Changes in leaf protein and pigment contents and photosynthetic activities during senescence of detached maize leaves: influence of different ultraviolet radiations

Senescence induced loss in pigments and proteins of detached maize (Zea mays L. cv. Col) leaves was significantly enhanced on the exposure of leaves to different ranges of ultraviolet (UV) radiation. Compared to UV-A (320-400 nm) and UV-B (280-320 nm), the UV-C (200-320 nm) was the most damaging for the pigments and macromolecules. A severe decline in photosystem (PS) 2 mediated photoreduction during senescence of detached leaves exposed to UV irradiation suggested a damage of the system. The PS1 mediated photoreduction of methylviologen with 2,6-dichlorophenol indophenol as electron donor was stimulated by UV-A and UV-B radiations, suggesting a reorganisation of the PS1 complex. These results were fortified by the values of fast and slow kinetics of chlorophyll (Chl) a fluorescence transients.     

Inhibition of photosynthetic activity by UV-B radiation in radish seedlings

Inhibition of primary photosynthetic reactions by UV-B radiation (280 nm-320 nm) was demonstrated in radish leaves ( Raphanus sativus cv. Saxa Treib). Detached radish cotyledons from 10-day-old seedlings were irradiated with continuous white light and increasing UV-B irradiances using cut-off filters with increasing transmission for shorter wavelengths (WG 360, WG 345, WG 320, WG 305, WG 295, WG 280). Photosynthetic activity measured in terms of chlorophyll fluorescence induction (Kautsky effect) after 2, 4, 6, 8 and 24 h irradiation decreased in a wavelength dependent way with increasing UV-B irradiance and irradiation time.
Radish seedlings grown for 10 days from the time of germination under the same UV-B irradiation conditions exhibited similar reductions of the variable fluorescence as detached cotyledons irradiated for short time periods. They additionally had lower initial fluorescence at high UV-B radiation levels, although the chlorophyll content per leaf area increased. In contrast to short term experiments, the plastoquinone and flavonoid content increased with increasing UV-B irradiance when based on leaf area.     

Changes in leaf protein and pigment contents and photosynthetic activities during senescence of detached maize leaves: influence of different ultraviolet radiations

Senescence induced loss in pigments and proteins of detached maize (Zea mays L. cv. Col) leaves was significantly enhanced on the exposure of leaves to different ranges of ultraviolet (UV) radiation. Compared to UV-A (320-400 nm) and UV-B (280-320 nm), the UV-C (200-320 nm) was the most damaging for the pigments and macromolecules. A severe decline in photosystem (PS) 2 mediated photoreduction during senescence of detached leaves exposed to UV irradiation suggested a damage of the system. The PS1 mediated photoreduction of methylviologen with 2,6-dichlorophenol indophenol as electron donor was stimulated by UV-A and UV-B radiations, suggesting a reorganisation of the PS1 complex. These results were fortified by the values of fast and slow kinetics of chlorophyll (Chl) a fluorescence transients. This revised version was published online in June 2006 with corrections to the Cover Date.     

Evaluation of the role of damage to photosystem II in the inhibition of CO2 assimilation in pea leaves on exposure to UV-B radiation

Mature pea (Pisum sativum L., cv. Meteor) leaves were exposed to two levels of UV-B radiation, with and without supplementary UV-C radiation, during 15 h photoperiods. Simultaneous measurements of CO₂ assimilation and modulated chlorophyll fluorescence parameters demonstrated that irradiation with UV-B resulted in decreases in CO₂ assimilation that are not accompanied by decreases in the maximum quantum efficiency of photosystem II (PSII) primary photochemistry. Increased exposure to UV-B resulted in a further loss of CO₂ assimilation and decreases in the maximum quantum efficiency of PSII primary photochemistry, which were accompanied by a loss of the capacity of thylakoids isolated from the leaves to bind atrazine, thus demonstrating that photodamage to PSII reaction centres had occurred. Addition of UV-C to the UV-B treatments increased markedly the rate of inhibition of photosynthesis, but the relationships between CO₂ assimilation and PSII characteristics remained the same, indicating that UV-B and UV-C inhibit leaf photosynthesis by a similar mechanism. It is concluded that PSII is not the primary target site involved in the onset of the inhibition of photosynthesis in pea leaves induced by irradiation with UV-B.     

