Effects of spring and summer levels of UV-B radiation on the growth and reproduction of a temperate perennial forb

Narrow-leaved plantain (Plantago lanceolata L.), a perennial forb, flowers for virtually the full length of the growing season in temperate latitudes and as a result it is exposed to widely variable intensities of shortwave (UV-B) radiation. In order to determine effects of spring and summer levels of UV-B exposure on growth and development, representatives of 42 maternal families were grown for 85 days at 3.2 and 6.4 kJ m^–2 day^–1 BE₃₀₀, levels corresponding to early spring and mid-summer in central Illinois. Impacts on early vegetative stages were most pronounced; early vegetative growth was decreased by higher levels of UV-B and both leaf angle (a measure of erectness) and leaf hair density were increased. At harvest, vegetative growth was significantly affected by higher levels of UV-B as well; the mass of senescent leaves and crown tissue were both decreased. Although exposure to higher levels of UV-B decreased inflorescence number by nearly 15%, it did not significantly alter reproductive biomass. Significant variation attributable to maternal families was present in nearly all measurements and the range of variation among families was wider than among UV-B treatments. A marginally significant (p=0.07) maternal family by UV-B interaction was found for the number of inflorescences, suggesting that, within populations of this plant, some small amount of genetic variation exists to allow for differential reproductive performance under a regime simulating spring and summer differences in UV-B exposure. For the most part, however, in this cosmopolitan species the level of adaptation to natural levels of variation in UV-B radiation does not differ dramatically among maternal families.     

Short-term UV-B radiation and ozone exposure effects on aromatic secondary metabolite accumulation and shoot growth of flavonoid-deficient Arabidopsis mutants

The presence of UV-absorptive substances in the epidermal cells of leaves is thought to protect mesophyll tissues from the harmful effects of UV-B radiation. We examined the influence of short-term UV-B exposures on UV-absorptive (330 nm) sinapates and flavonols, and on shoot growth of the Arabidopsis wild type ecotype Landsberg erecta and two mutants. 114 deficient in chalcone synthase, and 115 , deficient in chalcone/flavonone isomerase. Sequential ozone exposures were used to determine the effects of oxidative stress The levels of sinapates and flavonols on a leaf fresh weight basis increased substantially in the wild type and sinapates increased in the 114 mutant in vegetative vegetative/reproductive transitional and reproductive stage plants in response to short-term (48h) UV-B radiation. When UV-B was discontinued the levels generally decreased lo pre-exposure levels after 48 h in vegetative/reproductive but not in reproductive plants. Exposure to ozone before or alter UV-B treatment did not consistently affect the levels of these UV-absorptive compounds. Dry matter accumulation was less affected by UV-B at the vegetative and reproductive stages than at the vegetative/reproductive stage. At the vegetative/reproductive stage, shoot growth of all 3 genotypes was retarded by UV-B. Growth was not retarded by short-term ozone exposure alone but when exposure to ozone followed UV-B exposure, growth was reduced in all genotypes. Leaf cupping appeared on 115 plants exposed to UV-B.     

Differential stimulation of an arid-environment winter ephemeral Dimorphotheca pluvialis (L.) Moench by ultraviolet-B radiation under nutrient limitation

A South African winter ephemeral D. pluvialis was exposed, under low and high nutrient conditions, to four different daily doses of biologically effective UV-B radiation. These simulated different depletions (range 0–30%) in the ozone layer at the southerly distribution limit (33° 56′S) of this species, and included daily UV-B doses received at the northerly distribution limit (26° 38′S) without ozone depletion. Growth inhibition by increased UV-B radiation was observed during early vegetative stages, but only under low nutrient conditions. Thereafter, net CO₂ assimilation rate, growth and reproduction were stimulated by an increase in UV-B radiation, though doses above those approximating a 20% ozone depletion appeared to be inhibitory. Differential stimulation occurred in the two nutrient treatments. Under high nutrient conditions, photosynthesis (specifically carboxylation efficiency), and numbers of leaves, inflorescences and diaspores per plant, and leaf areas increased, but leaf thickness decreased with increased UV-B radiation. Under low nutrient conditions, dry masses of leaves, stems, inflorescences and diaspores, and total above-ground dry masses increased with increased UV-B radiation. Foliar organic carbon and nitrogen concentrations and foliar concentrations of UV-B absorbing compounds were unaffected by increased UV-B radiation, but foliar P concentrations declined. Diaspore viability declined with increased UV-B radiation. The net effect was a 35 to 43% reduction in viable diaspore production under high nutrient conditions at UV-B doses equivalent to those currently received at the northerly distribution limit during the reproductive phase. It is concluded that anticipated increases in UV-B radiation could reduce regeneration success, and seedling survival in areas of low soil fertility, particularly at lower latitudes, and consequently increase the risk of localized population extinctions from stochastic causes.     

