The involvement of LHCII-associated polyamines in the response of the photosynthetic apparatus to low temperature

The influence of low temperature on the structure and function of the photosynthetic apparatus was investigated in Phaseolus vulgaris L. Ten-day-old plants (grown at 26 degrees C) have been exposed to low temperature (6 degrees C) for 52 h and, then, transferred to the initial temperature (26 degrees C) for additional 30 h. Biochemical and physico-chemical measurements performed in the low temperature-treated plants showed that the response of the photosynthetic apparatus to low temperature is affected by the changes occurring in the pattern of LHCII-associated putrescine (Put) and spermine (Spm) which adjust the size of LHCII. The decrease of Put/Spm ratio, mainly due to the reduction in the quantity of LHCII-associated Put led to an increase of the LHCII, especially of the oligomeric forms. These alterations in the structure of the photosynthetic apparatus combined with the reduction in the photosynthetic electron transfer rate resulted in the inactivation of active reaction centers and the increase of dissipated energy which diminished the photosynthetic efficiency and the maximal photosynthetic rate. The transfer of plants at 26 degrees C after the low temperature treatment showed that, structurally and functionally, the photosynthetic mechanism recovered quite fast to the initial condition.     

Mycosporine-like amino acids in the marine dinoflagellate Gyrodinium dorsum: induction by ultraviolet irradiation

Cultures of the marine dinoflagellate Gyrodinium dorsum have been exposed to polychromatic radiation (photosynthetically active radiation and UV) from a solar simulator for up to 72 h. Different irradiance spectra in the ultraviolet are produced by inserting cut-off filters between lamp and samples. The mycosporine-like amino acid (MAA) content and composition are investigated by spectroscopic and chromatographic analysis. The study reveals that G. dorsum contains a complex mixture of several aminocyclohexenimine-MAAs and one aminocyclohexenone-MAA. UV irradiation around 320 nm induces an increase in the concentration of all MAAs in the samples. In contrast, exposure to short-wavelength UV-B radiation results in decreased overall MAA production. Furthermore, there is a spectral shift in the absorption of the MAA mixture towards shorter wavelengths, indicating that short-wavelength UV-B induces an altered MAA composition. The amount of MAAs is normalized to the chlorophyll a concentration.     

Spermine-Induced Morphogenesis and Effect of Partial Immersion System on the Shoot Cultures of Banana

Contribution of exogenous polyamines (PAs) and polyamine-inhibitors on plantlet regeneration patterns of banana (cv. Nanjanagudu Rasabale-AAB) was studied and the performance of regenerated shoots in temporary immersion system was evaluated. The rhizome explants (without shoot bud) of in vitro shoots produced a mixture of embryogenic and nonembryogenic calli on modified MS medium. The analyses of endogenous pools of polyamines showed higher levels of PAs in embryogenic than in nonembryogenic calli. Supplementation of various levels of (10-50 microM) spermine (Spm), spermidine (Spd), and putrescine (Put) to cultures with secondary embryogenesis showed that about 50% of embryogenic calli rapidly produced secondary embryos only in the presence 40 microM Spm but not in other treatments. The crucial role of Spm was further confirmed by the use of 0.1 mM each of alpha-DL-Difluromethylornithine and alpha-DL-Difluromethylarginine along with Spm where the presence of inhibitors concomitantly inhibited the secondary embryogenesis. The shoots obtained from the embryogenic cultures were checked for their performance on solid medium (SM) and partial immersion system (PIS). The rate of shoot multiplication was higher in PIS than in SM throughout 6 weeks culture period. Uniformity in elongation of all the shoot buds was observed in PIS but not in SM. Evaluation for the acclimatization, survival under greenhouse conditions revealed the better performance of PIS-derived plants than those from SM.     