Photomorphogenic regulation of increases in UV-absorbing pigments in cucumber (Cucumis sativus) and Arabidopsis thaliana seedlings induced by different UV-B and UV-C wavebands

Brief (1–100 min) irradiations with three different ultraviolet-B (UV-B) and ultraviolet-C (UV-C) wave bands induced increases the UV-absorbing pigments extracted from cucumber ( Cucumis sativus L.) and Arabidopsis . Spectra of methanol/1% HCl extracts from cucumber hypocotyl segments spanning 250–400 nm showed a single defined peak at 317 nm. When seedlings were irradiated with 5 kJ m⁻² UV-B radiation containing proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm; full-spectrum UV-B, FS-UVB), tissue extracts taken 24 h after irradiation showed an overall increase in absorption (91% increase at 317 nm) with a second defined peak at 263 nm. Irradiation with 1.1 kJ m⁻² UV-C (254 nm) caused similar changes. In contrast, seedlings irradiated with 5 kJ m⁻² UV-B including only wavelengths longer than 290 nm (8% of UV-B between 290 and 300 nm; long-wavelength UV-B, LW-UVB) resulted only in a general increase in absorption (80% at 317 nm). The increases in absorption were detectable as early as 3 h after irradiation with FS-UVB and UV-C, while the response to LW-UVB was first detectable at 6 h after irradiation. In extracts from whole Arabidopsis seedlings, 5 kJ m⁻² LW-UVB caused only a 20% increase in total absorption. Irradiation with 5 kJ m⁻² FS-UVB caused the appearance of a new peak at 270 nm and a concomitant increase in absorption of 72%. The induction of this new peak was observed in seedlings carrying the fah 1 mutation which disrupts the pathway for sinapate synthesis. The results are in agreement with previously published data on stem elongation indicating the existence of two response pathways within the UV-B, one operating at longer wavelengths (>300 nm) and another specifically activated by short wavelength UV-B (<300 nm and also by UV-C).     

Short term exposure with ultraviolet radiations: A strategy to improve resistance against root-infecting fungi in Luffa cylindrica (L.) Roem

Ultraviolet (UV) radiation is a component of the solar radiations that alter various physiological and biochemical processes in plants. There have been interests in UV-C and UV-B radiations because of their effects on plant physiology. In this study, we investigated the effect of short term UV irradiance on both biochemical parameters and pathogenicity of several root-infecting fungi in Luffa cylindrica. Plant seedlings were exposed once to UV-B and UV-C radiation for 0, 1, 2, 3, 4, and 5 h. After exposure, plant seedlings were transferred to a potting soil that contained natural populations of root-infecting fungi for 30 days. Initially, the plant height and weight enhanced with the increase of exposure time but then plants showed slower growth at the highest time (5 h) of exposure. Colonization of Macrophomina phaseolina, Rhizoctonia solani, and Fusarium species was reduced when plants were exposed to UV radiation at various time intervals. We also found increased levels of chlorophyll ´a`, chlorophyll ‘b’, and carotenoids in plants exposed to radiation. An increase in protein content was also recorded under UV-B and UV-C exposure. Enhanced catalase (CAT) activity was noted after maximum time exposure with UV-C irradiance. Ascorbate peroxidase (APX) activity was increased with the exposure time to UV radiation. We conclude that short time UV irradiation causes alteration in photosynthetic pigments and stress enzymes activities in L. cylindrica that play a major role in the improvement of resistance against root-infecting fungi.     

