Effects of supplemental UV-B radiation on primary photosynthetic carboxylating enzymes and soluble proteins in leaves of C3 and C4 crop plants

Effects of increased UV-B radiation on activities of primary photosynthetic carboxylating enzymes and on contents of soluble proteins were studied in soybean (Glycine max [L.] Merr. cv. Bragg), pea (Pisum sativum L. cv. Little Marvel), tomato (Lycopersicon esculentum L. cv. Rutgers), and sweet corn (Zea mays L. cv. Golden Cross Bantam). The purpose was to evaluate the responses of agronomic crops to increases in solar UV-B radiation. Plants were grown and exposed under greenhouse conditions for 6 h daily to supplemental UV-B radiation which was provided by Westinghouse FS-40 fluorescent sun lamps filtered with 0.127-mm film of cellulose acetate (UV-B treated) or Mylar S (Mylar control). Three UV-B levels were tested: 1.09 (treatment T₁), 1.36 (treatment T₂), and 1.83 (treatment T₃) UV-B_seu where 1 UV-B_seu equals 16.0 mW-m² weighted by EXP-[(λ-265)/21]². These UV-B levels corresponded to 6%,21%, and 36%, respectively, of decrease in stratospheric ozone content, based on the interpolations of UV-B irradiances at a solar elevation angle of 60°. Leaves of plants of soybean, pea, and tomato exposed to UV-B radiation were generally low in RuBP carboxylase activity. On a fresh weight basis, all three UV-B radiation levels significantly reduced the enzyme activity in soybean and pea, whereas tomato plants showed significant reduction in RuBP carboxylase activity only when exposed to 1.83 and 1.36 UV-B_seu. An apparent decrease in soluble proteins was observed in leaf extracts of soybean and pea plants exposed to 1.36 and 1.83 UV-B_seu whereas higher amounts of proteins were detected in leaves of tomato plants grown under UV-B radiation. Leaves of sweet corn plants grown under Mylar control were low in PEP carboxylase activity and proteins as compared with those of control plants receiving no supplemental UV and UV-B treatment. Activities of PEP carboxylase in crode extracts from leaves of sweet corn were significantly suppressed under 1.36 and 1.83 UV-B_seu as compared with the no UV control. Some stimulation of PEP carboxylase activity was observed in corn plants exposed to 1.09 UV-B_seu.     

Photosynthesis performance in sweet almond [<Emphasis Type="Italic">Prunus dulcis</Emphasis> (Mill) D. Webb] exposed to supplemental UV-B radiation

Due to anthropogenic influences, solar UV-B irradiance at the earth’s surface is increasing. To determine the effects of enhanced UV-B radiation on photosynthetic characteristics of Prunus dulcis, two-year-old seedlings of the species were submitted to four levels of UV-B stress, namely 0 (UV-B_c), 4.42 (UV-B₁), 7.32 (UV-B₂) and 9.36 (UV-B₃) kJ m⁻² d⁻¹. Effects of UV-B stress on a range of chlorophyll (Chl) fluorescence parameters (FPs), Chl contents and photosynthetic gas-exchange parameters were investigated. UV-B stress promoted an increase in minimal fluorescence of dark-adapted state (F₀) and F₀/F_m, and a decrease in variable fluorescence (F_v, F_v/F_m, F_v/F₀ and F₀/F_m) due to its adverse effects on photosystem II (PSII) activity. No significant change was observed for maximal fluorescence of dark-adapted state (F_m). Enhanced UV-B radiation caused a significant inhibition of net photosynthetic rate (P _N) at UV-B₂ and UV-B₃ levels and this was accompanied by a reduction in stomatal conductance (g _s) and transpiration rate (E). The contents of Chl a, b, and total Chl content (a+b) were also significantly reduced at increased UV-B stress. In general, adverse UV-B effects became significant at the highest tested radiation dose 9.36 kJ m⁻² d⁻¹. The most sensitive indicators for UV-B stress were F_v/F₀, Chl a content and P _N. Significant P<0.05 alteration in these parameters was found indicating the drastic effect of UV-B radiation on P. dulcis.     