Photoacclimation in spathiphyllum

We studied photoacclimation in Spathiphyllum grown at an irradiance of 40 or 420 micromol/m2 s (LL or HL, respectively). All parameters studied responded to acclimation. Leaves at LL, in contrast to HL, were thinner and oriented perpendicular to the incident light, had more chlorophyll per g f w, fewer stomata on the upper leaf surface and a reduced layer of mesophyll cells. Their chloroplasts at HL had wider grana with less thylakoids per granum, and better organized photosystems than at LL. PSI and PSII activities per mg chlorophyll ( Vmax ), and PSI and PSII content (total activity per g f w), were lower at LL than at HL and so was the light requirement for saturation of the PSI or PSII partial photoreactions, suggesting that fewer photosystems with larger antenna size prevail at LL, but many more with smaller antenna size at HL. Analysis of chlorophyll distribution among the thylakoid pigment-protein complexes showed less antenna chlorophyll serving PSII (CPa+LHCP1+LHCP3) than that serving PSI (CPIa+CPI+LHCP2) at LL as compared to HL, and thus a lower PSII/PSI ratio at LL, in agreement with the general finding that LL plants, with larger PSII antenna size, have lower PSII/PSI ratio. The increase in PSI antenna size at LL was correlated with the increase in the distribution of chlorophyll in pigment-protein complexes serving PSI, and a very large chlorophyll/protein molar ratio in the isolated CPI complex. On the other hand, the PSII antenna chlorophyll (CPa+LHCP1+LHCP3) on a g f w basis, and the chlorophyll a/b ratio remained more or less constant at LL or HL. This may reflect our finding that Spathiphyllum contains mainly the 27 kDa inner LHCII antenna protein, the size of which remains unaffected by photoacclimation. The increase in the distribution of chlorophyll in pigment-protein complexes serving PSII at HL, therefore, reflects the higher population of PSII at HL. Very high PSI activity was found at HL, which we attribute to the highly organized small in size PSI.     

Influence of PAR and UV-A in determining plant sensitivity and photomorphogenic responses to UV-B radiation

The role of photosynthetically active radiation (400-700 nm) (PAR) in modifying plant sensitivity and photomorphogenic responses to ultraviolet-B (280-320 nm) (UV-B) radiation has been examined by a number of investigators, but few studies have been conducted on ultraviolet-A (320-400 nm) (UV-A), UV-B and PAR interactions. High ratios of PAR-UV-B and UV-A-UV-B have been found to be important in ameliorating UV-B damage in both terrestrial and aquatic plants. Growth chamber and greenhouse studies conducted at low PAR, low UV-A and high UV-B often show exaggerated UV-B damage. Spectral balance of PAR, UV-A and UV-B has also been shown to be important in determining plant sensitivity in field studies. In general, one observes a reduction in total biomass and plant height with decreasing PAR and increasing UV-B. The protective effects of high PAR against elevated UV-B may also be indirect, by increasing leaf thickness and the concentration of flavonoids and other phenolic compounds known to be important in UV screening. The quality of PAR is also important, with blue light, together with UV-A radiation, playing a key role in photorepair of DNA lesions. Further studies are needed to determine the interactions of UV-A, UV-B and PAR.     

Elevated ultraviolet-B radiation induces cross-protection to cold in leaves of Rhododendron under field conditions

Previously, we have shown a cold-hardening response in Rhododendron 'English Roseum' exposed to elevated ultraviolet-B radiation (UV-B, 280-320 nm) under growth chamber conditions. We have conducted the present study under field conditions to provide for a higher ratio of photosynthetically active radiation to UV-B (PAR:UV-B) than is possible in the laboratory and to more accurately reflect natural conditions of solar irradiance. Leaf disks taken after 3 months from UV-B-exposed plants exhibited a greater tolerance to freezing temperatures than those from control plants that received no supplemental UV-B exposure during this time. Leaf disks taken from UV-B-irradiated plants survived temperatures below -8 degrees C, whereas control disks were killed at -6 degrees C. Cold hardiness did not significantly increase until September, when environmental cues such as decreasing day length and night temperatures also may have enhanced hardening. Our field findings confirm our previous laboratory study, demonstrating that elevated UV-B induces cross-protection to cold in Rhododendron leaf tissues.     

