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.     

Penetration and effects of solar UV-B on phytoplankton and macroalgae

The effects of short wavelength solar radiation on aquatic ecosystems were studied in several marine and freshwater systems. The spectral distribution and the penetration of solar radiation into different water types (coastal and oceanic waters of the Baltic Sea, North Sea, Atlantic and Mediterranean) were investigated. Penetration of short wavelength solar radiation strongly depends on the content of dissolved and particulate substances as well as the concentration of phytoplankton. The primary producers often show a typical vertical distribution within the euphotic zone and are reached as well as affected by the penetrating UV-B radiation. The effect of this radiation was both determined in phytoplankton and macroalgae. Measuring pulse amplitude modulated (PAM) fluorescence indicated that major biomass producers were severely inhibited by surface radiation and even impaired at their natural growth site. Likewise, photosynthetic oxygen production was affected by extended exposure to solar radiation.     

Effects of enhanced UV-B radiation on pea (Pisum sativum L.) grown under field conditions in the UK

A new modulated lamp system is described. This system has successfully provided an ultraviolet-B (UV-B) supplement in proportion to ambient UV-B. The modulated system was used to simulate the UV-B environment resulting from an annual mean reduction of 15% in the stratospheric ozone under UK field conditions, but taking account of seasonal variation in depletion. The effects of this enhanced level of UV-B on the growth, physiology and yield of four cultivars of pea were assessed. Enhanced UV-B resulted in small reductions in the number of stems and total stem length per plant (respectively 4.7 and 8.7%). There were also significant decreases in the dry weight of peas (10.1%), pods (10.3%) and stems (7.8%) per plant. UV-B treatment had no effect on the number of peas per pod or average pea weight, but did significantly reduce (12.1%) the number of pods per plant. This decrease in pod number was partly due to enhanced abscission of pods during the final month of plant growth. UV-B treatment had no significant effect on chlorophyll fluorescence characteristics or CO₂assimilation rate per unit leaf area. These results are consistent with previous controlled environment experiments, and suggest that reduction in yield may be due to direct effects of UV-B on plant growth rather than a decrease in photosynthetic capacity per unit leaf area.     

Enhanced UV-B radiation increases the reproductive effort in the Mediterranean shrub Cistus creticus under field conditions

Seedlings of the Mediterranean shrub Cistus creticus L. were grown in the field under ambient or ambient plus supplemental UV-B radiation (simulating a 15% ozone depletion over Patras, 38.3°W, 29.1°E) for 20 months. During this period, measurements of photosynthetic capacity, photochemical efficiency of PS II, chlorophylls and carotenoids were performed once per season. Supplemental UV-B radiation had no significant effect on these parameters nor on the total, above ground biomass accumulation, plant height and leaf specific mass measured at plant harvest. It was observed, however, that UV-B supplementation increased the number of seeds per fruit as well as mean individual seed mass. As a result, seed number and total seed mass per plant were considerably increased. Germination rates of produced seeds were not affected. We may conclude that C. creticus is a UV-B resistant plant whose competitive ability may be improved by enhanced UV-B radiation through an increase in its reproductive effort and a higher contribution to the seed bank.     

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.     

Row orientation effect on UV-B,UV-A and PAR solar irradiation components in vineyards at Tuscany,Italy

Besides playing an essential role in plant photosynthesis, solar radiation is also involved in many other important biological processes. In particular, it has been demonstrated that ultraviolet (UV) solar radiation plays a relevant role in grapevines (Vitis vinifera) in the production of certain important chemical compounds directly responsible for yield and wine quality. Moreover, the exposure to UV-B radiation (280–320 nm) can affect plant–disease interaction by influencing the behaviour of both pathogen and host. The main objective of this research was to characterise the solar radiative regime of a vineyard, in terms of photosynthetically active radiation (PAR) and UV components. In this analysis, solar spectral UV irradiance components, broadband UV (280–400 nm), spectral UV-B and UV-A (320–400 nm), the biological effective UVBE, as well as the PAR (400–700 nm) component, were all considered. The diurnal patterns of these quantities and the UV-B/PAR and UV-B/UV-A ratios were analysed to investigate the effect of row orientation of the vineyard in combination with solar azimuth and elevation angles. The distribution of PAR and UV irradiance at various heights of the vertical sides of the rows was also studied. The results showed that the highest portion of plants received higher levels of daily radiation, especially the UV-B component. Row orientation of the vines had a pronounced effect on the global PAR received by the two sides of the rows and, to a lesser extent, UV-A and UV-B. When only the diffused component was considered, this geometrical effect was greatly attenuated. UV-B/PAR and UV-A/PAR ratios were also affected, with potential consequences on physiological processes. Because of the high diffusive capacity of the UV-B radiation, the UV-B/PAR ratio was significantly lower on the plant portions exposed to full sunlight than on those in the shade.     