Comparative study of the action of ultraviolet-C and ultraviolet- B radiation on photosynthetic electron transport

The effect of ultraviolet-C (UV-C, mainly 254 nm radiation) and ultraviolet-B (UV-B, 290-320 nm) radiation on the photosynthetic electron transport reactions has been investigated. The rates of Hill activity mediated by ferricyanide and dichlorodimethoxy-p-benzoquinone (DCDMQ) were differently sensitive to UV-C but equally inhibited by UV-B. Replacement of water with diphenylcarbazide was ineffective in restoring the activity of dichlorophenol indophenol (DCPIP) Hill reaction in UV-B treated chloroplasts, but had significant effect in UV-C treated chloroplasts.
Photobleaching of carotenoids in the presence of carbonyl cyanide-m-chlorophenyl-hydrazone, an indicator of the photochemical reaction associated with the reaction centre of photosystem II, was suppressed and is paralleled by the changes in Hill activity only in UV-B-treated chloroplasts. Carotenoid photobleaching occurred even in UV-C treated chloroplasts showing no measurable Hill activity. UV-C and UV-B irradiation diminished variable fluorescence. With UV-B treated, but not with UV-C treated chloroplasts, an increase in the fluorescence yield was observed upon the addition of 3-(3,4-dichIorophenyl)-l,l-dimethylurea (DCMU) and/or Na dithionite.
Photosystem I activity was found to be unaffected by both UV-C and UV-B radiation at the fluences tested. Kinetics of P700 photooxidation and dark reversal in UV treated chloroplasts indicate that only the electron flow from photosystem II to photosystem I is impaired. It is concluded that while UV-B radiation inactivates specifically the photosystem II reaction centre, UV-C radiation acts at plastoquinone, the quencher Q, and the water oxidizing enzyme system.     

Growth and morphological responses to different UV wavebands in cucumber (Cucumis sativum) and other dicotyledonous seedlings

We examined the influence of short-term exposure of different UV wavebands on the fine-scale kinetics of hypocotyl growth of dim red light-grown cucumbers (Cucumis sativus L.) and other selected dicotyledonous seedlings to evaluate: (1) whether responses induced by UV-B radiation (280-320 nm) are qualitatively different from those induced by UV-A (320-400 nm) radiation, and (2) whether different wavebands within the UV-B elicit different responses. Responses to brief (30 min) irradiations with 3 different UV wavebands all included transient inhibition of elongation during irradiation followed by wavelength specific responses. Irradiations with proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm) induced inhibition of hypocotyl elongation within 20 min of onset of irradiation, while UV-B including only wavelengths longer than 290 nm (and only 8% of UV-B between 290 and 300 nm) induced inhibition of hypocotyl elongation with a lag of 1-2 h. The response to short wavelength UV-B was persistent for at least 24 h, while the response to long wavelength UV-B lasted only 2-3 h. The UV-A treatment induced reductions in elongation rates of approximately 6-9 h following exposure followed by a continued decline in rates for the following 15-18 h. Short wavelength UV-B also induced positive phototropic curvature in both cucumber and Arabidopsis seedlings, and this response was present in nph-1 mutant Arabidopsis seedlings defective in normal blue light phototropism. Reciprocity was not found for the response to short wavelength UV-B. The short wavelength and long wavelength UV-B responses differed in dose-response relationships and both short wavelength responses (phototropic curvature and elongation inhibition) increased sharply at wavelengths below 300 nm. These results indicate that different photosensory processes are involved in mediating growth and morphological responses to short wavelength UV-B (280-300 nm), long wavelength UV-B (essentially 300-320 nm) and UV-A. The existence of two separate types of hypocotyl inhibition responses to UV-B, with one that depends on the intensity of the light source, provides alternate interpretations to findings in other studies of UV-B induced photomorphogenesis and may explain inconsistencies between action spectra for inhibition of stem growth.     