Polyamines interaction with thylakoid proteins during stress

The involvement of polyamines in plant responses to abiotic stresses is well investigated, while there has been few reports on the specific mode of action of polyamines on the photosynthetic apparatus. The objective of this review is thus to examine the mode of interaction of polyamines with proteins of photosystem II core and LHCII, including methylamine (monoamine) as a simplified model to better understand the mode of action of polyamines. Spectroscopic methods used to determine the binding mode of amines with PSII proteins showed that amines such as spermine, putrescine and methylamine interact with protein (H-bonding) through polypeptide C=O, C-N and N-H groups with major perturbations of protein secondary structure as the concentration of amines was raised. High concentration of amines added to PSII-enriched submembrane fractions causes a significant loss of PSII activity. However, at lower concentration, polyamines, especially spermine, improve the photosynthetic functions under stress. We concluded from this review that besides the conjugation of polyamines with LHC polypeptides, polyamines are likely to interact with extrinsic proteins and the hydrophilic part of intrinsic proteins of PSII by electrostatic interaction. This could stabilize the conformation of proteins under various stresses. However, at high concentration of polyamines a strong inhibition of PSII activity is observed.     

Use of UV-A Energy for Photosynthesis in the Red Macroalga Gracilaria lemaneiformis

UV radiation is known to inhibit photosynthetically active radiation (PAR)-driven photosynthesis; however, moderate levels of UV-A have been shown to enhance photosynthesis and growth rates of some algae. Here, we have shown that UV-A alone could drive photosynthetic utilization of bicarbonate in the red alga Gracilaria lemaneiformis as evidenced in either O₂ evolution or carbon fixation as well as pH drift. Addition of UV-B inhibited the apparent photosynthetic efficiency, raised the photosynthetic compensation point and photosynthesis-saturating irradiance level, but did not significantly affect the maximal rate of photosynthetic O₂ evolution. The electron transport inhibitor, DCMU, inhibited the photosynthesis completely, reflecting that energy of UV-A was transferred in the same way as that of PAR. Inorganic carbon acquisition for photosynthesis under UV alone was inhibited by the inhibitors of carbonic anhydrase. The results provided the evidence that G. lemaneiformis can use UV-A efficiently to drive photosynthesis based on the utilization of bicarbonate, which could contribute significantly to the enhanced photosynthesis in the presence of UV-A observed under reduced levels of solar radiation.     

Regulation of the excitation energy utilization in the photosynthetic apparatus of chlorina f2 barley mutant grown under different irradiances

Acclimation of the photosynthetic apparatus of chlorophyll b-less barley mutant chlorina f2 to low light (100 micromolm(-2)s(-1); LL) and extremely high light level (1000 micromolm(-2)s(-1); HL) was examined using techniques of pigment analysis and chlorophyll a fluorescence measurements at room temperature and at 77 K. The absence of chlorophyll b in LL-grown chlorina f2 resulted in the reduction of functional antenna size of both photosystem II (by 67%) and photosystem I (by 21%). Chlorophyll fluorescence characteristics of the LL-grown mutant indicated no impairment of the utilization of absorbed light energy in photosystem II photochemistry. Thermal dissipation of excitation energy estimated as non-photochemical quenching of minimal fluorescence (SV(0)) was significantly higher as compared to the wild-type barley grown under LL. Despite impaired assembly of pigment-protein complexes, chlorina f2 was able to efficiently acclimate to HL. In comparison with chlorina f2 grown under LL, HL-grown chlorina f2 was characterized by unaffected maximal photochemical efficiency of photosystem II (F(V)/F(M), doubled content of both beta-carotene and the xanthophyll cycle pigments and considerably reduced efficiency of excitation energy transfer from carotenoids to chlorophyll a. The enormous xanthophyll cycle pool size was however associated with reduced SV(0) capacity. We suggest that the substantial part of the xanthophyll cycle pigments is not bound to the remaining pigment-protein complexes and acts as filter for excitation energy, thereby contributing to the efficient photoprotection of chlorina f2 grown under HL.     

