AMELIORATION OF UV-B DAMAGE UNDER HIGH IRRADIANCE. II: ROLE OF BLUE LIGHT PHOTORECEPTORS

Abstract Sensitivity of plants to UV-B radiation (280–315 nm) is often reduced at high background irradiance. Interpretation of plant responses to potential increases in solar UV-B requires improved understanding of interactions between UV-B and other environmental parameters. In this study, photosynthetically active radiation (PAR, 400–700 nm) was kept approximately constant (38 mol m-² per day) while the daily blue light fluence (BL, 400–500 nm) was varied between 0.23 and 2.68 mol m-². Two lines of cucumber (cvs Ashley and Poinsett) with differential sensitivity to UV-B were compared. At low BL, 3 days of UV-B treatment (21 kJ m-² biologically effective radiation per 10 h per day) caused severe inhibition of growth in a developing leaf in both cultivars. Growth effects were detectable sooner and were accompanied by chlorotic lesions in the sensitive cultivar (cv Poinsett). Supplemental BL progressively reduced symptoms, consistent with an important role for BL photoreceptor(s) in prevention or repair of UV-B damage. Ultraviolet-induced increases in UV-absorbing compounds on an area basis were significant within 24 h of the start of the treatment but were independent of BL fluence over the range tested, suggesting that bulk accumulation of screening pigments did not contribute to BL-dependent amelioration of UV damage. However, BL did stimulate net increases in extractable UV-absorbing compounds on a total leaf busis, while high-performance liquid chromatography analysis indicated that BL and UV-B acted synergistically to increase specific components. Thus, the data do not necessarily exclude UV-absorbing compounds from an important role in overall UV-B protection nor do they rule out some more specific function for these compounds (e.g. antioxidants). Finally, BL effects on UV-B alteration of leaf growth and accumulation of UV-absorbing compounds were not saturated under the conditions used here, suggesting that BL may contribute to interactions between UV-B and natural levels of background irradiance. Caution is urged in the interpretation of data on UV-B effects obtained under conditions of low BL irradiance.     

INFLUENCE OF PHOTON FLUX DENSITY IN THE 400–700 nm WAVEBAND ON INHIBITION OF PHOTOSYNTHESIS BY UV-B (280–320 nm) IRRADIATION IN SOYBEAN LEAVES: SEPARATION OF INDIRECT AND IMMEDIATE EFFECTS

Abstract— Visible radiation can substantially influence the degree to which plant photosynthesis is inhibited by UV-B radiation. This study was designed to separate the immediate effects of visible radiation on UV-B photosynthetic inhibition from the indirect influence of visible irradiation on morphological and physiological properties of leaves during leaf development. Soybean plants were pretreated in growth chambers with either high or low visible irradiance (750 and 70 μmol m^-2s^-1 quantum flux in the 400–700 nm waveband, respectively) during the development of leaves used subsequently for UV irradiation. Test leaves still attached to the plant were exposed to 5 h of polychromatic UV-B irradiation and the photosynthetic capacity (net CO₂ exchange) was determined before and after the UV irradiation. During the UV irradiation, plants from both pretreatment groups received either high or low visible flux. Development of leaves in the high visible flux pretreatment conditions resulted in thicker leaves, higher chlorophyll a/b ratios, more UV-absorbing pigments, and reduced sensitivity to the UV-B irradiation. However, higher visible flux during the UV-B irradiation resulted in greater depression of photosynthesis by the UV-B irradiation. The relative magnitude of photosynthetic depression under these treatment combinations was the same when photosynthesis was measured under either light-limited or light-saturated conditions.     

EFFECTIVENESS OF UV-B RADIATION ON THE GROWTH AND PHYSIOLOGY OF FIELD-GROWN SOYBEAN MODIFIED BY WATER STRESS

Abstract— Soybeans [ Glycine max (L) Merr. cv Essex] were grown in field plots during May-October 1985 under ambient and an enhanced level of ultraviolet-B (UV-B) radiation (supplemental daily dose: 5.1 effective kJ m^-2). They were either subjected to water stress or supplementally irrigated, resulting in a 2.0 MPa lower soil water potential in stressed plots. Increased levels of UV-B radiation reduced leaf area, total plant dry weight and net photosynthesis under well-watered conditions, but no significant UV-B effects were detected in plants concurrently subjected to water stress. The insensitivity of growth and net photosynthesis to UV-B radiation in water-stressed plants may be related to anatomical and biochemical changes induced by water stress. These include an increase in the concentration of UV absorbing compounds in leaf tissues and leaf thickening.     