Consequences of depletion of stratospheric ozone for terrestrial Antarctic ecosystems: the response of Deschampsia antarctica to enhanced UV-B radiation in a controlled environment

Mini UV lamps were installed over antarctic plants at Léonie Island, Antarctic peninsula, and shoot length measurements of Deschampsia antarctica were performed during the austral summer January–February 1999.We studied the response of the antarctic hairgrass, Deschampsia antarctica to enhanced UV-B. In a climate room experiment we exposed tillers of Deschampsia antarctica, collected at Léonie Island, Antarctic peninsula, to ambient and enhanced levels of UV-B radiation. In this climate room experiment with 0, 2.5 and 5 kJ m^–2 day^–1 UV-B_BE treatments we observed that length growth of shoots at 2.5 and 5 kJ m^–2 day^–1 UV-B_BE was markedly reduced compared to 0 kJ m^–2 day^–1 UV-B_BE. In addition, there was an increased number of shoots and increased leaf thickness with enhanced UV-B. The Relative Growth Rate (RGR) was not affected by UV-B, possibly because reduced shoot length growth by enhanced UV-B was compensated by increased tillering. Light response curves of net leaf photosynthesis of plants exposed to 5 kJ m^–2 day^–1 UV-B_BE did not differ from those exposed to 0 kJ m^–2 day^–1 UV-B_BE. The content of UV-B absorbing compounds of plants exposed to increasing UV-B did not significantly change.Mini UV-B lamp systems were installed in the field, to expose the terrestrial antarctic vegetation at Léonie Island to enhanced solar UV-B. In that study, the increment of shoot length of tagged plants of Deschampsia antarctica during the January-February 1999 at Léonie Island, was recorded and compared to shoot length growth under controlled conditions.The consequences of enhanced UV-B radiation as a result of ozone depletion for the terrestrial antarctic ecosytems are discussed.     

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

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

Response of oxidative stress defense systems in rice (Oryza sativa) leaves with supplemental UV-B radiation

The impact of elevated ultraviolet-B radiation (UV-B, 280–320 nm) on membrane systems and lipid peroxidation, and possible involvement of active oxygen radicals was investigated in leaves of two UV-B susceptible rice cultivars (Oryza sativa L. cvs IR74 and Dular). Rice seedlings were grown in a greenhouse for 10 days and then treated with biologically effective UV-B (UV-B_BE) radiation for 28 days. Oxidative stress effects were evaluated by measuring superoxide anion (O₂) generation rate, hydrogen peroxide (H₂O₂) content, malondialdehyde (MDA) concentration and relative electrolyte conductivity (EC) for IR74 and Dular at 0 (control), 6 or 13 kJ m^?2 day^?1 UV-B_BE. Significant increases in these parameters were found in rice plants grown at 13 vs 0 kJ m^?2 day^?1 UV-B_BE after 28 days; indicating that disruption of membrane systems may be an eventual reason for UV-B-induced injury in rice plants. There was a positive correlation between O₂^? generation and increases in EC or MDA in leaves. Activities of enzymatic and nonenzymatic free radical scavengers were measured for IR74 after 7, 14, 21 and 28 days of exposure to 13 or 0 UV-B_BE to evaluate dynamics of these responses over time. Activities of catalase and superoxide dismutase (but not ascorbate peroxidase) and concentrations of ascorbic acid and glutathione were enhanced by 13 vs 0 UV-B_BE after 14 days of UV-B exposure. Further exposure to 28 days of UV-B was associated with a decline in enzyme activities and ascorbic acid, but not glutathione. It is suggested that UV-B-induced injury may be associated with disturbance of active oxygen metabolism through the destruction and alteration of both enzymatic and nonenzymatic defense systems in rice.     

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.     

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 supplemental ultraviolet-B on indoleacetic acid and calmodulin in the leaves of rice (Oryza sativa L.)

IR68 and Dular rice cultivars were grown under ambient, 13.0 (simulating 20% ozone depletion) and 19.1 (simulating 40% ozone depletion) kJ m^-2 day^-1 of biologically effective ultraviolet-B (UV-B_BE) for 4 weeks. Plant height and leaf area were significantly reduced by supplemental UV-B_BE radiation. Greater reduction in leaf area than of plant height was observed. A decrease in indole-3-acetic acid (IAA) content and increase in peroxidase and IAA oxidase activities of UV-B treated plants in both cultivars were observed compared with ambient control. Calmodulin content also decreased after plants were treated with high supplemental UV-B for two weeks and medium UV-B treatment for four weeks. The results indicated that peroxidase and IAA oxidase activities in rice leaves were stimulated by supplemental UV-B, resulting in the destruction of IAA which in turn may cause inhibition of rice leaf growth. Although the mechanism is unclear, calmodulin is most likely involved in leaf growth.     