The damage repair role of He-Ne laser on plants exposed to different intensities of ultraviolet-B radiation

Light-grown broad bean (Vicia faba L.) seedlings were subjected to different intensities of UV-B radiation (0, 0.05, 0.15, 0.45, 0.90, 1.45 and 1.98 W m(-2)) for 7 h under photosynthetically active radiation (70 micromol m(-2) s(-1)) and then exposed to He-Ne laser (632.8 nm, 5.43 mW mm(-2)) radiation for 5 min or red light radiation for 4 h without ambient light radiation. When He-Ne laser radiated leaves were treated using lower intensity UV-B, the activities of superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11) and catalase (EC 1.11.1.6) improved significantly. Moreover, the UV-B-injured plants treated with laser light recovered faster from UV-B treatment because the concentration of malondialdehyde and the rate of electrolyte leakage from leaf disks reached control levels (no UV-B or laser treatment) early compared with those exposed only to ambient light or in dark conditions. Laser treatment, however, had no repair effect on seedling damage induced by higher UV-B radiation (1.45 and 1.98 W m(-2)), even with higher laser flux rates and longer laser treatment. In addition, the red light treatment had no repair effect on UV-B-induced damage. Meanwhile, the long-term physiological effect of He-Ne laser treatment on UV-B damaged plants was presented and evaluated. The results showed that the laser had a long-term positive physiological effect on the growth of UV-B-damaged plants. With the exception of the severe damage caused by higher UV-B radiation, a laser with the proper flux rate and treatment time can repair UV-B-induced damage and shorten the recovery time.     

Effect of ultraviolet-B radiation on salt marsh vegetation: Trends of the genus Salicornia along the Americas

The effects of natural UV-B radiation on growth, photosynthetic and photoprotective pigment composition of different Salicornia species were analyzed in salt marshes at three different sites along the Americas (Puerto Rico, southern Brazil and Patagonia, Argentina). Plants were exposed to different levels of UV-B radiation for 1-2 years in situ as well as in outdoor garden UV-B exclusion experiments. Different UV-B levels were obtained by covering plants with UV-B opaque (blocked 93-100% of ambient UV-B) and UV-B attenuating (near-ambient) filters (reduced 20-25% of UV-B). Unfiltered plants were exposed to natural irradiance. UV-B filters had significant effects on temperature and photosynthetic pigments (due to changes in PAR; 400-700 nm). The growth of Salicornia species was inhibited after 35 to 88 days of exposure to mean UV-B radiation dosages between 3.6 and 4.1 kJ m(-2) day(-1). The highest number of branches on the main shoot (S. bigelovii and S. gaudichaudiana) and longest total length of the branches (S. gaudichaudiana) were observed in the UV-B opaque treatment. Salicornia species responded to increasing levels of UV-B radiation by increasing the amount of UV-B absorbing pigments up to 330%. Chromatographic analyses of seedlings and adult S. bigelovii plants found seven different UV-B absorbing flavonoids that are likely to serve as UV-B filtering pigments. No evidence of differential sensitivity or resilience to UV-B radiation was found between Salicornia species from low-mid latitudes and a previously published study of a high-latitude population.     

Physiological responses of Acropora cervicornis to increased solar irradiance

The effects of increased UV radiation (UV-B [280-320 nm] + UV-A [320-400 nm]; hereafter UVR) on the growth, production of photosynthetic pigments and photoprotective mycosporine-like amino acids (MAAs) were studied in the threatened Caribbean coral Acropora cervicornis transplanted from 20 to 1 m depth in La Parguera, Puerto Rico. The UVR exposure by the transplanted colonies was significantly higher than that at 20 m, while photosynthetically active radiation (PAR) only increased by 9%. Photosynthetic pigments, quantified with HPLC, as well as linear extension rates and skeletal densities, were significantly reduced 1 month after transplantation to 1 m depth, while MAAs increased significantly despite immediate paling experienced by transplanted colonies. While these colonies showed a significant reduction in photosynthetic pigments, there were no significant reductions in zooxanthellae densities suggesting photoacclimation of the coral's symbionts to the new radiation conditions. The results suggest that while corals might be able to survive sudden increases in UVR and PAR, their skeletal structure can be greatly debilitated due to a reduction in the photosynthetic capacity of their symbionts and a possible relocation of resources.     

