UV-B light induces an adaptive response to UV-C exposure via photoreactivation activity in Euglena gracilis.

Phytoplankton such as Euglena are constantly exposed to solar light which is used for photosynthesis. Although the solar ultraviolet (UV) induces DNA damage such as cyclobutane-pyrimidine dimers (CPDs), many kinds of living organisms can repair CPDs by photoreactivation (PR) utilizing the near-UV/blue light component in sunlight. Euglena cells are known to possess such PR activity. In the present paper, the formation of CPDs induced by UV-C exposure and the photoreactivation PR repair of these CPDs by UV-A are demonstrated. To clarify the adaptive responses prior UV-B irradiation on PR activity, cells were cultured in the dark or under UV-B light. When the cells were cultured in the dark for 3 d prior to UV-C exposure, PR activity decreased. When the cells were cultured under UV-B light, however, PR activity increased. These results suggest that exposing the cells to UV-B prior to exposure to UV-C induced an adaptive response towards DNA damage caused by UV-C exposure, and this UV-C induced damage was repaired through PR activity.     

The effect of UV-B and UV-C radiation on Hibiscus leaves determined by ultraweak luminescence and fluorescence induction

The effects of UV-C (254 nm) and UV-B (280-320 nm) on chlorophyll fluorescence induction and ultraweak luminescence (UL) in detached leaves of Hibiscus rosa-sinensis L. were investigated. UL from leaves exposed to UV-B and UV-C radiation reached a maximum 72 h after irradiation. In both cases most of the light was of a wavelength over 600 nm. An increase in the percentage of long wavelength light with time was detected. UV radiation increased peroxidase activity, which also reached a maximum 72 h after irradiation. UV-B and UV-C both reduced variable chlorophyll fluorescence. No effect on the amount of chlorophyll or UV screening pigments was observed with the short-term irradiation used in this investigation.     

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.     

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).     

Action spectra for killing and mutation of Chinese hamster cells exposed to mid- and near-ultraviolet monochromatic light

We have examined the response of Chinese hamster V79 cells to monochromatic light of selected wavelengths in the mid- to near-UV region, using cell survival and induction of mutants resistant to 6-thioguanine (6-TG) or ouabain (OUA) as end points. As the wavelength increased from 313 to 405 nm, the induction of mutants resistant to 6-TG and to OUA decreased to a greater degree than did cell survival. Cells resistant to OUA were induced with considerably lesser efficiency at wavelengths of 313 and 334 nm than cells resistant to 6-TG. No mutants resistant to either 6-TG or OUA were induced by 405-nm light, and no mutants resistant to OUA were induced by 365-nm light. Thus, cell killing and mutation induction have different action spectra, and furthermore, action spectra for mutation induction at the HGPRT and Na+/K+-ATPase loci are different from each other. These observations imply important differences in the cellular mechanisms, and/or lesions, for cell inactivation, induction of 6-TG and OUA resistance for V79 cells exposed to near-UV monochromatic light.     

Repair of UV-B induced damage of Photosystem II via de novo synthesis of the D1 and D2 reaction centre subunits in Synechocystis sp. PCC 6803

The repair of ultraviolet-B radiation induced damage to the structure and function of Photosystem II was studied in the cyanobacterium Synechocystis sp. PCC 6803. UV-B irradiation of intact Synechocystis cells results in the loss of steady-state oxygen evolution, an effect accompanied by a parallel loss of both D1 and D2 protein subunits of the Photosystem II reaction centre. Transfer of the UV-irradiated cells to normal growth conditions under visible light results in partial recovery of the inhibited oxygen evolving activity and restoration of the lost D1 and D2 proteins. The extent of recovery decreases with increasing degree of damage: after 50% inhibition, the original activity is completely restored within 2 hours. In contrast, after 90–95% inhibition less than half of the original activity is regained during a 4 hour recovery period. The translation inhibitor lincomycin completely blocks the recovery process if added after the UV-B treatment, and accelerates the kinetics of activity loss if added before the onset of UV-B irradiation. Substantial retardation of recovery and acceleration of activity loss is also observed if the very low intensity short wavelength contribution (<290 nm) is not filtered out from the UV-B light source. It is concluded that in intact cells UV-B induced damage of the Photosystem II complex can be repaired. This process is the first example of simultaneous D1 and D2 protein repair in Photosystem II, and considered to function as an important defence mechanism against detrimental UV-B effects in oxygenic photosynthetic organisms. De novo synthesis of the D1 and D2 reaction centre subunits is a key step of the repair process, which itself can also be inhibited by ultraviolet light, especially by the short wavelength UV-C components, or by high doses of UV-B.     