Different response of two reindeer forage plants to enhanced UV-B radiation: modification of the phenolic composition

The long-term effects of enhanced UV-B radiation on the content and composition of leaf phenolics in Epilobium angustifolium L. and Eriophorum russeolum Fries ex Hartman were studied in northern Finland (68°N) using two UV-B enhancement experiments, both simulating UV-B_CIE radiation and corresponding to a 20% loss of ozone layer. High proportions of hydrolyzable tannins (69%) and condensed tannins (66%) characterized both Epilobium and Eriophorum leaves, respectively. No UV treatment effect was detected in the content or composition of Epilobium leaf soluble phenolics, whereas significant UV effects were detected in Eriophorum leaves in a developmental-specific manner. At the end of the growing season, the proportion of total soluble phenolics was higher in leaves exposed to enhanced UV-A and UV-B radiation than in the control leaves, but the phenolic composition was not significantly modified. This study introduces a new example on plants’ phenolic response to UV radiation being species-specific and detectable only at certain developmental stages. Possible consequences of increased phenolic content in forage plants for selection and digestibility by reindeer are, however, not yet known.     

Growth and reproduction of Antarctic vascular plants in response to warming and UV radiation reductions in the field

Along the west coast of the Antarctic Peninsula springtime ozone depletion events can lead to a two-fold increase in biologically effective UV-B radiation (UV-B_BE) and summer air temperatures have risen ≈1.5°C during the past 50 years. We manipulated levels of UV radiation and temperature around Colobanthus quitensis (a cushion-forming plant, Caryophyllaceae) and Deschampsia antarctica (a tussock grass) along the Peninsula near Palmer Station for two field seasons. Ambient levels of UV were manipulated by placing filters that either transmitted UV (filter control), absorbed UV-B (reducing diurnal levels of UV-B_BE by about 82%), or absorbed both UV-B and UV-A (reducing UV-B_BE and UV-A_BE by about 88 and 78%, respectively) on frames over naturally growing plants from November to March. Half the filters of each material completely surrounded the frames and raised diurnal and diel air temperatures around plants by an average of 2.3°C and 1.3°C, respectively. Reducing UV or warming had no effect on leaf concentrations of soluble UV-B absorbing compounds, UV-B absorbing surface waxes or chlorophylls. Warming had few effects on growth of either species over the first season. However, over the second field season warming improved growth of C. quitensis, leading to a 50% increase in leaf production (P < 0.10), a 26% increase in shoot production, and a 6% increase in foliar cover. In contrast, warming reduced growth of D. antarctica, leading to a 20% decline in leaf length, a 17% decline in leaf production (P < 0.10), and a 5% decline in foliar cover. Warming improved sexual reproduction in both species, primarily through faster development of reproductive structures and greater production of heavier seeds. Over the second field season, the percentage of reproductive structures that had reached the most developed (seed) stage in C. quitensis and D. antarctica was 20% and 15% higher, respectively, under warming. Capsules of C. quitensis produced 45% more seeds under warming and these seeds were 11% heavier. Growth of D. antarctica was improved when UV was reduced and these effects appeared to be cumulative over field seasons. Over the second season, tillers produced 55% more leaves and these leaves were 32% longer when UV-B was reduced. Tillers produced 137% more leaves that were 67% longer when both UV-B and UV-A were reduced. The effects of UV reduction were not as pronounced on C. quitensis, although over the second season cushions tended to be 17% larger and produce 21% more branches when UV-B was reduced, and tended to be 27% larger and produce 38% more branches when both UV-B and UV-A were reduced (P < 0.10). Few interactions were found between UV reduction and warming, although in the absence of warming, reducing UV led to slower development of reproductive structures in both species. The effects of warming and UV reduction were species specific and were often cumulative over the two field seasons, emphasizing the importance of long-term field manipulations in predicting the impacts of climate change. Received: 4 August 1998 / Accepted: 1 December 1998     

Changes in leaf expansion and epidermal screening effectiveness in Liquidambar styraciflua and Pinus taeda in response to UV-B radiation