Effect of Enhanced UV-B Radiation and Low-Energy N+ Ion Beam Radiation on the Response of Photosynthesis, Antioxidant Enzymes, and Lipid Peroxidation in Rice (Oryza sativa) Seedlings

To understand the effect of enhanced UV-B radiation and low-energy N⁺ ion beam radiation on the response of photosynthesis, antioxidant enzymes, and lipid peroxidation in rice seedlings, Oryza sativa was exposed to three different doses of low-energy N⁺ ion beam and enhanced UV-B alone and in combination. Enhanced UV-B caused a marked decline in some photosynthetic parameters (net photosynthetic rate, transpiration rate, and stomatal conductance) and photosynthetic pigments, whereas it induced an increase in hydrogen peroxide (H₂O₂) accumulation, the rate of superoxide radical production, and the content of malondialdehyde (MDA). Enhanced UV-B also induced an increase in the activity of antioxidant enzymes (superoxide dismutase [SOD], peroxidase (POD), and catalase [CAT]) and some nonenzymatic antioxidants such as proline. Under the combined treatment of enhanced UV-B and low-energy N⁺ ion beam at the dose of 3.0?×?10¹⁷ N⁺ cm^?2, the activity of antioxidant compounds (SOD, POD, CAT, proline, and glutathione), photosynthetic pigments, and some photosynthetic parameters (net photosynthetic rate, transpiration rate, and stomatal conductance) increased significantly; however, the MDA content, H₂O₂ accumulation, and rate of superoxide radical production showed a remarkable decrease compared with the enhanced UV-B treatment alone. These results implied that the appropriate dose of low-energy N⁺ ion beam treatment may alleviate the damage caused by the enhanced UV-B radiation on rice.     

Plant responses to current solar ultraviolet-B radiation and to supplemented solar ultraviolet-B radiation simulating ozone depletion: an experimental comparison

Field experiments assessing UV-B effects on plants have been conducted using two contrasting techniques: supplementation of solar UV-B with radiation from fluorescent UV lamps and the exclusion of solar UV-B with filters. We compared these two approaches by growing lettuce and oat simultaneously under three conditions: UV-B exclusion, near-ambient UV-B (control) and UV-B supplementation (simulating a 30% ozone depletion). This permitted computation of "solar UV-B" and "supplemental UV-B" effects. Microclimate and photosynthetically active radiation were the same under the two treatments and the control. Excluding UV-B changed total UV-B radiation more than did supplementing UV-B, but the UV-B supplementation contained more "biologically effective" shortwave radiation. For oat, solar UV-B had a greater effect than supplemental UV-B on main shoot leaf area and main shoot mass, but supplemental UV-B had a greater effect on leaf and tiller number and UV-B-absorbing compounds. For lettuce, growth and stomatal density generally responded similarly to both solar UV-B and supplemented UV-B radiation, but UV-absorbing compounds responded more to supplemental UV-B, as in oat. Because of the marked spectral differences between the techniques, experiments using UV-B exclusion are most suited to assessing effects of present-day UV-B radiation, whereas UV-B supplementation experiments are most appropriate for addressing the ozone depletion issue.     

Expression of a ketolase gene mediates the synthesis of canthaxanthin in Synechococcus leading to tolerance against photoinhibition, pigment degradation and UV-B sensitivity of photosynthesis

The potential of ketocarotenoids to protect the photosynthetic apparatus from damage caused by excess light and UV-B radiation was assessed. Therefore, the cyanobacterium Synechococcus was transformed with a foreign beta-carotene ketolase gene under a strong promoter leading to the accumulation of canthaxanthin. This diketo carotenoid is absent in the original strain. Most of the newly formed canthaxanthin was located in the thylakoid membranes. The endogenous beta-carotene hydroxylase was unable to interact with the ketolase. Therefore, only traces of astaxanthin were found. The transformant was treated with strong light (500 or 1200 mumol m-2 s-1) and with UV-B radiation. In contrast to a nontransformed strain the overall photosynthesis, measured as oxygen evolution, was protected from inhibition by light of 500 mumol m-2 s-1 and UV-B radiation of 6.8 W m-2. Furthermore, degradation in the light of chlorophyll and carotenoids at an irradiance of 1200 mumol m-2 s-1, which was substantial in the nontransformed control, was prevented. These results indicate that in situ canthaxanthin, which is formed at the expense of zeaxanthin and replaces this hydroxy carotenoid within the photosynthetic apparatus, is a better protectant against solar radiation. These findings are discussed on the basis of the in vitro properties such as inactivating peroxyl radicals, quenching of singlet oxygen and oxidation stability of these different carotenoid structures.     