The response of Vicia faba to enhanced UV-B radiation under low and near ambient PAR levels

The effects of enhanced UV-B are often overestimated in greenhouse studies due to low levels of photosynthetically active radiation (PAR). For this reason, we studied effects of enhanced UV-B (12 kJ m^–2 d^–1) at low and near ambient PAR levels on young vegetative plants of Vicia faba, in the greenhouse. It was hypothesized that near ambient PAR levels could reduce the negative UV-B effects on growth, due to higher amounts of UV-B absorbing compounds in the leaves and to morphological changes attenuating UV-B damage.We found that effects of enhanced UV-B on the growth were not negative. We found an increase in biomass in response to enhanced UV-B at low and near ambient PAR levels. The increase in biomass was related to increased branching, which leads to a higher interception of PAR. Enhanced irradiance of both PAR and UV-B had similar photomorphogenic effects: thicker and smaller leaves and reduced plant height and internode length. Moreover, the concentration of UV-B absorbing compounds was increased. We conclude that in this study effects of enhanced UV-B were mainly photomorphogenic effects, which were also induced by radiation in the PAR region.     

Effects of enhanced UV-B radiation on a weedy forb (Plantago lanceolata) and its interactions with a generalist and specialist herbivore

Assessments of potential impacts of global climate change often focus exclusively on plants; however, as the base of most food webs, plants generally experience abiotic stresses concomitantly with biotic stresses. Longleaf plantain, Plantago lanceolata L., is a cosmopolitan temperate perennial weed that experiences a wide range of environmental conditions throughout its range. We examined the impacts of elevated levels of exposure to shortwave (UV-B) radiation on this plant, on two herbivores associated with this plant, and on the plant-herbivore interaction. Plantains were grown at 6 and 12 kJ m^–2 d^–1 BE₃₀₀ UV-B radiation and concentrations of iridoid glycosides (aucubin and catalpol), verbascosides, and nitrogen were measured. In terms of plant impacts, we found that iridoid glycoside concentrations were unchanged by elevated UV-B radiation, whereas, in one experiment, the concentration of verbascosides in young leaves and levels of nitrogen in old leaves increased under elevated UV-B radiation. Variation in plant chemistry due to leaf age and maternal family was greater than variation due to UV-B exposure. When caterpillars were fed excised leaves from plants grown under elevated UV-B, growth and survivorship of the specialist herbivore, Precis coenia Hbn. (Lepidoptera: Nymphalidae), were unaltered and growth of the generalist herbivore, Trichoplusia ni (Hbn.) (Lepidoptera: Noctuidae), was accelerated. When the caterpillars were reared on potted plants at high and low levels of UV-B radiation, growth and survivorship of P. coenia were unchanged while growth of T. ni was significantly depressed by elevated UV-B. Elevated UV-B altered allocation patterns of above-ground biomass in these plants; masses of crowns and reproductive tissue were reduced. UV-B levels, however, did not affect distribution of damage to foliage inflicted by either species. In two additional experiments with artificial diet, designed to test the direct effect of UV-B radiation on caterpillars, growth and survivorship of P. coenia were unaltered while survivorship of T. ni was significantly depressed when caterpillars were exposed to elevated UV-B radiation. These studies collectively demonstrate that higher trophic level impacts of UV-B-induced changes in plants depend on the identity of the herbivore and its degree of adaptation not only to variation in hostplant quality but also variation in its light environment.     

