously decreased with increasing collecting depth when sampled in mid June, and algae taken in late August from the same depths contained on average 30–45% higher MAA concentrations, indicating a seasonal effect as well. The presence of increasing MAA contents with decreasing depth correlated with a more insensitive photosynthetic capacity under both UV-A and UV-B treatments. Populations of D. ramentacea collected from 1 m depth, with one fully exposed to solar radiation and the other growing protected as understorey vegetation underneath the kelp Laminaria saccharina, exhibited quantitatively different MAA compositions in the apices. The exposed seaweeds contained 2.5-fold higher MAA values compared with the more shaded algae. Moreover, the exposed isolates showed a strong tissue gradient in MAAs, pigments, and proteins. The green apices contained 5-fold higher MAA contents than the red bases. Transplantation of D. ramentacea from 2 m depth to the surface induced the formation and accumulation of MAAs after 1 week exposure to the full solar spectrum. Control samples which were treated with the solar spectrum without UV-A+B or with solar without UV-B showed unchanged MAA contents, indicating a strong UV-B effect on MAA metabolism. All data well supported the suggested physiological function of MAAs as natural UV sunscreens in macroalgae.     

UV-B radiation and selenium affected energy availability in green alga <Emphasis Type="Italic">Zygnema</Emphasis>

Green alga Zygnema was exposed to three concentrations of selenium and two levels of UV-B radiation. The combined effects of both treatments on energy availability; photochemical quantum yield and respiratory potential were studied. Our findings show that traces of selenium enhance metabolic process connected with photochemical quantum yield and mitochondrial respiration. Surprisingly, selenium does not diminish the effects of UV-B radiation; on the contrary, the combined action of UV-B radiation and traces of selenium leads to pronounced negative effects on photochemical quantum yield and the respiratory potential. Selenium is involved in the activation of energy resources in green alga Zygnema. The importance of selenium for activity of the mitochondria is possibly an evolutionary recollection from an endosymbiotic bacterium.     

Influence of high light and UV-B radiation on photosynthesis and D1 turnover in atrazine-tolerant and -sensitive cultivars of Brassica napus

An atrazine-tolerant mutant and an atrazine-sensitive cultivar of Brassica napus L. were grown under visible radiation (400 mumol m-2 s-1, photosynthetically active radiation, PAR) and then subjected to treatment conditions. These included short-term high PAR (1600 mumol m-2 s-1) which was given for 4 h either alone or in combination with an enhanced level of UV-BBE radiation (4.6 kJ m-2 h-1 biologically effective UV-B, 280-320 nm). Recovery from the radiation treatment was studied for 4 h under the light conditions for growth. Since it is known that the atrazine-tolerant mutant is susceptible to photoinhibition, one of the aims of the present study was to determine the effects of a supplemental, enhanced level of UV-B radiation with regard to the mutant. The results indicate an additive effect of UV-B radiation on Fv/Fm, photochemical yield and photosynthetic oxygen evolution during both exposure and recovery, and also a higher susceptibility of the mutant to photoinhibitory PAR conditions alone and in combination with UV-B, which may have implications in a changing environment. Both cultivars also showed a higher D1 turnover during the radiation stress than during recovery, as shown by immunoblotting and 35S-methionine incorporation measurements.     

A new filter that accurately mimics the solar UV-B spectrum using standard UV lamps: the photochemical properties, stabilization and use of the urate anion liquid filter

The physiological effects unique to solar ultraviolet (UV)-B exposure (280-315 nm) are difficult to accurately replicate in the laboratory. This study evaluates the effectiveness of the sodium urate anion in a liquid filter that yields a spectrum nearly indistinguishable from the solar UV-B spectrum while filtering the emissions of widely used UV-B lamps. The photochemical properties and stability of this filter are examined and weighed against a typical spectrum of ground-level solar UV-B radiation. To test the effectiveness of this filter, light-saturated photosynthetic oxygen evolution rates were measured following exposure to UV-B filtered either by this urate filter or the widely used cellulose acetate (CA) filter. The ubiquitous marine Chlorophyte alga Dunaliella tertiolecta was tested under identical UV-B flux densities coupled with ecologically realistic fluxes of UV-A and visible radiation for 6 and 12 h exposures. These results indicate that the urate-filtered UV-B radiation yields minor photosynthetic inhibition when compared with exposures lacking in UV-B. This is in agreement with published experiments using solar radiation. In sharp contrast, radiation filtered by CA filters produced large inhibition of photosynthesis.     

Photoinhibition in Chlamydomonas reinhardtii: Effect on state transition,intersystem energy distribution and Photosystem I cyclic electron flow

The energy distribution, state transitions and photosynthetic electron flow during photoinhibition of Chlamydomonas reinhardtii cells have been studied in vivo using photoacoustics and modulated fluorescence techniques. In cells exposed to 2500 W/m² light at 21 °C for 90 min, 90% of the oxygen evolution activity was lost while photochemical energy storage as expressed by the parameter photochemical loss (P.L.) at 710–720 nm was not impaired. The energy storage vs. modulation frequency profile indicated an endothermic step with a rate constant of 2.1 ms. The extent of the P.L. was not affected by DCMU but was greatly reduced by DBMIB. The regulatory mechanism of the state 1 to state 2 transition process was inactivated and the apparent light absorption cross section of photosystem II increased during the first 20 min of photoinhibition followed by a significant decrease relative to that of photosystem I. These results are consistent with the inactivation of the LHC II kinase and the presence of an active cyclic electron flow around photosystem I in photoinhibited cells.Abbreviations PS I, PS II Photosystem I and Photosystem II respectively - P.L. photochemical loss - DCMU 3-(3,4-dichlorophenyl-1,1-dimethyl urea - LHC II light harvesting chlorophyll a,b-protein complex of PS II - DBMIB 2,5 dibromo-3-methyl-6-isopropyl-p-benzoquinone     

Field research on the effects of UV-B filters on terrestrial Antarctic vegetation

Patches of vegetation of six common species growing on Léonie Island (67°35 S, 68°20 W), Antarctic Peninsula region were covered with either UV-B transparent perspex screens or UV-B absorbing screens. Uncovered plots served as a control. Temperature and relative humidity were monitored during the austral summer under and outside the screens. The mean effective PSII quantum efficiency showed significant differences among the species, but not between the UV-B treatments. It was concluded that the temperature and the moisture status of the vegetation obscured any possible influence of UV-B treatment on the tteffective PSII quantum efficiency. he usefulness of various UV-B exclusion and supplementation methods used to study the influence of UV-B in the field is discussed.