RESPONSE DIFFERENCES BETWEEN TWO SOYBEAN CULTIVARS WITH CONTRASTING UV-B RADIATION SENSITIVITIES

Abstract— Soybeans (Glycine max [L.] Men. cvs. Essex and Williams) were grown in an unshaded greenhouse under two levels of biologically effective ultraviolet-B (UV-B_BE) radiation (effective daily dose: 0 and 11.5 kJ m^-2) for 34 days. Ultraviolet-B radiation reduced leaf area and total plant mass in Essex but these parameters were unaffected in Williams. Differences in both anatomical and biochemical characteristics were found between cultivars. Some of these differences were inherently distinct between cultivars while others were variably induced by UV treatment. Specific leaf weight. an estimate of leaf thickness, was unchanged in Essex but increased in Williams with UV-B irradiation. The relative increase in concentration of UV-absorbing compounds in leaf tissues after UV-B irradiation was greater in Williams. The composition of UV-absorbing compounds in leaf tissues differed between the two cultivars but was unaffected by UV-B radiation. Although total soluble proteins and total peroxidase activity were similar between cultivars, several electrophoretically distinct peroxidase activities were detected. Therefore, the intraspecific variation in UV-B sensitivity found in soybean appears to be correlated with a suite of anatomical and biochemical differences, including leaf thickness, composition and concentration of UV-absorbing compounds in leaf tissues, and possibly differences in peroxidase activities.     

Effects of UV-B radiation on growth, photosynthesis, UV-B-absorbing compounds and NADP-malic enzyme in bean (Phaseolus vulgaris L.) grown under different nitrogen conditions.

The effects of UV-B radiation on growth, photosynthesis, UV-B-absorbing compounds and NADP-malic enzyme have been examined in different cultivars of Phaseolous vulgaris L. grown under 1 and 12 mM nitrogen. Low nitrogen nutrition reduces chlorophyll and soluble protein contents in the leaves and thus the photosynthesis rate and dry-matter accumulation. Chlorophyll, soluble protein and Rubisco contents and photosynthesis rate are not significantly altered by ambient levels of UV-B radiation (17 microW m-2, 290-320 nm, 4 h/day for one week). Comparative studies show that under high nitrogen, UV-B radiation slightly enhances leaf expansion and dry-matter accumulation in cultivar Pinto, but inhibits these parameters in Vilmorin. These results suggest that the UV-B effect on growth is mediated through leaf expansion, which is particularly sensitive to UV-B, and that Pinto is more tolerant than Vilmorin. The effect of UV-B radiation on UV-B-absorbing compounds and on NADP-malic enzyme (NADP-ME) activity is also examined. Both UV-B radiation and low-nitrogen nutrition enhance the content of UV-B-absorbing compounds, and among the three cultivars used, Pinto exhibits the highest increases and Arroz the lowest. The same trend is observed for the specific activity and content of NADP-ME. On a leaf-area basis, the amount of UV-B-absorbing compounds is highly correlated with the enzyme activity (r2 = 0.83), suggesting that NADP-ME plays a key role in biosynthesis of these compounds. Furthermore, the higher sensitivity of Vilmorin than Pinto to UV-B radiation appears to be related to the activity of NADP-ME and the capacity of the plants to accumulate UV-B-absorbing compounds.     

Responses of a marine red tide alga Skeletonema costatum (Bacillariophyceae) to long-term UV radiation exposures

UV radiation (280–400 nm) is known to affect phytoplankton in negative, neutral and positive ways depending on the species or levels of irradiation energy. However, little has been documented on how photosynthetic physiology and growth of red tide alga respond to UVR in a long-term period. We exposed the cells of the marine red tide diatom Skeletonema costatum for 6 days to simulated solar radiations with UV-A (320–400 nm) or UV-A + UV-B (295–400 nm) and examined their changes in photosynthesis and growth. Presence of UV-B continuously reduced the effective photosynthetic quantum yield of PSII, and resulted in complete growth inhibition and death of cells. When UV-B or UV-B + UV-A was screened off, the growth rate decreased initially but regained thereafter. UV-absorbing compounds and carotenoids increased in response to the exposures with UVR. However, mechanisms for photoprotection associated with the increased carotenoids or UV-absorbing compounds were not adequate under the continuous exposure to a constant level of UV-B (0.09 W m^?2, DNA-weighted). In contrast, under solar radiation screened off UV-B, the photoprotection was first accomplished by an initial increase of carotenoids and a later increase in UV-absorbing compounds. The overall response of this red tide alga to prolonged UV exposures indicates that S. costatum is a UV-B-sensitive species and increased UV-B irradiance would influence the formation of its blooms.     