CARBON UPTAKE IN A MARINE DIATOM DURING ACUTE EXPOSURE TO ULTRAVIOLET B RADIATION: RELATIVE IMPORTANCE OF DAMAGE AND REPAIR1

Experiments on a marine diatom, Thalassiosira pseudonana (Hustedt) clone 3H, demonstrate that under moderate photon flux densities (75 μmol quanta·m^?2·s^?1) of visible light the inhibition of photosynthesis by supplemental ultraviolet (UV) radiation (UV-B: 280–320 nm) is well described as a hyperbolic function of UV-B irradiance for time scales of 0.5–4 h. Results are consistent with predictions of a recently developed model of photosynthesis under the influence of UV and visible irradiance. Although net destruction of chlorophyll occurs during a 4-h exposure to UV-B, and the effect is a function of exposure, the principal effect of UV-B is a decrease in chlorophyll-specific photosynthetic rate. The dependence of photoinhibition on dosage rate, rather than cumulative dose, and the hyperbolic shape of the relationship are consistent with net photoinhibition being an equilibrium between damage and repair. The ratio of damage to repair is estimated by a mathematical analysis of the inhibition of photosynthesis during exposures to UV-B. A nitrate-limited culture was much more sensitive to UV-B than were the nutrient-replete cultures, but the kinetics of photoinhibition were similar. The analysis suggests that the nutrient-limited culture was more sensitive than the nutrient-replete cultures because repair or turnover of critical proteins associated with photosynthesis is inhibited. An inhibitor of chloroplast protein synthesis was used to suppress repair processes. Photoinhibition by UV-B was enhanced, and inhibition was a function of cumulative dose, as would be expected if damage were not countered by repair. The fundamental importance of repair processes should be considered in the design of field experiments and models of UV-B effects in the environment, especially in the context of vertical mixing. Repair processes must also be considered whenever biological weighting functions are developed.     

Photomorphogenetic responses to UV radiation and short-term red light in lettuce seedlings

Effects of red light (R), far-red light (FR) and UV radiation on growth and greening of lettuce seedlings (Latuca sativa L., cv. Berlinskii) have been investigated. UV-B and UV-C inhibited hypocotyl elongation and stimulated cotyledonary growth. R in combination with UV-B and UV-C partly eliminated these effects, but FR increased those and reversed the R effect. Chlorophyll accumulation was inhibited by UV-B and UV-C. In comparison with cotyledonary growth, R strengthened the UV inhibitory effect, and FR reversed this effect of R. Thus, UV and phytochrome system modify the effects of each other on hypocotyl and leaf growth in lettuce seedlings depending on the level of active phytochrome formed.     

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.     

The two major spore DNA repair pathways, nucleotide excision repair and spore photoproduct lyase, are sufficient for the resistance of Bacillus subtilis spores to artificial UV-C and UV-B but not to solar radiation.

Bacterial endospores are 1 to 2 orders of magnitude more resistant to 254-nm UV (UV-C) radiation than are exponentially growing cells of the same strain. This high UV resistance is due to two related phenomena: (i) DNA of dormant spores irradiated with 254-nm UV accumulates mainly a unique thymine dimer called the spore photoproduct (SP), and (ii) SP is corrected during spore germination by two major DNA repair pathways, nucleotide excision repair (NER) and an SP-specific enzyme called SP lyase. To date, it has been assumed that these two factors also account for resistance of bacterial spores to solar UV in the environment, despite the fact that sunlight at the Earth's surface consists of UV-B, UV-A, visible, and infrared wavelengths of approximately 290 nm and longer. To test this assumption, isogenic strains of Bacillus subtilis lacking either the NER or SP lyase DNA repair pathway were assayed for their relative resistance to radiation at a number of UV wavelengths, including UV-C (254 nm), UV-B (290 to 320 nm), full-spectrum sunlight, and sunlight from which the UV-B portion had been removed. For purposes of direct comparison, spore UV resistance levels were determined with respect to a calibrated biological dosimeter consisting of a mixture of wild-type spores and spores lacking both DNA repair systems. It was observed that the relative contributions of the two pathways to spore UV resistance change depending on the UV wavelengths used in a manner suggesting that spores irradiated with light at environmentally relevant UV wavelengths may accumulate significant amounts of one or more DNA photoproducts in addition to SP. Furthermore, it was noted that upon exposure to increasing wavelengths, wild-type spores decreased in their UV resistance from 33-fold (UV-C) to 12-fold (UV-B plus UV-A sunlight) to 6-fold (UV-A sunlight alone) more resistant than mutants lacking both DNA repair systems, suggesting that at increasing solar UV wavelengths, spores are inactivated either by DNA damage not reparable by the NER or SP lyase system, damage caused to photosensitive molecules other than DNA, or both.     