Effects of UV-B on flavonoids, ferulic acid, growth and photosynthesis in barley primary leaves

Barley (Hordeum vulgare L.) was grown with UV-B (280–320 nm) at levels simulating 25 nr 5% ozone depletion on the date of the summer solstice al 40°N latitude, with UV-A (320–400 nm), or with no supplemental irradiation. In plant growth chambers providing 300 μmol m^?2 s^?1 photosynthetically active radiation (PAR). UV-B-grown leaves elongated more slowly than controls but reached the same final length 1 day later. Leal specific fresh weight (mass leaf area^?1) was significantly increased by UV-B after the 7th day of growth. IV-B did not significantly affect leaf area, fresh weight, dry weight, total chlorophylls, total carotenoids or photosynthetic quantum efficiency. CO₂ assimilation was decreased by UV-B only at internal CO₂ levels above 250 μl l^?1. By the 8th day of growth, UV-B increased flavonoid (saponarin and lutonarin) accumulation in both the lower epidermis and the mesophyll: about 40% of the saponarin and 20% of the lutonarin were in the lower epidermis under all experimental conditions. Glasshouse conditions proved too variable for reproducible determination of growth and photosynthesis but were reliable for determining developmental changes in flavonoid (saponarin and lutonarin) accumulation and provided up to 800 μmol m^?2 s^?1 PAR. In the glasshouse UV-B-grown leaves had more flavonoids than controls al all stages from 5 to 30 days after planting: ca 509 more saponarin and 100% more lutonarin. Levels of soluble (vacuolar) ferulic acid esters were similar under all conditions on day 5. and on day 20 or later, but were significantly higher in UV-B-grown plants on days 10 and 15. UV-B decreased insoluble (cell-wall-bound) ferulic acid esters on a whole leaf basis but significantly increased this fraction in the lower epidermis. UV-A had no significant effects on growth, photosynthesis or ferulic acid, but it slightly increased flavonoid accumulation. The results are discussed in terms of secondary phenolics as a tissue-specific, developmentally regulated adaptive response to UV-B.     

Carry-over of enhanced ultraviolet-B exposure effects to successive generations of a desert annual: interaction with atmospheric CO2 and nutrient supply

The performance of fifth generation offspring of a desert annual (Dimorphotheca sinuata DC.) were compared in the absence of UV-B, under variable atmospheric CO₂ and nutrient supply, after four consecutive generations of concurrent exposure of their progenitors to UV-B at ambient (seasonal range: 2.55–8.85 kJ m^–2 d^–1) and enhanced (seasonal range: 4.70–11.41 kJ m^–2 d^–1) levels. Offspring of progenitors grown under elevated UV-B exhibited a diminished photosynthetic rate, a consequence of a reduced leaf density, and diminished foliar levels of carotenoids, polyphenolics and anthocyanins. Conversely, nonstructural carbohydrate and chlorophyll b levels were increased. Altered physiology was accompanied by reduced apical dominance and earlier flowering, features generally considered under photomorphogenic control, increased branching and inflorescence production and greater partitioning of biomass to reproductive structures, but diminished seed production. Many of these changes were magnified under nutrient limitation and intensified under atmospheric CO₂ enriched conditions. The latter disagrees with current opinion that elevated CO₂ may reduce detrimental UV-B effects, at least over the long-term. Observed correlations between seed production and polyphenolic, especially anthocyanin, levels in offspring, and indications of diminished lignification (thinner leaves, less robust stems and fewer lignified seeds set) all pointed to the involvement of the phenylpropanoid pathway in seed formation and plant structural development and its disruption during long-term UV-B exposure. Comparisons with earlier generations revealed trends with cumulative generations of enhanced UV-B exposure of increasing chlorophyll b and nonstructural carbohydrates, decreasing polyphenolics and biomass allocation to vegetative structures, and diminishing seed production despite increasing biomass allocation to reproductive structures. Notwithstanding some physiological compensation (increased chlorophyll b), the accumulation and persistence of these ostensibly inherited changes in physiological and reproductive performance suggest a greater impact of elevated UV-B on vegetation, primary production and regeneration over the long-term than presently envisaged.     