Peroxidation of lipids and growth inhibition induced by UV-B irradiation

Cotyledons excised from dark-grown seedlings of cucumber (Cucumis sativus L.) were cultured in vitro under UV radiation at different wavelengths, obtained by passage of light through cut-off filters with different transmittance properties. Growth and the synthesis of chlorophyll (Chl) in cotyledons were inhibited and malondialdehyde was accumulated upon irradiation at wavelengths below 320 nm. Exogenous application of scavengers of free radicals reversed the growth inhibition induced by UV-B. Measurement of the fluorescence of Chl a suggested that electron transfer in photosystems was affected by UV-B irradiation. On the basis of these results, the involvement is postulated of active species of oxygen in damages to thylakoid membranes and the growth inhibition that are induced by UV-B irradiation.Abbreviations Chl chlorophyll - F_m maximal fluorescence (dark) - F_m maximal fluorescence (light) - F_v variable fluorescence (dark) - F_v variable fluorescence (light) - MDA malondialdehyde - O₂ Superoxide radical - PS photosystem - q_N non-photochemical quenching of fluorescence - q_P photochemical quenching of fluorescence - UV-B_BE biologically effective UV-B radiation - WL(T = 0.5) wavelength at which 50% transmittance occurs     

Effects of solar ultraviolet radiation on the potential efficiency of photosystem II in leaves of tropical plants

The effects of solar ultraviolet (UV)-B and UV-A radiation on the potential efficiency of photosystem II (PSII) in leaves of tropical plants were investigated in Panama (9°N). Shade-grown tree seedlings or detached sun leaves from the outer crown of mature trees were exposed for short periods (up to 75 min) to direct sunlight filtered through plastic or glass filters that absorbed either UV-B or UV-A+B radiation, or transmitted the complete solar spectrum. Persistent changes in potential PSII efficiency were monitored by means of the dark-adapted ratio of variable to maximum chlorophyll a fluorescence. In leaves of shade-grown tree seedlings, exposure to the complete solar spectrum resulted in a strong decrease in potential PSII efficiency, probably involving protein damage. A substantially smaller decline in the dark-adapted ratio of variable to maximum chlorophyll a fluorescence was observed when UV-B irradiation was excluded. The loss in PSII efficiency was further reduced by excluding both UV-B and UV-A light. The photoinactivation of PSII was reversible under shade conditions, but restoration of nearly full activity required at least 10 d. Repeated exposure to direct sunlight induced an increase in the pool size of xanthophyll cycle pigments and in the content of UV-absorbing vacuolar compounds. In sun leaves of mature trees, which contained high levels of UV-absorbing compounds, effects of UV-B on PSII efficiency were observed in several cases and varied with developmental age and acclimation state of the leaves. The results show that natural UV-B and UV-A radiation in the tropics may significantly contribute to photoinhibition of PSII during sun exposure in situ, particularly in shade leaves exposed to full sunlight.     

Measurement of leaf epidermal transmittance of UV radiation by chlorophyll fluorescence

In higher plants one of the important functions of the leaf epidermis is the effective screening of ultraviolet-B (280–320 nm, UV-B) radiation, due mostly to phenolic compounds. The assessment of the contribution of this function is necessary for an evaluation of the impact of increasing UV-B radiation. A method is proposed to estimate epidermal transmittance on the basis of chlorophyll fluorescence measurements. Fluorescence of chlorophyll induced by UV-A (320–400 nm, measuring beam centered at 366 nm, half band width 32 nm) or UV-B (measuring beam centered at 314 nm, half band width 18 nm) is compared to that induced by a blue-green measuring light (475 nm, half band width 140 nm). It is shown that the ratios of UV-and blue-green (BG)-induced fluorescence, F(UV-A)/F(BG) and F(UV-B)/F(BG), are relatively constant among leaf samples of various species ( Vicia faba, Spinacia oleracea, Rumex scutatus ) from which the epidermis was removed. In epidermis-free leaves no significant differences were found between adaxial and abaxial leaf sides, suggesting that leaf structure has negligible influence on the F(UV)/F(BG) ratios. On the other hand, fluorescence excitation ratios varied over a vast range when intact leaves from different species and habitats were investigated. Ratios were low in sun leaves and relatively high in shade- and greenhouse-grown leaves. By relating these results to those obtained with epidermis-free leaves, epidermal transmittances for UV-B radiation could be estimated, with values ranging between 1 and 45%. The data demonstrate a large adaptability of epidermal UV-A and UV-B transmittance in higher plants. The proposed method may prove a versatile and relatively simple tool for investigating epidermal UV transmittance complementing established methods.     