Growth and pigment production in two subarctic grass species under four different UV-B irradiation levels

The aim of the experiment was to test the sensitivity of grasses from a subarctic region to a range of different ultraviolet-B (UV-B, 280–320 nm) levels. Seedlings of Calama-grostis lapponica and tillers of Calamagrostis purpurea were grown for 50 days in a greenhouse at 4 different levels of UV-B_BE radiation (no UV-B, ambient UV-B, UV-B corresponding to 15 and 25% ozone depletion, respectively, at 68°N, northern Sweden on July 27th).
In C. purpurea total plant dry weight was significantly reduced at the 15% level compared to all other treatments, whilst tiller number was reduced when compared to the non UV-B treatment only. Reduced growth at the 15% level was accompanied by significant increases in shoot to root ratio (S:R), leaf area ratio (LAR) and leaf weight ratio (LWR), which were due to changes in both specific leaf area (SLA) and root growth. Similarly, in C. lapponica there were reductions in total dry weight at the 15% level, although differences in partitioning were only observed in LWR which was significantly greater than in the non UV-B treatment.
There was a difference in tiller number in C, lapponica between all treatments. A reduced number of tillers was observed with increasing UV-B exposure from zero UV-B to the 15% level, whilst tiller number was significantly higher at the 25% level than in all other treatments.
In C, lapponica , differences were apparent in the amount of UV absorbing pigments. Plants exposed to ambient UV-B and the 15% level had significantly more UV-B absorbing compounds than those in the 25% and non UV-B treatments.
All these results suggest that the 15% ozone depletion level was the most harmful to plant growth and few damaging effects were observed at the higher 25% ozone depletion level.     

The importance of the ratio UV-B/photosynthetic active radiation (PAR) during leaf development as determining factor of plant sensitivity to increased UV-B irradiance: effects on growth, gas exchange and pigmentation of bean plants (Phaseolus vulgaris cv. Label)

To evaluate the effect of different naturally occurring irradiation conditions on the sensitivity of bean (Phaseolus vulgaris cv. Label) to increased UV-B levels, plants were grown under six different light treatments. In the control series (at ambient levels of UV-B), UV-B and visible light were decreased in parallel, resulting in three different total irradiation treatments with the same UV-B/PAR ratio. A second series with a 15% increase in UV-B irradiation at each PAR level was used to investigate the effect of UV-B under the varying total irradiance levels. The different total irradiance levels resulted in large differences in total dry weight, specific leaf weight, photosynthesis-light response and pigment concentrations. Nevertheless, the 15% increase in UV-B resulted in equal reductions in total dry weight (from 24.5 to 34.3%) and effective photosynthesis for all light levels. The accumulation of protective pigments in the primary bean leaves was strongly correlated to the total irradiance level (200% increase from the lowest to the highest light level), but was not influenced by increasing UV-B levels. As the UV-B/PAR ratio outside increases with decreasing total irradiance (when induced by cloud cover) this implies that low radiation levels are potentially dangerous to some plants, even though the UV-B levels may seem negligible.     

外源NO对不同UV-B辐射天数处理的小麦叶片总蛋白的影响

研究外源NO对不同UV-B辐射天数处理的小麦叶片总蛋白的影响.采用蒸馏水和0.1 mmol/L 硝普钠（SNP,一种NO供体）浸种24 h,试验设置不同的处理组：CK、CK＋B、SNP、SNP＋B.待幼苗生长7天后取其叶片进行总蛋白的提取及SDS-PAGE凝胶图像的分析.不同处理对叶片总蛋白含量、电泳图谱及电泳条带数目的影响不同;外源NO能显著增加不同UV-B处理天数下小麦叶片总蛋白的含量,且SNP和SNP＋B处理组的电泳条带均较CK和CK＋B处理组的电泳条带变宽且颜色变深.外源NO能显著增加UV-B辐射下小麦叶片的总蛋白含量.同时,不同处理组电泳条带数目的变化反应出一些蛋白的降解和合成,最终也体现了不同处理组蛋白含量的变化.     