Quantification of polyamines in human erythrocytes using a new near-infrared cyanine 1-(epsilon-succinimidyl-hexanoate)-1'-methyl-3,3,3',3'-tetramethyl-indocarbocyanine-5,5'-disulfonate potassium with CE-LIF detection

A novel near-infrared (NIR) cyanine 1-(epsilon-succinimidyl-hexanoate)-1'-methyl-3,3,3',3'-tetramethyl-indocarbocyanine-5,5'-disulfonate potassium (MeCy5-OSu) has been developed in our laboratory. Simultaneous determination of MeCy5-OSu-derivatized polyamines spermine (Spm), spermidine (Spd), cadaverine (Cad), and putrescine (Put) based on the separation by CE combined with diode LIF detection has been accomplished. The highest derivatization efficiency was achieved in 0.2 mol/L borate buffer (pH 8.8) for 20 min at 25 degrees C. Polyamine derivatives were separated within 14 min in the phosphate running buffer (pH 3) containing 50 mmol/L phosphoric acid, 40 mmol/L SDS, and 35% methanol v/v. Linearity of response was obtained in the range of 10-200 nmol/L. The detection limits (S/N = 3) for Spm, Spd, Cad, and Put were 0.8, 1, 3, and 2 nmol/L, respectively. The proposed method has been successfully applied to the analysis of polyamines in erythrocytes of two healthy persons and one cancer patient. Average recoveries for erythrocyte samples were 93.6-106% and coefficients of variation ranged from 1.8 to 5.4%. The analysis of polyamines in erythrocytes can be used for studying the relationship between their changes and the carcinogenesis process involved in erythrocytes.     

Role of white light in reversing UV-B-mediated effects in the N2-fixing cyanobacterium Anabaena BT2

The effects of various irradiances of artificial UV-B (280-315 nm) in the presence or absence of visible light (photosynthetically active radiation) on growth, survival, 14CO2 uptake and ribulose 1,5-bisphosphate carboxylase (RuBISCO) activity were studied in the N2-fixing cyanobacterium Anabaena BT2. We tested the hypothesis whether or not visible radiation offers any protection against UV-B-induced deleterious effects on growth and photosynthesis in Anabaena BT2. Attempts were also made to determine the irradiances of UV-B where inhibitory effects could be mitigated by simultaneous irradiation with visible light. Exposure of cultures to 0.2 W m(-2) or higher irradiance of UV-B caused inhibition of growth and survival and growth ceased above 1.0 W m(-2). 14CO uptake and RuBISCO activity were found to be more sensitive to UV-B and around 60% reduction in 14CO2 uptake and RuBISCO activity occurred after exposure of cultures to 0.4 W m(-2) for 1 h. However, growth, 14CO2 uptake and RuBISCO activity were nearly normal when UV-B (0.4 W m(-2)) and visible light (14.4 W m(-2)) were given simultaneously. Blue radiation (450 nm) was found to be the most effective in photoreactivation against UV-B, better than UV-A or any other light wavelength band. Our results demonstrate that the studied cyanobacterium possesses active photoreactivation mechanism(s) against UV-B-mediated damage which in turn probably allow survival under natural conditions in spite of being continuously exposed to the UV-B component present in the solar radiation. Continued growth of many algae and cyanobacteria in the presence of intense solar UV-B radiation under natural conditions seems to be due to the active role of photoreactivation.     