Effect of UV-B Radiation on Leaf Optical Properties Measured with Fibre Optics

Changes in the internal light microenvironment in leaves ofplants of Brassica campestris L. cv. Emma, B. carinata L., andMedicago saliva L. cv. Armour in response to exposure to UV-B(UV-B, 280–320 nm) radiation were measured using a fibreopticmicroprobe. Plants were exposed for 2 weeks either to high visiblelight or to supplemental ultraviolet-B radiation. The spectral regime (400–700 nm; PAR) was measured eithermidway through the leaf palisade or the spongy mesophyll. Afterexposure to UV-B radiation leaves of Brassica campesiris attenuatedtransmitted light more than the controls. At the same time bothforward and back scattered light increased in the palisade andspongy mesophylls. In contrast, UV-treatment of Medicago salivaleaves increased light transmission into the palisade, whilethe back scattered component showed little change. Leaves ofcariiwla showed little change in response to UV. Other responsesto UV-B radiation included increases in leaf thickness, decreasedtotal chlorophyll content, and changes in UV-B screening pigmentsand chlorophyll fluorescence induction kinetics. Brassica campestriswas most sensitive to exposure to enhanced levels of UV-B radiation,whereas leaves of B. carinata were the least sensitive. Ourdata indicate that exposure to UV-B radiation altered the lightmicroenvironment within leaves of the species different ways.These changes appeared to be caused by alterations in pigmentcontent and leaf anatomy. In turn, the altered distributionof PAR within the leaf could influence photosynthesis. Key words: Brassica campestris, Brassica carinata, fibre optics, light scattering, Medicago saliva, optical properties, ozone depletion, photosynthesis, ultraviolet radiation     

Protection by the epidermis of photosynthesis against UV-C radiation estimated by chorophyll a fluorescence

The epidermis of Argenteum mutant of Pisum sativum L. and Vicia faba L. was shown to be effective in protecting mesophyll photosynthesis from UV-C irradiation (peak 254 nm; 1.5 W m⁻²). These plants were chosen because it is easy to peel the epidermis from both sides of the leaf in Argenteum and the abaxial side in Vicia . Chlorophyll a fluorescence induction was decreased to the same extent by UV-C radiation in both leaf sides when the epidermis was removed. The fluorescence from the leaf with the epidermis inact was not affected. With irradiation of leaves with higher intensity (3.4 W m⁻²) of UV-C, the variable fluorescence was decreased by UV-C impinging on the abaxial side, but not the adaxial side in either of the plant species. A methanol extract from the mutant Pisum epidermis had a high absorbance in the UV region, and the absorbance was more than two-fold larger in adaxial epidermis than in abaxial epidermis. These results indicate that the epidermal layer, which contains substances that have high UV absorbance, protects mesophyll photosynthesis against UV radiation.     

The effects of salicylic acid on pigment contents in ultraviolet radiation stressed pepper plants

Pepper (Capsicum annuum L.) plants were sprayed with salicylic acid (SA) and treated with ultraviolet radiation UV-A (320–390 nm), UV-B (312 nm), and UV-C (254 nm) of 6.1, 5.8, and 5.7 W m⁻², respectively. UV significantly reduced contents of chlorophyll (Chl) a and b, and carotenoids (Car). SA treatment moderated Chl and Car reduction in plants treated with UV-B and UV-C. The quantity of antocyanins, flavonoids, rutin, and UV-absorbing compounds in plants that were treated with UV-B, UV-C, and SA were significantly increased. Foliar spray of SA counteracted the UV effects on pepper.     