Absorption or screening of ultraviolet-B (UV-B) radiation by the epidermis may be an important protective method by which plants avoid damage upon exposure to potentially harmful UV-B radiation. In the present study we examined the relationships among epidermal screening effectiveness, concentration of UV-absorbing compounds, epidermal anatomy and growth responses in seedlings of loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styraciflua L.). Seedlings of each species were grown in a greenhouse at the University of Maryland under either no UV-B radiation or daily supplemental UV-B radiation levels of 4, 8 or 11 kJ m^?2 of biologically effective UV-B (UV-B_BE) radiation. Loblolly pine seedlings were subsequently grown in the field under either ambient or supplemental levels of UV-B radiation. At the conclusion of the growing season, measurements of epidermal UV-B screening effectiveness were made with a fiber-optic microprobe. In loblolly pine, less than 0.5% of incident UV-B radiation was transmitted through the epidermis of fascicle needles and about 1% was transmitted in primary needles. In contrast, epidermal transmittance in sweetgum ranged from about 20% in leaves not preconditioned to UV-B exposure, to about 10% in leaves grown under UV-B radiation. The concentration of UV-absorbing compounds was unaffected by UV-B exposure, but generally increased with leaf age. Increases in epidermal thickness were observed in response to UV-B treatment in loblolly pine, and this accounted for over half of the variability in UV-B screening effectiveness. In spite of the low levels of UV-B penetration into the mesophyll, delays in leaf development (both species) and final needle size (loblolly pine) were observed. Seedling biomass was reduced by supplemental UV-B radiation in loblolly pine. We hypothesize that the UV-induced growth reductions were manifested by changes in either epidermal anatomy or epidermal secondary chemistry that might negatively impact cell elongation.     

Genetics of resistance to Fusarium oxysporum f.sp. cucumerinum races 1 and 2 in cucumber line Wisconsin-2757

The mode of inheritance of resistance to Fusarium oxysporum f.sp. cucumerinum races 1 and 2 in Wisconsin-2757 (WI-2757), a gynoecious cucumber (Cucumis sativus L.), was determined by analysing segregation of F₁, F₂ and BC₁ populations of crosses with susceptible cultivar Straight-8. Resistance to either race 1 or race 2 in WI-2757 was conferred by a single dominant gene. In allelism tests, resistance to either race in WI-2757 was determined by the gene Fcu-1, which also confers resistance in line SMR-18.     

Current and elevated levels of UV-B radiation have few impacts on yields of perennial forage crops

Two experiments assessed the effect of current and elevated levels of ultraviolet_B (UV-B) radiation on forage crop production. The effect of current levels of UV-B radiation was assessed by comparing the growth of eight cultivars of four legume and four grass species for three growing seasons (1994–96) under simulated sward conditions in the field. An exclusion system using cellulose diacetate or polyester covers provided comparable growing conditions for the plants, except for the presence or absence of ambient UV-B radiation, respectively. The second experiment studied the effect of elevated levels of UV-B on eight cultivars of two legume and two grass species in the greenhouse under simulated sward conditions. Natural lighting with sufficient supplemental light was used to provide ideal growing conditions for a 16-h day length. Separate sets of UV lights were installed to provide UV-B levels at approximately the same intensity as would be found in mid summer, and 33% and 66% more than this value. A fourth treatment consisted of removing UV-B radiation by using a polyester filter. Plant production was measured in both experiments. In 1994, field herbage yields from all young grass and legume seedlings were not significantly affected by the exclusion of ambient UV-B radiation, with the exception of alfalfa. Intra-specific variations with alfalfa yields were found for reduced levels of UV-B radiation. In general, these trends persisted as stands matured during two post seeding years. In the second experiment, no significant differences were observed for all tested species with increasing levels of UV-B radiation, except with some alfalfa cultivars and one birdsfoot trefoil cultivar. Collectively, these results demonstrate that in the northern latitudes young and mature plants of the studied species are resistant to current and potentially higher levels of solar UV-B radiation, with the exception of some alfalfa cultivars. The yield of these cultivars increased under enhanced levels of UV-B radiation in the greenhouse and decreased when UV-B was excluded in the field.     