POTENTIAL ERRORS IN THE USE OF CELLULOSE DIACETATE AND MYLAR FILTERS IN UV-B RADIATION STUDIES

Abstract— The increase in UV-B radiation(290–320 nm) penetrating to the earth's surface as a result of the chemical depletion of the stratospheric ozone layer is an important environmental concern. In most studies using artificial UV-B sources, the determination of enhanced UV-B radiation effects on plants relies on equivalent UV-A radiation(320–400 nm) from the experimental UV-B fluorescent lamp source, filtered with either cellulose diacetate (CA) to create UV-B treatments, or with type S Mylar or polyester (PE) to create controls (no UV-B). The spectral irradiance in the UV-A was measured in the dark below lamps at two daily UV-B irradiance levels (14.1 and 10.7 W m^-2) with CA and PE at two ages. Highly significant differences in UV-A radiation (P 0.01) were measured below the treatment/control pairs at both fluence rates and filter ages. Filter aging was observed, which reduced the UV-A irradiance, especially for PE. The total daily ambient UV-A irradiance was also determined in the glasshouse at three seasons: the fall equinox, summer and winter, from which the total daily UV-A (lamp + ambient) irradiances were calculated. The addition of low to moderate ambient irradiance removed the treatment/control differences in the longwave UV-A(350–400 nm); however, the treatment/contro1 differences remained in the shortwave UV-A(320–350 nm), which was restricted by the glass, and in the total UV-A. The treatment/control differences persisted in the shortwave UV-A for the higher irradiance level, even under high summer ambient light. Also, spectral ratios (UVB:UV-A and shortwave: longwave UV-A) for all treatment groups decreased as the ambient UV-A radiation increased. Therefore, a range of experimental conditions exist where PE-covered lamps do not provide adequate control for UV-A irradiance, relative to the CA treatment, for glasshouse/growth chamber experiments. Potential complications in the interpretation of plant response exist for UV-B experiments conducted under low ambient light conditions (e.g. growth chambers; glasshouse in winter) or high daily UV-B irradiances (e.g. 14 kJ m^-2) for those plant responses that are sensitive to UV-A radiation.     

The Influence of Photosynthetically Active Radiation on the Effects of Ultraviolet-B Radiation on Arabidopsis thaliana

Ultraviolet-B (UVB;280–320 nm) radiation is a small but biologically significant portion of the solar spectrum reaching the earth's surface. Research interests have been fostered because UVB has been increasing in recent years due to depletion of stratospheric ozone. Ultraviolet-B that penetrates into plant tissue may damage important cellular macromolecules. Although there has been considerable research on the effects of UVB on plants, the influence of the level of photosynthetically active radiation (PAR;400–700 nm) on effects of UVB requires further definition as a prelude to studies of UVB sensitivity and defense mechanisms. Arabidopsis thaliana wildtype ecotype Landsberg erecta (LER), which is relatively insensitive to UVB, and the relatively sensitive LER-based mutant transparent testa-5 (tt5), were grown under 100 or 250 μmol m^?2 s^?1 PAR and then exposed to O or 7 kJ m^?2 day ^?1 UVB_BE under these PAR levels. Plants exposed to UVB had reduced dry weight and leaf area and higher levels of UV-absorbing compounds in leaf tissue. The level of PAR did influence the effects of UVB, with the higher level of PAR prior to UVB exposure reducing sensitivity of LER to UVB. In contrast to other studies, higher PAR supplied simultaneously with UVB increased rather than decreased sensitivity of both genotypes to UVB. These results demonstrate the importance of controlling and comparing PAR levels when undertaking studies of UVB sensitivity, as effects of UVB on plants are influenced by the PAR levels plants are growing under prior to and during exposure to UVB.     