Effects of Natural Intensities of Visible and Ultraviolet Radiation on Epidermal Ultraviolet Screening and Photosynthesis in Grape Leaves

Grape (Vitis vinifera cv Silvaner) vine plants were cultivated under shaded conditions in the absence of ultraviolet (UV) radiation in a greenhouse, and subsequently placed outdoors under three different light regimes for 7 d. Different light regimes were produced by filters transmitting natural radiation, or screening out the UV-B (280-315 nm), or screening out the UV-A (315-400 nm) and the UV-B spectral range. During exposure, synthesis of UV-screening phenolics in leaves was quantified using HPLC: All treatments increased concentrations of hydroxycinnamic acids but the rise was highest, reaching 230% of the initial value, when UV radiation was absent. In contrast, UV-B radiation specifically increased flavonoid concentrations resulting in more than a 10-fold increase. Transmittance in the UV of all extracted phenolics was lower than epidermal UV transmittance determined fluorimetrically, and the two parameters were curvilinearly related. It is suggested that curvilinearity results from different absorption properties of the homogeneously dissolved phenolics in extracts and of the non-homogeneous distribution of phenolics in the epidermis. UV-B-dependent inhibition of maximum photochemical yield of photosystem II (PSII), measured as variable fluorescence of dark-adapted leaves, recovered in parallel to the buildup of epidermal screening for UV-B radiation, suggesting that PSII is protected against UV-B damage by epidermal screening. However, UV-B inhibition of CO(2) assimilation rates was not diminished by efficient UV-B screening. We propose that protection of UV-B inactivation of PSII is observed because preceding damage is efficiently repaired while those factors determining UV-B inhibition of CO(2) assimilation recover more slowly.     

Differential Responses of Growth and Photosynthesis in Cyamopsis Tetragonoloba L. Grown under Ultraviolet-B and supplemental long-wavelength radiations

Cyamopsis tetragonoloba L. seedlings were subjected to continuous ultraviolet (UV)-B radiation for 18 h and post-irradiated with "white light" (WL) and UV-A enhanced fluorescent radiations. UV-B treatment alone reduced plant growth, pigment content, and photosynthetic activities. Supplementation of UV-A promoted the overall seedling growth and enhanced the synthesis of chlorophyll and carotenoids with a relatively high photosystem 1 activity. Post UV-B irradiation under WL failed to photoreactive the UV-B damage whereas a positive photoregulatory effect of UV-A was noticed in electron transport rates and low temperature fluorescence emission spectra.     

Role of salicylic acid in regulating ultraviolet radiation-induced oxidative stress in pepper leaves

The effect of salicylic acid (SA) counteracting the UV-A, UV-B, and UV-C-induced action on pepper (Capsicum annuum L.) plants was studied. For this purpose, the activities of antioxidant enzymes (peroxidase, polyphenol oxidase, ascorbate peroxidase, catalase, and glutathione reductase) were measured. Plants were sprayed with SA and treated with UV-A (320–390 nm), UV-B (312 nm), and UV-C (254 nm) radiation with a density of 6.1, 5.8, and 5.7 W/m². The activities of antioxidant enzymes were enhanced in leaves in response to UV-B and UV-C radiation. SA treatment moderated an increase in the activities of some antioxidant enzymes (peroxidase, ascorbate peroxidase, catalase, and glutathione reductase) in plants that were treated with UV radiation. The activity of antioxidant enzyme polyphenol oxidase in plants that were treated with UV-B, UV-C, and SA was significantly increased. The aim of the present study was to investigate the possible protective effect of SA treatment on UV-A, UV-B, and UV-C stress.     