Solar UV-B and visible radiation in tropical forest gaps: measurements partitioning direct and diffuse radiation

Solar ultraviolet-B (UV-B) radiation penetrates plant canopies to a different degree than photosynthetically active radiation (PAR) because UV-B is diffused to a greater degree by the atmosphere. We measured both global (total) and diffuse solar radiation in canopy gaps of a semideciduous tropical forest in Panama. Measurements were simultaneously made in the UV-B and PAR wavebands. Compared to unobstructed measurements taken outside the forest, the sunlit portions of gaps were depleted in the proportion of UV-B relative to PAR, especially at midday. Shaded areas, in contrast, were always richer in UV-B relative to PAR, but the magnitude of the change varied greatly. Presumably this variation was due to the differences in the directional nature of diffuse solar UV-B radiation as compared to diffuse PAR. Measurements in the gaps showed substantial reductions in the proportion of radiation in the diffuse components of both the UV-B and PAR wavebands. However, because of the greater proportion of UV-B which is diffuse, it tended to predominate in shaded areas. Similar patterns were seen in measurements taken at temperate latitudes. Response of shade- and gap-dwelling plants to these high UV-B:PAR ratios has received little attention.     

Influences of light and UV-B on growth and sporulation of the green alga Ulva pertusa Kjellman

The cosmopolitan presence of Ulva spp. is partly due to its great reproductive ability, but relatively little information is available for the radiation conditions triggering reproduction. In the present study, we investigated the effect of photon irradiance, photoperiod, and spectral qualities of light on growth and reproduction of Ulva pertusa.During 8-day culture of discs cut from marginal parts of the thallus of U. pertusa, the size of the thallus discs was greatest at 10 μmol m⁻² s⁻¹, while saturation of reproduction occurred at 30 μmol m⁻² s⁻¹. The minimum photon irradiance allowing growth and reproduction was 5 and 10 μmol m⁻² s⁻¹, respectively. Rapid increases in the size and subsequent initiation of sporulation were observed in samples transferred to saturating irradiance from 5 μmol m⁻² s⁻¹ or darkness for 9 days. When exposed to different photoperiods (8:16-, 12:12-, 16:8-h LD and continuous light regimes) combined with different photon irradiances (10 and 100 μmol m⁻² s⁻¹), U. pertusa thallus showed that the thallus size attained at the low irradiance was similar in daylengths longer than 12 h per day, while the surface area increased in parallel with increasing light duration at the high irradiance. The degree of sporulation at 10 μmol m⁻² s⁻¹ varied, ranging from no sporulation in 8:16-h LD to 80% in 16:8-h LD and continuous light. On the other hand, there was no remarkable difference in the degree of sporulation between the photoperiods except for slightly smaller percentage sporulation in 8:16-h LD at 100 μmol m⁻² s⁻¹.At 5 μmol m⁻² s⁻¹, the growth of U. pertusa was markedly lower in green than in blue or red light, but there was no sporulation in any spectral quality. The degree of sporulation at 20 μmol m⁻² s⁻¹ was almost twice as much in blue or red as in green light.The size of plants irradiated with 1.0 W m⁻² of UV-B for 20-40 min increased by 18-21% relative to control, whereas higher UV irradiance caused inhibition of growth. There was a significantly lower incidence of sporulation in UV-B-irradiated plants with the degree of reduction being greater in those exposed to higher UV doses. The total biologically effective UV-B dose for 50% inhibition of sporulation was 0.085 Dose_eff kJ m⁻². The time required to achieve 50% inhibition would be longer than 13 h at depths below 1 m in Ahnin coastal waters. The vertical attenuation coefficient of PAR (λ=400-700 nm) and UV-B (λ=300-320 nm) in April 1998 at Ahnin on the eastern coast of Korea was 0.21 m⁻¹ for K_PAR and 0.54 m⁻¹ for K_UV-B. A large reduction of light quantity and rapid disappearance of blue wavelength occurred by shading from overlying thalli.Percentage inhibition of sporulation was only 14-18% at 150-200 μmol m⁻² s⁻¹ compared with 70% at 10 μmol m⁻² s⁻¹, when plants were incubated under different irradiances of PAR immediately after UV-B exposures.These different photoadaptive strategies for sporulation may in part account for the great ecological success of U. pertusa.     

Response of photosynthesis to high light and drought for Arabidopsis thaliana grown under a UV-B enhanced light regime

Arabidopsis thaliana grown in a light regime that included ultraviolet-B (UV-B) radiation (6 kJ m⁻² d⁻¹) had similar light-saturated photosynthetic rates but up to 50% lower stomatal conductance rates, as compared to plants grown without UV-B radiation. Growth responses of Arabidopsis to UV-B radiation included lower leaf area (25%) and biomass (10%) and higher UV-B absorbing compounds (30%) and chlorophyll content (52%). Lower stomatal conductance rates for plants grown with UV-B radiation were, in part, due to lower stomatal density on the adaxial surface. Plants grown with UV-B radiation had more capacity to down regulate photochemical efficiency of photosystem II (PSII) as shown by up to 25% lower φ_PSII and 30% higher non-photochemical quenching of chlorophyll fluorescence under saturating light. These contributed to a smaller reduction in the maximum photochemical efficiency of PSII (F _v/F _m), greater dark-recovery of F _v/F _m, and higher light-saturated carbon assimilation and stomatal conductance and transpiration rates after a four-hour high light treatment for plants grown with UV-B radiation. Plants grown with UV-B were more tolerant to a 12 day drought treatment than plants grown without UV-B as indicated by two times higher photosynthetic rates and 12% higher relative water content. UV-B-grown plants also had three times higher proline content. Higher tolerance to drought stress for Arabidopsis plants grown under UV-B radiation may be attributed to both increased proline content and decreased stomatal conductance. Growth of Arabidopsis in a UV-B-enhanced light regime increased tolerance to high light exposure and drought stress.     