Monitoring of Solar Irradiance in the High Andes

Solar radiation has been measured in the high Andes near Laguna Lejia (latitude 23° 26′ 23.30" S, longitude 67° 38′ 14.29" W) at an elevation of 4715 m between December 2016 and December 2017. Irradiances were monitored in four wavelength channels: PAR (400–700 nm), UV-A (315–400 nm), UV-B (295–315 nm) and short-wavelength UV-B (295–310 nm) with a new radiometer. In addition, ambient temperatures were recorded. Record values have been found for PAR (exceeding 600 W m⁻²), UV-A (close to 95 W m⁻²), UV-B (3.13 W m⁻²) and short-wavelength UV-B (0.144 W m⁻²) during Austral spring. The winter irradiance values slightly exceeded 50% of these values. Maximal cloud effects due to multiple reflections were 45, 38, 32 and 35% higher than values under cloudless skies for PAR, UV-A, UV-B and short-wavelength UV-B, respectively. Record irradiance for this site shows a UV index reaching and exceeding 20, which is due to low solar zenith angles, the altitude, low water vapor and aerosol concentrations in the atmosphere as well as low total column ozone concentrations.     

RAPID STOMATAL RESPONSE TO RED LIGHT IN Zea mays

Gas exchange techniques were employed to study responses of stomatal conductance to pulses of red and blue light in the grass, Zea mays. Zea mays exhibited conductance increases following brief (< 1 min) pulses of either red or blue light, in contrast to other species (e.g. Commelina communis; Assmann, 1988, Plant Physiol. 87 , 226–231) that exhibit consistent conductance responses only to pulses of blue light. Red light pulses of 450 μmol m^?2s^?1 for x min or 225 μmol m^?2s^?1 for 2x min were used to probe the fluence dependence of the red light response. The red light-stimulated conductance increase was constant for a given fluence, and increased with increasing total fluence. The conductance response to red light was larger in field grown plants (maximum growth irradiance ? 1600 μmol m-²s^?l) than in plants raised in growth chambers (maximum growth irradiance ? 150 μmol m^?2s^?1).     

PLANT PROTECTIVE RESPONSE TO ENHANCED UV-B RADIATION UNDER FIELD CONDITIONS: LEAF OPTICAL PROPERTIES and PHOTOSYNTHESIS

Abstract— Plants of Vicia faba were grown in the field during early to midsummer while receiving two levels of supplemental UV-B radiation. Light-saturated photosynthesis and stomatal diffusive conductance of intact leaves did not show any indications of UV-radiation damage. Supplemental UV treatment did cause increased concentrations of UV-absorbing flavonoid pigments in leaf tissues and decreased epidermal transmittance of UV radiation. While epidermal transmittance was reduced 30% under the low-UV treatment, the high-UV treatment failed to elicit as large a change. However, total amounts of flavonoids in other leaf tissues did increase in response to the high-UV treatment (up to 12% greater per unit leaf area than for control plants). This may have been a major factor protecting underlying photosynthetic tissues.     

Alteration of foliar flavonoid chemistry induced by enhanced UV-B radiation in field-grown Pinus ponderosa, Quercus rubra and Pseudotsuga menziesii

Chromatographic analyses of foliage from several tree species illustrate the species-specific effects of UV-B radiation on both quantity and composition of foliar flavonoids. Pinus ponderosa, Quercus rubra and Pseudotsuga menziesii were field-grown under modulated ambient (1x) and enhanced (2x) biologically effective UV-B radiation. Foliage was harvested seasonally over a 3-year period, extracted, purified and the flavonoid fraction applied to a mu Bondapak/C(18) column HPLC system sampling at 254 nm. Total flavonoid concentrations in Quercus rubra foliage were more than twice (leaf area basis) that of the other species; Pseudotsuga menziesii foliage had intermediate levels and P. ponderosa had the lowest concentrations of total flavonoids. No statistically significant UV-B radiation-induced effects were found in total foliar flavonoid concentrations for any species; however, concentrations of specific compounds within each species exhibited significant treatment effects. Higher (but statistically insignificant) levels of flavonoids were induced by UV-B irradiation in 1- and 2-year-old P. ponderosa foliage. Total flavonoid concentrations in 2-year-old needles increased by 50% (1x ambient UV-B radiation) or 70% (2x ambient UV-B radiation) from that of 1-year-old tissue. Foliar flavonoids of Q. rubra under enhanced UV-B radiation tended to shift from early-eluting compounds to less polar flavonoids eluting later. There were no clear patterns of UV-B radiation effects on 1-year-old P. menziesii foliage. However, 2-year-old tissue had slightly higher foliar flavonoids under the 2x UV-B radiation treatment compared to ambient levels. Results suggest that enhanced UV-B radiation will alter foliar flavonoid composition and concentrations in forest tree species, which could impact tissue protection, and ultimately, competition, herbivory or litter decomposition.     