Dose- and Wavelength-Dependent Oxidation of Crystallins by UV Light-Selective Recognition and Degradation by the 20S Proteasome

The lens of the human eye is a suitable model for age-related alterations at the molecular level. Age-related cataract formation is closely related to the accumulation of oxidatively altered proteins. In this study the influence of UV-A, UV-B, and UV-C irradiation on the proteolytic susceptibility of -, β_L-, and β_H-crystallins by the isolated 20S proteasome was investigated. The proteins were irradiated with 280, 300, and 350 nm monochromatic light. Changes of the physical properties of the crystallins were characterized by absorbance measurements at 280 nm, fluorescence spectra, and SDS-PAGE-electrophoresis. The proteolytic susceptibility of crystallins was maximal after irradiation at 280 nm and three- to fivefold lower at 300 nm. Irradiation at 350 nm was not able to initiate proteolysis, probably due to protein-aggregate formation of higher molecular weight, as shown by SDS-PAGE. The damage of crystallins by UV-C light might be a signal for its proteolytic degradation by the 20S proteasome, whereas UV-B and UV-A do not increase the proteolytic susceptibility to the same extent but promote the formation of crosslinked proteins. Therefore, irradiation with UV, which is not followed by an increase in the proteolytic susceptibility, is accompanied by the formation of crosslinked proteins. It was concluded, that also long UV-B and UV-A may be involved in age-related alterations of the human lens and cataract formation.     

Induction and Characterization of UV Resistant Calothrix braunii

A UV resistant mutant of Calothrix braunii has been isolated after repeated exposure to UV-C (254 nm) radiation. LD₅₀ for wild type against UV-B was 1.74 hand 100% lethality was achieved after 3.5 h. Whereas, UV resistant mutant showed LD₅₀ at 3.33 h and loss of complete survival after 5 h exposure. The growth rate of mutant was about 29 per cent greater than that of the wild type. The photosynthetic pigments, chlorophyll a and phycoerythrin were stimulated by 36.2 and 41.2 per cent, respectively over wild type. There were no differences in nitrogenase activity nitrate reductase activity, extracellular ammonia production and of plasmid DNA.     

Physiological and structural changes in the chloroplast of the green alga Micrasterias denticulata induced by UV-B simulation

Exposure of postmitotic growing and non-growing cells of the unicellular green alga Micrasterias denticulata to different UV-B cut-off wavelengths together with simulated sunlight in a sun simulator has revealed a marked resistence of the algae against strong irradiation. While down to a cut-off wavelength of 284 nm irradiated during the most sensitive stage of cell development chloroplast ultrastructure remains unaffected, severe changes in arrangement and structure of stroma and grana thylakoids occur only at the lowest cut-off wavelengths of 280 and 275 nm. The structural alterations end up in a more or less complete desintegration of grana and stroma thylakoids with the remaining membraneous structures appearing in negative staining thus indicating drastic changes in membrane composition. Photosynthetic activity determined by chlorophyll fluorescence (ratio of variable to maximal fluorescence) and oxygen evolution responded more sensitively to UV-B irradiation. With decreasing UV cut-off wavelengths and prolonged incubation a decrease of photochemistry of PS II occured reaching its lowest values after 60 min at 275 and 280 nm. Oxygen production was even maintained under strong UV irradiation with a cut-off wavelenght of 275 nm up to 15 min. With prolonged UV-B treatment any activity was lost. HPLC separations of pigments exhibited the appearance of break-down products (mainly derivatives of chl b and chl a) with decreasing cut-off wavelength and increasing exposure time. The xanthophyll cycle pigments seemed to be unaffected at least for an irradiation period of 60 to 90 min at low UV cut-offs. Possible mechanisms of UV stress avoidance or protection are discussed with regard to the varying altitudes of the natural habitats of the algae.