Intraspecific variation in sensitivity to ultraviolet-B radiation in endogenous hormones and photosynthetic characteristics of 10 wheat cultivars grown under field conditions

Field studies were conducted to determine the potential of altering endogenous hormones and photosynthetic characteristics and intraspecific variation in sensitivity of 10 wheat (Triticum aestivum) cultivars (four tolerant, two middle sensitive and four sensitive) to enhanced ultraviolet-B (UV-B, 280–315 nm) radiation under field conditions. The supplemental UV-B radiation was 5.00 kJ m⁻², simulating a depletion of 20% stratospheric ozone. Responses were cultivar-specific. Out of the 10 tested wheat cultivars, six showed significant decrease in IAA content. UV-B radiation significantly increased ZR content in two wheat cultivars and significantly decreased in five cultivars. ABA content of three wheat cultivars was increased significantly, while that of five cultivars was decreased significantly. UV-B radiation significantly increased the stomatal conductance of three cultivars, and significantly decreased that of four cultivars. Intercellular CO₂ concentrations were significantly increased in five cultivars and significantly decreased in one cultivar (Mianyang 20). Transpiration rate of three cultivars significantly increased, while that of three cultivars significantly decreased. UV-B radiation significantly decreased the net photosynthetic rate of six cultivars. Intraspecific differences were found for the different measured parameters. For seven measured parameters, UV-B radiation had significant effects on five wheat cultivars, while no effect on the others. Significant correlations were observed between net photosynthetic rate and stomatal conductance, intercellular CO₂ concentrations and transpiration rate in eight cultivars. UV-B radiation might change stomatal conductance, intercellular CO₂ concentrations and transpiration rate, thus resulting in changes in net photosynthetic rate.     

UV-B response of cucumber seedlings grown under metal halide and high pressure sodium/deluxe lamps

UV-B-sensitive (Poinsett) and -insensitive (Ashley) cultivars of cucumber ( Cucumis sativus L.) were grown in growth chambers at 600 μmol m⁻²s⁻¹ of photosynthetically active radiation provided by metal halide (MH) or high pressure sodium/deluxe (HPS/DX) lamps. Plants were irradiated 15 days from seeding for 6 h per day under 18. 2 kJ m⁻² day⁻¹ of biologically effective UV-B (UV-B_BE) radiation. One of the most pronounced effects of UV-B was a 27 to 78% increase in phenylalanine ammonialyase (PAL) activity. UV-B also increased total polyamines. Catalase and superoxide dismutase varied greatly in their response to UV-B. There were no interactive effects on PAL or catalase activity, or total polyamines. There was a UV × PAR source interaction for superoxide dismutase activity. UV-B increased chlorosis and decreased height, dry weight and leaf area. Stem elongation, biomass production, leaf enlargement and chlorosis were greater under HPS/DX lamps than under MH lamps. Chlorosis was greater in Poinsett than in Ashley and in lower leaves than in upper ones. Aside from chlorosis, there were no interactive effects of UV-B, PAR source or cultivar on any of the growth parameters measured, suggesting that the growth response of cucumber seedlings to UV-B is unaffected by PAR source or cultivar. Similarly, except for SOD activity, the biochemical response to UV-B was also not influenced by PAR source or cultivar.     

UV-B and PAR in a grass (Lolium perenne L.) canopy

The penetration of natural and artificial UV-B_BE (Biologically Effective UV-B, Caldwell 1971) and PAR (400–700 nm) in a grass canopy with increasing LAI was followed during 2 months. Overall, the transmission of UV-B_BE sunlight is significantly higher than of PAR sunlight. This is mainly due to the higher proportion of diffuse light in the UV-B. Under cloudy conditions no difference between UV-B_BE and PAR could be found. Sun angle and intensity of the radiation were less important in determining the penetration of light. Artificial light penetrates much more through the canopy, resulting in higher irradiation levels in the lower part of the canopy, but a lower UV-B_BE/PAR ratio (since UV-B transmittance of the leaves is lower). The UV-B_BE/PAR ratio reaching the leaves was influenced by LAI, sun angle, percent diffuse light and leaf angle. The large differences in UV-B_BE/PAR ratio per unit leaf area under natural and artificial light conditions are important in understanding the influence of UV-B on plants.     

Anatomical,physiological, and biochemical traits involved in the UV-B radiation response in highbush blueberry

The effects of a long-term simulated spring-summer UV-B daily course on some anatomical, physiological, and biochemical features were studied in new and old leaves of blueberry (Vaccinium corymbosum L.) cultivars Legacy, Brigitta, and Bluegold. The results show that under UV-B exposure, leaf thickness increased in Bluegold due to an increased intercellular cavities. By contrast, Brigitta maintained its leaf thickness. The net photosynthetic rate was not significantly affected by the UV-B radiation in any of the cultivars; however, Brigitta presented a better photosystem II performance, since this cultivar had more efficient photochemistry under the UV-B radiation. In addition, Brigitta also maintained enhanced total phenol and total anthocyanin content compared to the other cultivars. In conclusion, Brigitta was more resistant to the UV-B radiation than the other two cultivars.     