Variability of UVR effects on photosynthesis of summer phytoplankton assemblages from a tropical coastal area of the South China Sea

From June to September 2005, we carried out experiments to determine the ultraviolet radiation (UVR) -induced photoinhibition of summer phytoplankton assemblages from a coastal site of the South China Sea. Variability in taxonomic composition was determined throughout the summer, with a peak chlorophyll a (chl a approximately 20 microg chl a L(-1)) dominated by the diatom Skeletonema costatum that was detected early in the study period; the rest of the time samples were characterized by monads and flagellates, with low chl a values (1-5 chl a microg L(-1)). Surface water samples were placed in quartz tubes, inoculated with radiocarbon and exposed to solar radiation for 2-3 h to determine photosynthetic rates under three quality radiation treatments (i.e. PAB, 280-700 nm; PA, 320-700 nm and P, 400-700 nm) using different filters and under seven levels of ambient irradiance using neutral density screens (P vs E curves). UVR inhibition of samples exposed to maximum irradiance (i.e. at the surface) varied from -12.2% to 50%, while the daytime-integrated UVR-related photoinhibition in surface seawater varied from -62% to 7%. The effects of UVR on the photosynthetic parameters P(B)(max) and E(k) were also variable, but UV-B accounted for most of the observed variability. During sunny days, photosynthesis of microplankton (>20 microm) and piconanoplankton (<20 microm) were significantly inhibited by UVR (mostly by UV-B). However, during cloudy days, while piconanoplankton cells were still inhibited by UVR, microplankton cells used UVR (mostly UV-A) as the source of energy for photosynthesis, resulting in higher carbon fixation in samples exposed to UVR than the ones exposed only to photosynthetically active radiation (PAR). Our results indicate that size structure and cloudiness clearly condition the overall impact of UVR on phytoplankton photosynthesis in this tropical site of South China. In addition, model predictions for this area considering only PAR for primary production might have underestimated carbon fixation due to UVR contribution.     

Relationship between the organization of the PS II super complex and the functions of the photosynthetic apparatus

The chlorophyll fluorescence and the photosynthetic oxygen evolution (flash-induced oxygen yield patterns and oxygen bursts under continuous irradiation) were investigated in the thylakoid membranes with different stoichiometry and organization of the chlorophyll-protein complexes. Data show that the alteration in the organization of the photosystem II (PS II) super complex, i.e. the amount and the organization of the light-harvesting chlorophyll a/b protein complex (LHCII), which strongly modifies the electric properties of the membranes, influences both the energy redistribution between the two photosystems and the oxygen production reaction. The decrease of surface electric parameters (charge density and dipole moments), associated with increased degree of LHCII oligomerization, correlates with the strong reduction of the energy transfer from PS II to PSI. In the studied pea thylakoid membranes (wild types Borec, Auralia and their mutants Coeruleovireus 2/16, Costata2/133, Chlorotica XV/1422) with enhanced degree of oligomerization of LHCII was observed: (i) an increase of the S(0) populations of PS II in darkness; (ii) an increase of the misses; (iii) an alteration of the decay kinetics of the oxygen bursts under continuous irradiation. There is a strict correlation between the degree of LHCII oligomerization in the investigated pea mutants and the ratio of functionally active PS II alpha to PS II beta centers, while in thylakoid membranes without oligomeric structure of LHCII (Chlorina f2 barley mutant) the PS II alpha centers are not registered.     

Laser pretreatment protects cells of broad bean from UV-B radiation damage

In order to determine the role of lasers in the stress resistance of broad bean (Vicia faba L.) to ultraviolet-B (UV-B) radiation, the embryos in seeds were exposed to He-Ne laser or CO2 laser radiation. Afterwards they were cultivated in Petri dishes in a constant temperature incubator until the lengths of epicotyls were nearly 3 cm. The epicotyls were then exposed to 1.02, 3.03 or 4.52 kJ m(-2) UV-B radiation, respectively, under 70 micromol m(-2) s(-1) photosynthetically active radiation (PAR) in a growth cabinet. Changes in the concentration of malondialdehyde (MDA), ascorbic acid (AsA) and UV-B absorbing compounds (absorbance at 300 nm) were measured to test the effects of laser pretreatment. The results showed that laser pretreatment of embryos enhanced UV-B stress resistance in the epicotyls of the broad bean by decreasing the MDA concentration and increasing the content of AsA and UV-B absorbing compounds. We suggest that those changes in MDA, AsA and UV-B absorbing compounds were responsible for the increase in stress resistance observed in the broad bean. This is the first investigation reporting the use of laser pretreatment to protect the cells of the broad bean from UV-B-induced damage.