Growth enhancement of soybean (Glycine max) upon exclusion of UV-B and UV-B/A components of solar radiation: characterization of photosynthetic parameters in leaves

Exclusion of UV (280–380 nm) radiation from the solar spectrum can be an important tool to assess the impact of ambient UV radiation on plant growth and performance of crop plants. The effect of exclusion of UV-B and UV-A from solar radiation on the growth and photosynthetic components in soybean (Glycine max) leaves were investigated. Exclusion of solar UV-B and UV-B/A radiation, enhanced the fresh weight, dry weight, leaf area as well as induced a dramatic increase in plant height, which reflected a net increase in biomass. Dry weight increase per unit leaf area was quite significant upon both UV-B and UV-B/A exclusion from the solar spectrum. However, no changes in chlorophyll a and b contents were observed by exclusion of solar UV radiation but the content of carotenoids was significantly (34–46%) lowered. Analysis of chlorophyll (Chl) fluorescence transient parameters of leaf segments suggested no change in the F _v/F _m value due to UV-B or UV-B/A exclusion. Only a small reduction in photo-oxidized signal I (P700⁺)/unit Chl was noted. Interestingly the total soluble protein content per unit leaf area increased by 18% in UV-B/A and 40% in UV-B excluded samples, suggesting a unique upregulation of biosynthesis and accumulation of biomass. Solar UV radiation thus seems to primarily affect the photomorphogenic regulatory system that leads to an enhanced growth of leaves and an enhanced rate of net photosynthesis in soybean, a crop plant of economic importance. The presence of ultra-violet components in sunlight seems to arrest carbon sequestration in plants. An erratum to this article can be found at     

The influence of enhanced UV-B radiation on the spring geophyte Pulmonaria officinalis

Pulmonaria officinalis is an understorey spring geophyte, which starts its vegetative period before full foliation of the tree storey. During its early growth phase it is exposed to full solar radiation, therefore the enhanced UV-B radiation could present a threat to this species. An outdoor experiment in which potted plants were exposed to below ambient, ambient, and above ambient (corresponding to 17% ozone reduction) UV-B radiation, was conducted in order to evaluate the radiation effects. The amount of photosynthetic pigments and photochemical efficiency of PSII were not affected, but the amount of UV-B absorbing compounds was lower in plants grown under reduced UV-B. This change was measurable after only fourteen days in reproductive shoots, while in the vegetative shoots, it was not detectable until after three months. The leaves of P. officinalis are variegated and the light green spots became less transparent to PAR under enhanced UV-B. The results reveal that under simulated 17% ozone depletion the harmful effects of UV-B on the measured parameters were negligible.     

Effects of enhanced UV-B radiation on the hormonal content of vegetative and reproductive tissues of two tomato cultivars and their relationships with reproductive characteristics

The effects of enhanced UV-B radiation on hormone changes in vegetative and reproductive tissues of tomato (Lycopersicon esculentum Mill.) and their relationships with reproductive characteristics were studied. Two cultivars, TongHui (TH) and XiaGuang (XG), were grown in the field for one growing season under ambient (Control), ambient plus 2.54 kJ m^–2 d^–1 (T1) or ambient plus 4.25 kJ m^–2 d^–1 (T2) of supplemental ultraviolet-B (280–320 nm). The number of open flowers increased significantly in the TH cultivar under T2 while it declined in the XG cultivar under T1. Although pollen germination from both cultivars was inhibited by UV-B treatment, fruit number was enhanced in the TH cultivar at both UV-B doses and in the XG cultivar at the low dose. On the other hand, seed size (dry weight) was reduced in the XG cultivar by both UV-B doses and in the TH cultivar at the low UV-B dose. The final germination rates of seeds from control and UV-B treated plants of both cultivars showed no significant differences (p > 0.05), while germination was delayed in the TH cultivar at both doses of UV-B and in the XG cultivar only for T2. To determine the mechanism of UV-B's effects on developmental processes, hormone concentrations in leaves, pistils and seeds were analyzed using ELISA on partially purified extracts. The results suggested that enhanced UV-B radiation induced hormone changes in both vegetative and reproductive tissues. The alteration of flower number may be associated with the changes of ZR in leaves under enhanced UV-B radiation and the delayed germination may due to the changes in seed ABA and GAs.     