Morphological and physiological responses of nine southern U.S. rice cultivars differing in their tolerance to enhanced ultraviolet-B radiation

The impact of climatic change on crop production is a major global concern. One of the climatic factors, ultraviolet-B radiation (UV-B; 280–320 nm), which is increasing as a result of depletion of the global stratospheric ozone layer, can alter crop productivity. As the initial step in development of UV-B tolerant rice cultivars for the southern U.S., in this study we screened popular southern U.S. rice cultivars for variation in tolerance to elevated UV-B radiation with respect to morphological, phenological and physiological parameters. Plants grown in the greenhouse at the Texas AgriLife Research and Extension Center in Beaumont, Texas, U.S. were exposed to 0, 8 or 16 kJ m⁻² day⁻¹ UV-B radiation for 90 days. Our results showed differences among southern US rice cultivars in response to UV-B treatments with respect to leaf photosynthetic rate (P_n), leaf phenolic concentration, pollen germination (PG), spikelet fertility (SF), leaf number, leaf area, and yield. For most of the cultivars, plants exposed to enhanced UV-B radiation showed decreased P_n, PG, SF and yield and increased spikelet abortion and leaf phenolic concentration compared to the plants grown in a UV-B-free environment. In this study, cultivar ‘Clearfield XL729’ performed better than the other cultivars under enhanced UV-B radiation.     

The effects of UV-B radiation on loblolly pine. 3. Interaction with CO2 enhancement

Projected depletions in the stratospheric ozone layer will result in increases in solar ultraviolet-B radiation (290–320 nm) reaching the earth's surface, These increases will likely occur in concert with other environmental changes such as increases in atmospheric carbon dioxide concentrations. Currently very little information is available on the effectiveness of UV-B radiation within a CO₂-enriched atmosphere, and this is especially true for trees. Loblolly pine (Pinus taeda L.) seedlings were grown in a factorial experiment at the Duke University Phytotron with either 0, 8.8 or 13.8 kJ m⁻² of biologically effective UV-B radiation (UV-B_BE). The CO₂ concentrations used were 350 and 650 μmol mol⁻¹. Measurements of chlorophyll fluorescence were made at 5-week intervals and photosynthetic oxygen evolution and leaf pigments were measured after 22 weeks, prior to harvest. The results of this study demonstrated a clear growth response to CO₂ enrichment but neither photosynthetic capacity nor quantum efficiency were altered by CO₂. The higher UV-B irradiance reduced total biomass by about 12% at both CO₂ levels but biomass partitioning was altered by the interaction of CO₂ and UV-B radiation. Dry matter was preferentially allocated to shoot components by UV-B radiation at 350 μmol mol⁻¹ CO₂ and towards root components at 650 μmol mol⁻¹ CO₂. These subtle effects on biomass allocation could be important in the future to seedling establishment and competitive interactions in natural as well as agricultural communities.     

Stratospheric ozone reduction and ecosystem processes: enhanced UV-B radiation affects chemical quality and decomposition of leaves of the dune grassland species Calamagrostis epigeios

This study reports changes in the plant's chemical composition and the decomposition of this plant material under enhanced solar UV-B radiation. Calamagrostis epigeios, a dominant grass species in the dune grassland in The Netherlands, was grown outdoor on an experimental field under ambient and enhanced solar UV-B (5 and 7.5 kJ m^-2 day^-1 UV-B_BE, respectively), corresponding to about 15% stratospheric ozone depletion. After one growing season aerial plant parts were harvested. The decomposition of this harvested leaf material was studied in a dune grassland and on the above mentioned experimental field under ambient (5 kJ m^-2 day^-1 UV-B_BE) and enhanced (7.5 kJ m^-2 day^-1 UV-B_BE) radiation, using litter bags. The chemical quality of the leaves grown under enhanced solar UV-B changed. There was an increase in the leaf content of lignin, while no significant changes occurred for the content of -cellulose, hemicellulose and tannins under enhanced UV-B. In the field, the rate of decomposition of leaf material grown under enhanced UV-B (with an increased content of lignin) was reduced. The content of lignin of the decomposing leaf material increased, but less under exposure to enhanced UV-B. The latter may be explained by photodegradation of the lignin. The consequences of enhanced UV-B radiation for carbon fluxes in the dune grassland ecosystem are discussed.