Photosynthetic,Biochemical and Secondary Metabolite Changes in a Medicinal Plant Chlorophytum borivillianum (Safed musli) against Low and High Doses of UV-B Radiation

Plants are inevitably grown in presence of sunlight, therefore bound to be exposed to natural UV-B radiation. Several studies have already been conducted with UV-B and medicinal plants and only few studies showed dose dependent variation. The present study aims to find out the variations and adaptation in Chlorophytum borivillianum under two different doses of UV-B radiation; ambient + low (3.2 kJm⁻² d⁻¹) and high (7.2 kJm⁻² d⁻¹) UV-B dose, denoted as LD and HD, respectively. Reduction in photosynthetic rate was higher at HD, while plants receiving LD displayed nonsignificant variation. During vegetative and reproductive stage, significant reduction (P ≤ 0.001) in stomatal conductance was obtained when exposed to HD-eUV-B. F_v/F_m showed more reductions in HD-eUV-B (12.6%) followed by LD-eUV-B (7.9%). Low and high doses of UV-B enhanced the anthocyanin content but the increase was significant in HD, indicates epidermal protection strategy by the plants. Under LD-eUV-B, the content of saponin, a major phytochemical constituent was enhanced by 26%. Phytochemical analysis of roots revealed reduction mostly in fatty acid components whereas the steroidal components (stigmasterol and sarsasapogenin) showed enhancement in response to LD. The study suggests the importance of LD-eUV-B in the stimulation of medicinal compounds in C. borivillianum.     

ULTRAVIOLET QUANTUM YIELDS OF PHOTOPHOSPHORYLATION IN Halobacterium halobium*

Quantum yields of photophosphorylation in Halobacterium halobium were determined for ultraviolet spectral bands between 276 and 365 nm, and at 565 nm wavelength, based on integral spectral cell absorptance, bacteriorhodopsin-specific cell absorptance and the corresponding quantum dose rates. In the ultraviolet, there is an almost linear decline of the quantum yields of photophosphorylation from 365 to 276 nm wavelength, despite the peak absorption of bacteriorhodopsin at 280 nm. The cycling quantum yield for 276 nm excitation of bacteriorhodopsin was determined as 4.5 ± 1.8%, which is about one fourth of the value of 19% for solubilized bacteriorhodopsin. Threshold energy fluence rates of 20 W m^?2 for UV-B radiation typify the photophosphorylation as three orders less sensitive than the sensory UV-B avoidance response that needs 0.02 W m^?2 as the threshold. Thus, UV-B avoidance appears as the dominating strategy for survival of the archaic bacterium H. halobium, rather than possible photoenergetic use of UV-B radiation and photorepair of UV-damage.     

UV-B-INDUCED INCREASE IN SPECIFIC LEAF WEIGHT OF CUCUMBER AS A CONSEQUENCE OF INCREASED STARCH CONTENT

Abstract Specific leaf weight (SLW), the ratio of leaf dry matter to area, often increases in plants exposed to elevated UV-B radiation (280–315 nm). Increased SLW can result from greater leaf thickness or increased leaf density ( e.g . accumulation of high density substances in cells). The basis for large increases in SLW was examined in the first and third leaves of cucumber differing in developmental stage at the start of UV treatment. Leaf 1 was approximately 50% fully expanded, while leaf 3 had just unfolded. It is shown here that up to 80% of the UV-generated change in SLW in leaf 1 was caused by accumulation of nonstructural carbohydrates, especially starch (increasing from 13 to 23% of total dry weight). Leaf 3 contained a much smaller proportion of nonstructural carbohydrates (less than 8%) and the effect on SLW was correspondingly less. As shown in the previous paper, UV-B inhibition of growth in leaf 3 was reversed by supplemental blue light (BL) in a fluence-dependent manner between 0.23 to 2.68 mol m ² perday. Fluence-response curves revealed that supplemental BL reversed both the UV-induced accumulation of starch and increase in SLW in leaf 1 over the same range. The data are consistent with a back-up of photosynthate into leaf 1 as a result of UV-B inhibition of growth in leaf 3. The data also demonstrate that increases in SLW cannot be assumed to represent increases in leaf thickness.     