Responses of Strawberry (Fragaria × ananassa) to Supplemental UV-B Radiation and Selenium Under Field Conditions

In greenhouse experiments, selenium (Se) has been shown to defend plants against detrimental effects of heavy UV-B radiation stress. The aim of this study was to investigate whether this positive effect can be found in open-field conditions with enhancement of UV-B radiation. In the experiment, conducted with strawberry (Fragaria×ananassa, cultivars “Jonsok” and “Polka”) over two growing seasons, plants were exposed to UV-B radiation (including UV-A) and cultivated without Se or supplied with Se added at two levels (0.1 and 1.0 mg kg⁻¹). The plants were monitored for growth, flavonoids, chlorophyll fluorescence, net photosynthesis as well as tissue and cell structure. Photosystem II was observed to be sensitive to UV-B stress under field conditions. In the leaves, a decrease in F_v/F_m was seen at the end of the growing season, implying a cumulative effect of UV-B stress. Several parameters, especially cell and tissue structures, were affected by UV-B and UV-A treatments, which proves the need for UV-A control in outdoor UV-B supplementation studies. Addition of Se did not ameliorate the harmful effects of UV-B but the lower Se-increment level increased leaf growth. The effects of UV-B and Se differed during the two experimental years, indicating the need to repeat experiments during several growing seasons.     

Enhanced UV-B radiation alleviates the adverse effects of summer drought in two Mediterranean pines under field conditions

The effects of enhanced UV-B (290-320 nm) radiation on two native Mediterranean pines (Pinus pinea L., Pinus halepensis Mill.) were recorded during a one-year field study. Plants received ambient or ambient plus supplemental UV-B radiation (simulating a 15% stratospheric ozone depletion over Patras. Greece, 38.3°N. 29.1°E) and only natural precipitation, i.e. they were simultaneously exposed to other natural stresses. particularly water stress during summer. Supplemental UV-B irradiation started in early February, 1993 and up to late June, no effects were observed on growth and photochemical efficiency of photosystem II, as measured by chlorophy II fluorescence induction. Water stress during the summer was manifested in the control plants as a decline in the ratio of variable to maximum fluorescence (F_v/F_m), the apparent photon yield for oxygen evolution (φ_l) and the photosynthetic capacity at 5% CO₂ (P_m). In addition, a partial needle loss was evident. Under supplemental UV-B radiation, however, the decreases in F_v/F_m, φ_i, and P_m. as well as needle losses were significantly less. Soon after the first heavy autumn rains. photosynthetic parameters in both control and UV-B treated plants recovered to similar values. but the transient summer superiority of UV-B irradiated plants resulted in a significant increase in their dry weight measured at plant harvest. during late January. 1994. Plant height. UV-B absorbing compounds, photosynthetic pigments and relative water content measured at late spring. late summer and at plant harvest, were not significantly affected by supplemental UV-B radiation. The results indicate that enhanced UV-B radiation may be beneficial for Mediterranean pines through a partial alleviation of the adverse effects of summer drought.     

The influence of ultraviolet-B radiation on the growth, pigment production and chlorophyll fluorescence of Norway spruce seedlings

Norway spruce (Picea abies (L.)Karst.) from seven seed sources was grown in a greenhouse with 8.3 and 14.7 kJ·m⁻²·d⁻¹ m UV-B_BE (biologically effective UV-B: 280–320 nm) irradiation, and with no supplemental irradiation as control. The seedlings total biomass (dry weight) and shoot growth decreased with high UV-B treatment but spruce from low elevation seed sources were more affected. The seedlings grown at the highest UV-B irradiance (14.7 kJ·m⁻²·d⁻¹) showed from 5 to 38% inhibition of total biomass and 15 to 70 % shoot growth inhibition. Norway spruce populations from higher altitude seed sources manifested greater tolerance to UV-B radiation compared to plants from low altitudes. Changes in phospholipids and protective pigments were also determined. The plants grown at the lower UV-B irradiance (8.3 kJ·m⁻²·d⁻¹) showed greater ability to concentrations UV-B-absorbing pigments then control plants. Chlorophyll a fluorescence parameter R_fd, (R_fd=(F_m-F_s)/F_s) showed a significant decrease in needles of UV-B treated plants and this correlated with the altitude of seed source. Exposure to UV-B affect levels of the ratio of variable to maximum fluorescence (F_v/F_m). Results from this study suggest that the response to increased levels of UV-B radiation is depended upon the ecotypic differentiation of Norway spruce and involved changes in metabolites in plant tissues.     