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.     

Oxidative DNA damage in cucumber cotyledons irradiated with ultraviolet light

DNA was isolated from the cotyledons of cucumber seedlings irradiated with ultraviolet (UV)-C (254 nm) or UV-B+UV-A (280–360 nm; maximum energy at 312 nm) at various fluence rates and durations. Following enzymatic hydrolysis of DNA, the content of 8-hydroxy-2-deoxyguanosine [(8-OHdG), 8-oxo-7,8-dihydro-2-deoxyguanosine], a well-established biomarker closely identified with carcinogenesis and aging in animal cells, was determined using a high-performance liquid chromatograph equipped with an electrochemical detector. The levels of 8-OHdG increased with UV-C and UV-B irradiation in a fluence-dependent manner. This increase was also observed in etiolated cotyledons that had been excised from dark-grown cucumber seedlings and then cultured in vitro under UV light: monochromatic UV light at 270 nm or 290 nm increased the 8-OHdG level considerably, while UV at wavelengths above 310 nm had only small effects. In situ detection of H₂O₂ and quantification of H₂O₂ in plant extracts revealed that H₂O₂ accumulated in cotyledons irradiated with UV light. These results suggest that UV irradiation induces oxidative DNA damage in plant cells.     

Phenolic compounds and antioxidant properties in the snow alga Chlamydomonas nivalis after exposure to UV light

The snow alga Chlamydomonas nivalis was collected from the Sierra Nevada, California, USA, and examined for its ability to produce phenolic compounds, free proline, and provide antioxidant protection factor in response to UV-A and UV-C light. Exposure of C. nivalis cells to UV-A light (365nm) for 5 days resulted in a 5–12% increase in total phenolics, where as exposure to UV-C light (254 nm) resulted in a 12–24% increase in phenolics after 7 days of exposure. Free proline was not affected by UV-A, but increased markedly after UV-C exposure. A three-fold increase in free proline occurred within two days after exposure to UV-C, but then dropped as cells became bleached. Antioxidant protection factor (PF) increased after treatment of cells with UV-A and remained constant throughout UV-C exposure. Spectral analysis of algal extracts revealed a decrease in absorption in the 215–225 nm region, short-term (2day) stimulation of pigment at 280 nm, and an increase in carotenoids (473 nm), after exposure to UV-A. Snow alga exposed to UV-C light had a different spectrum from that of UV-A exposed cells, i.e. an enhancement of three major peaks at 220, 260, and 280 nm, and loss of absorption in the carotenoid region.We report that UV light exposure, especially in the UV-C range, can stimulate phenolic-antioxidant production in aplanospores of C. nivalis effecting biochemical pathways related to proline metabolism. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light-Induced Anthocyanin Reduces the Extent of Damage to DNA in UV-Irradiated Centaurea cyanus Cells in Culture

Using suspension cultures of Centaurea cyanus L. cells, in whichthe biosynthesis of anthocyanin is induced by illumination withUV-containing white light and in which the level of pigmentcan be controlled, we examined the sensitivity of the cellsto both UV-B and UV-C irradiation and the formation of pyrimidinedimer as a result of exposure to UV light, with a special referenceto the level of accumulation of anthocyanin pigment in the cells.The sensitivity of the cells to UV-B or UV-C decreased as theiranthocyanin content increased. Furthermore, the extent of formationof pyrimidine dimers induced by irradiation with UV-B or UV-Clight was found to be reduced in cells with accumulated anthocyanin.Both the extent of resistance to UV irradiation and the reductionin the extent of formation of dimers as a result of exposureto UV light were correlated with the levels of accumulated anthocyaninpigment in the cells. The results show that anthocyanin, a kindof flavonoid, plays a role in protecting cells from the adverseeffects of UV light. (Received January 28, 1991; Accepted May 19, 1991)     

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.     