Responses in the morphology,physiology and biochemistry of Taxus chinensis var. mairei grown under supplementary UV-B radiation

The effects of supplemental UV-B radiation on Taxus chinensis var. mairei were studied. Leaf traits, gas exchange parameters and the concentrations of photosynthetic pigments, cellular defense system products, secondary metabolites and ultrastructure were determined. UV-B radiation significantly decreased leaf area (p < 0.05). Leaf number, secondary branch number, leaf weight per plant and leaf moisture all increased dramatically (p < 0.05). Neither the leaf weight nor the specific leaf weight (SLW) exhibited significant differences between ambient and enhanced UV-B radiation. Gas exchange parameters were all dramatically reduced by enhanced UV-B radiation (p < 0.05). The contents of chlorophyll and the chlorophyll a/b ratio were not distinctly affected by UV-B radiation, while carotenoids content significantly decreased (p < 0.05). Supplemental UV-B treatment induced significant flavonoid accumulation (p < 0.05), which was able to protect plant from radiation damage. Meanwhile, the appendage content, abaxial stomatal density, papilla density and particulate matter content in substomatic chambers increased noticeably by supplemental UV-B radiation, whereas the aperture size of single stomata was diminished. The number and area of plastoglobuli were apparently reduced by UV-B radiation, but stroma and grana lamellae were not destroyed. Our results demonstrated that T. chinensis var. mairei can activate several defense mechanisms against oxidative stress injury caused by supplemental UV-B radiation.     

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.     

PROTECTION OF THE D1 PHOTOSYSTEM II REACTION CENTER PROTEIN FROM DEGRADATION IN ULTRAVIOLET RADIATION FOLLOWING ADAPTATION OF Brassica napus L. TO GROWTH IN ULTRAVIOLET-B

As depletion of the stratospheric ozone layer continues, the biosphere will most likely be exposed to higher levels of ultraviolet-B (UV-B) irradiation (290–320nm). For plants, damage from UV-B can occur at several molecular targets with the photosynthetic apparatus being especially vulnerable. We are interested both in the mechanisms of UV-B-induced damage and identifying adaptation processes that can confer protection from UV-B. Toward this end, Brassica napus (oil seed rape) plants grown under visible light plus a low level of UV-B radiation (adapted plants) were compared to plants grown under visible light alone (control plants). Relative to the control plants, the adapted plants showed little evidence of damage at the levels of morphology or photosynthesis, indicating that B. napus has some tolerance of UV-B and that the plants may have protection mechanisms. Consistent with this, a strong UV-B adaptation process was observed in the plants-accumulation of flavonoids in the epidermis. These pigments seemed to screen a molecular target in the mesophyll. Namely, the D1 photosystem II reaction center protein, which is rapidly degraded in UV-B, was partially protected from degradation in UV-B in the adapted plants. Moreover, the extent that the half-life of the D1 protein increased in the adapted plants was on par with the elevation in total flavonoid concentrations. These experiments demonstrate that degradation of the D1 protein can be used as an in vivo assay of penetration of UV-B photons to the mesophyll.     

Seasonal Impacts of Solar UV Radiation on Photosynthesis of Phytoplankton Assemblages in the Coastal Waters of the South China Sea

We carried out experiments to evaluate seasonal changes in the impacts of UV radiation (UVR, 280–400 nm) on photosynthetic carbon fixation of phytoplankton assemblages. Surface water samples were obtained in the coastal area of the South China Sea, where chlorophyll a ranged 0.72–3.82 μg L⁻¹. Assimilation numbers (photosynthetic carbon fixation rate per chl a) were significantly higher during summer 2005 than those in spring and winter 2004. The mean values obtained under photosynthetically active radiation (PAR) were 2.83 (spring 2004), 4.35 (winter 2004) and 7.29 μg C (μg chl a)⁻¹ h⁻¹ (summer 2005), respectively. The assimilation numbers under PAR + UVR were 1.58, 2.71 and 5.28 μg C (μg chl a)⁻¹ h⁻¹, for spring, winter and summer, respectively. UVR induced less inhibition of photosynthesis during summer 2005 than during the other seasons, in spite of the higher UVR during summer. The seasonal differences in the productivity and photosynthetic response to UV were mainly due to changes in water temperature, while irradiance and vertical mixing explained >80% of the observed variability. Our data suggest that previous studies in the SCS using UV-opaque vessels might have overestimated the phytoplankton production by about 80% in spring, 61% in winter and 38% in summer.     

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.