Effect of Enhanced UV-B Radiation on Polyamine Content and Membrane Permeability in Cucumber Leaves

Cucumber plants (Cucumis sativus L., cv. Jingchun 3) were grown in a greenhouse under PAR illumination of 400–600 mol/(m² s) at 30/15°C (day/night) temperature. Two enhanced biologically effective UV-B radiation levels per day were applied: 8.82 kJ/m² (T₁) and 12.6 kJ/m² (T₂). Cucumber seedlings were irradiated 7 h per day for 25 days under T₁ and T₂. A comparative study of growth, membrane permeability, and polyamine content in cucumber leaves under T₁ and T₂ treatments was conducted. UV-B radiation resulted in the dose-dependent decrease in leaf area, dry weight of foliage, and plant height. The T₁ and T₂ treatments caused an increase in the contents of putrescine, spermine, and spermidine. However, the total polyamine content declined slightly when electrolyte leakage increased dramatically on the 18th day of treatment, especially after T₂ treatment. It can be concluded that polyamine accumulation in the cucumber leaves is an adaptive mechanism to the stress caused by UV-B radiation.     

Nitrate Absorption and Acetylene Reduction by Soybeans during Reproductive Development

Nodulated and unnodulated soybeans (Glycine max [L.] Merr.) were grown in N-free or N-containing nutrient solutions, respectively. Starting at the initial flowering stage, and throughout reproductive growth, the NO₃- absorption capacity of roots of intact plants from both treatments was determined in short-term uptake experiments. Acetylene reduction activity was determined for nodulated plants. Nitrate absorption rate, expressed on a root dry weight basis, was greatest at early flowering for both nodulated and unnodulated plants. At 33 days after germination, the NO₃^- absorption rate of unnodulated plants was twice as great as that of nodulated plants. During the remainder of the sampling period, NO₃^- absorption rates of both nodulated and unnodulated plants decreased progressively and similarly. Maximum nodule specific activity occurred 30 days after germination, or initial flowering. However, maximum total C₂H₂ reduction activity, oner plant basis, was observed during the early stages of pod-filling. Compared to unnodulated plants dependent on NO₃^- assimilation, nodulated plants were smaller, had less N in vegetative tissues, and produced less seed per plant. We suggest that the higher NO₃^- absorption rate of unnodulated soybean roots, particularly during early reproductive growth, may have reflected a more favorable supply of photosynthate translocated to the roots from larger, more vigorous, non-N-stressed shoots.     

PAR modulation of the UV-dependent levels of flavonoid metabolites in Arabidopsis thaliana (L.) Heynh. leaf rosettes: cumulative effects after a whole vegetative growth period

Long-term effects of ultraviolet (UV) radiation on flavonoid biosynthesis were investigated in Arabidopsis thaliana using the sun simulators of the Helmholtz Zentrum München. The plants, which are widely used as a model system, were grown (1) at high photosynthetically active radiation (PAR; 1,310 µmol m^?2?s^?1) and high biologically effective UV irradiation (UV-B_BE 180 mW m^?2) during a whole vegetative growth period. Under this irradiation regime, the levels of quercetin products were distinctively elevated with increasing UV-B irradiance. (2) Cultivation at high PAR (1,270 µmol m^?2?s^?1) and low UV-B (UV-B_BE 25 mW m^?2) resulted in somewhat lower levels of quercetin products compared to the high-UV-B_BE conditions, and only a slight increase with increasing UV-B irradiance was observed. On the other hand, when the plants were grown (3) at low PAR (540 µmol m^?2?s^?1) and high UV-B (UV-B_BE 180 mW m^?2), the accumulation of quercetin products strongly increased from very low levels with increasing amounts of UV-B but the accumulation of kaempferol derivatives and sinapoyl glucose was less pronounced. We conclude (4) that the accumulation of quercetin products triggered by PAR leads to a basic UV protection that is further increased by UV-B radiation. Based on our data, (5) a combined effect of PAR and different spectral sections of UV radiation is satisfactorily described by a biological weighting function, which again emphasizes the additional role of UV-A (315–400 nm) in UV action on A. thaliana.     