Sudden exposure to solar UV-B radiation reduces net CO(2) uptake and photosystem I efficiency in shade-acclimated tropical tree seedlings

Tree seedlings developing in the understory of the tropical forest have to endure short periods of high-light stress when tree-fall gaps are formed, and direct solar radiation, including substantial UV light, reaches the leaves. In experiments simulating the opening of a tree-fall gap, the response of photosynthesis in leaves of shade-acclimated seedlings (Anacardium excelsum, Virola surinamensis, and Calophyllum longifolium) to exposure to direct sunlight (for 20-50 min) was investigated in Panama (9 degrees N). To assess the effects of solar UV-B radiation (280-320 nm), the sunlight was filtered through plastic films that selectively absorbed UV-B or transmitted the complete spectrum. The results document a strong inhibition of CO(2) assimilation by sun exposure. Light-limited and light-saturated rates of photosynthetic CO(2) uptake by the leaves were affected, which apparently occurred independently of a simultaneous inhibition of potential photosystem (PS) II efficiency. The ambient UV-B light substantially contributed to these effects. The photochemical capacity of PSI, measured as absorbance change at 810 nm in saturating far-red light, was not significantly affected by sun exposure of the seedlings. However, a decrease in the efficiency of P700 photooxidation by far-red light was observed, which was strongly promoted by solar UV-B radiation. The decrease in PSI efficiency may result from enhanced charge recombination in the reaction center, which might represent an incipient inactivation of PSI, but contributes to thermal dissipation of excessive light energy and thereby to photoprotection.     

Chloroquine,a lysosomotropic agent,inhibits zygote formation in yeast

The effects of solar UV-B radiation on the green flagellate, Euglena gracilis, are measured under controlled conditions. Both photoorientation and motility are drastically impaired even after short exposure times of a few hours to sunlight not filtered by an ozone cuvette. Phototactic orientation starts to deteriorate after about 90 min and is completely lost after about 5 h. The percentage of motile cells in a population decreases likewise after an exposure of about 2 h and the velocity distributions shows a reduced speed of movement after an initial photokinetic increase. The damage is irreversible: in populations exposed for >2 h no living cell was found 24 h later. The UV-B sensitivity seems to be independent of the culture age at least over three weeks: While the percentage of motile cells changes with a peak at about 8 d, the relative UV-B induced inhibition is constant and depends only on the UV dose. DNA seems not to be the primary UV-B target since UV-B inhibition could not be repaired during subsequent dark or moderate light conditions even after low doses.     

Isolation of uvh1, an Arabidopsis mutant hypersensitive to ultraviolet light and ionizing radiation.

A genetic screen for mutants of Arabidopsis that are hypersensitive to UV light was developed and used to isolate a new mutant designated uvh1. UV hypersensitivity in uvh1 was due to a single recessive trait that is probably located on chromosome 3. Although isolated as hypersensitive to an acute exposure to UV-C light, uvh1 was also hypersensitive to UV-B wavelengths, which are present in sunlight that reaches the earth's surface. UV-B damage to both wild-type and uvh1 plants could be significantly reduced by subsequent exposure of UV-irradiated plants to photoreactivating light, showing that photoreactivation of UV-B damage is important for plant viability and that uvh1 plants are not defective in photoreactivation. A new assay for DNA damage, the Dral assay, was developed and used to show that exposure of wild-type and uvh1 plants to a given dose of UV light induces the same amount of damage in chloroplast and nuclear DNA. Thus, uvh1 is not defective in a UV protective mechanism. uvh1 plants were also found to be hypersensitive to ionizing radiation. These results suggest that uvh1 is defective in a repair or tolerance mechanism that normally provides plants with resistance to several types of DNA damage.     

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.