Influence of ultraviolet-B (UV-B) radiation on photosynthetic and growth characteristics in field-grown cassava (Manihot esculentum Crantz)

The effects of ultraviolet-B (UV-B between 290 and 320 nm) on photosynthesis and growth characteristics were investigated in field grown cassava (Manihot esculentum Crantz). Plants were grown at ambient and ambient plus a 5.5kJ m^?2 d^?1 supplementation of UV-B radiation for 95 d. The supplemental UV-B fluence used in this experiment simulated a 15% depletion in stratospheric ozone at the equator (0°N). Carbon dioxide exchange, oxygen evolution, and the ratio of variable to maximum fluorescence (F_v/F_m) were determined for fully expanded leaves after 64–76 d of UV-B exposure. AH plants were harvested after 95 d of UV-B exposure, assayed for chlorophyll and UV-B absorbing compounds, and separated into leaves, petioles, stems and roots. Exposure to UV-B radiation had no effect on in situ rates of photosynthesis or dark respiration. No difference in the concentration of UV-B absorbing compounds was observed between treatments. A 2-d daytime diurnal comparison of F_v to F_m ratios indicated a significant decline in F_v/F_m ratios and a subsequent increase in photoinhibition under enhanced UV-B radiation if temperature or PPF exceeded 35°C or 1800μmol m^?2 s^?1, respectively. However, UV-B effects on fluorescence kinetics appeared to be temporal since maximal photosynthetic rates as determined by oxygen evolution at saturated CO₂ and PPF remained unchanged. Although total biomass was unaltered with UV-B exposure, alterations in the growth characteristics of cassava grown with supplemental UV-B radiation are consistent with auxin destruction and reduced apical dominance. Changes in growth included an alteration of biomass partitioning with a significant increase in shoot/root ratio noted for plants receiving supplemental UV-B radiation. The increase in shoot/root ratio was due primarily to a significant decrease in root weight (–32%) with UV-B exposure. Because root production determines the harvest-able portion of cassava, UV-B radiation may still influence the yield of an important tropical agronomic species, even though photosynthesis and total dry biomass may not be directly affected.     

Studies on the effects of UV-B radiation on phytoplankton of sub-antarctic lakes and ponds

Summary Experiments were performed to determine the effects of UV-B (ultraviolet, 280–320 nm) radiation on motility and growth of phytoplankton from lakes and ponds in South Georgia. After 4 h of solar radiation and 4h artificial radiation (UV-B_BE 11.6 kJ m^-2 day^-1, UV-B lamps) the swimming velocity of Cryptomonas sp. decreased. The growth rate of Botryococcus, Lyngbya sp. and Stauraslrum sp. did not show any significant variations between the different light conditions. The UV-B component was reduced by filtering solar radiation through glass bottles und cellulose acetate. Cloudy days had only 30% of the radiation of clear days in both the PAR (photosynthetic active radiation) and UV-B regions. The ponds contained large amounts of humic substances, which are responsible for the absorbance in the UV region.     

Effect of UV-B radiation on the growth and antioxidant enzymes of Antarctic sea ice microalgae <Emphasis Type="Italic">Chlamydomonas</Emphasis> sp. ICE-L

The effect of ultraviolet-B (UV-B) radiation on Antarctic phytoplankton has become an attractive ecological issue as a result of annual springtime ozone depletion. The effects of UV-B radiation on the growth and antioxidant enzymes were investigated using Antarctic sea ice microalgae Chlamydomonas sp. ICE-L as the material in this study. The results demonstrated that UV-B radiation could notably inhibit the growth, especially at high UV-B radiation intensity (70 μW cm⁻²). Malondialdehyde and O₂ ^·− content in ICE-L increased rapidly in early days (1–3 days) exposed to UV-B radiation enhancement, then decreased rapidly. In the stress of UV-B radiation enhancement, the superoxide dismutase, peroxidase and Catalase activities of 1–4 days in ICE-L were obviously higher than those in the control, and their activities became higher at high UV-B radiation intensity (70 μW cm⁻²). These enzymes activity of 7 days would kept stable at low UV-B radiation intensity (35 μW cm⁻²), but kept high level at high UV-B radiation intensity (70 μW cm⁻²). However, the ascorbate peroxidase activity in ICE-L kept stable under the stress of UV-B radiation enhancement. The above experimental results indicated that the antioxidant enzyme system played an important role in the adaptation of Antarctic ice microalgae under the UV-B radiation change of Antarctic ecosystems.