不同风浪条件下太湖梅梁湾光合有效辐射的衰减

基于2003年7月12～17日在太湖梅梁湾进行的连续6d原位水下光场观测资料,分析了不同风浪条件下光合有效辐射(PAR)的衰减和真光层深度,探讨了影响水下光合有效辐射的主导因子.结果表明,整个观测期间向下PAR衰减系数为2.63～4.71·m^-1(均值为3.63±0.47·m^-1),对应的真光层深度为0.98～1.75m(均值为1.29±0.18m),显示1.5m以下深度浮游植物、沉水植物基本上无法获取足够的太阳光能进行光合作用.从小风浪到中风浪、大风浪向下PAR衰减系数分别是2.63、3.72和4.37·m^-1,衰减系数分别增加了41%、66%.透明度、PAR衰减系数、真光层深度与悬浮物浓度存在显著性线性相关,并且与悬浮物中无机颗粒物相关性最好,而与叶绿素a、脱镁叶绿素及溶解性有机碳相关性很低.多元逐步线性回归表明,叶绿素a和溶解性有机碳最先被剔除方程,说明在梅梁湾由于风浪扰动引起悬浮物浓度的改变是影响水下光场的主导因素.     

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.     

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.     

Effects of UV-B radiation and additional irrigation on the Mediterranean evergreen sclerophyll Ceratonia siliqua L. under field conditions

Seedlings of Ceratonia siliqua L. were grown for 1 year in the field under ambient or ambient plus supplemental UV-B radiation (corresponding to 15% ozone depletion over Patras) and received two levels of additional irrigation during the summer dry period. The experiment was started during February 1998 and two major samplings were performed, the first at the end of the dry period (September 1998) and the second at the end of the experiment (January 1999). Plants receiving additional irrigation showed significantly higher leaf number, plant height and chlorophyll content at the end of the summer, but these differences were abolished at the final harvest. Plants growing under enhanced UV-B radiation had significantly fewer leaves and less nitrogen content at the end of the dry period, but these effects were also abolished at the final harvest, during which significant UV-B induced increases in stem dry mass were observed. None of the other measured parameters (mean leaf area, leaf dry mass, leaf thickness, UV-B absorbing compounds, phenolics, tannins and photochemical efficiency of PSII) were affected by either treatment. Combined UV-B / water effects were not significant. We may conclude that although some minor responses to enhanced UV-B radiation were evident, C. siliqua is resistant against UV-B radiation damage at the level applied.     

Enhanced UV-B radiation under field conditions increases anthocyanin and reduces the risk of photoinhibition but does not affect growth in the carnivorous plant Pinguicula vulgaris

The effects of enhanced UV-B radiation were investigated in the carnivorous plant Pinguicula vulgaris in a field experiment performed in Abisko, North Sweden (68° 21' N, 18° 49' E, 380 m above sea level). Potted plants were exposed to either ambient or ambient plus supplemental UV-B radiation, simulating a 15% ozone depletion. No effect was observed on either the epicuticular (external) or cellular (internal) UV absorbing capacity of the leaves. However, the anthocyanin content was more than doubled by supplemental UV-B radiation. In laboratory experiments, the anthocyanin rich, UV-B treated leaves were less susceptible to a low temperature/high light photoinhibitory treatment, as judged by in vivo chlorophyll fluorescence measurements. Yet, this potential benefit did not considerably affect the growth of the plant in the field (leaf area and dry mass, reproductive dry mass, flowering frequency, senescence rates, dry mass of winter buds). However, there was a marginally significant increase in root dry mass and in the root to shoot ratio, which may underlie the significant increase in the nitrogen content of the leaves. We suggest that P. vulgaris is resistant against UV-B radiation damage and that the possible negative effects of additional UV-B radiation on the growth of these plants may have been effectively counterbalanced by the lower risk of photoinhibition, due to the concomitant increase in anthocyanins.     

UV-B胁迫小麦上麦长管蚜的生命表参数和取食行为

【目的】为明确UV-B胁迫小麦对麦长管蚜Sitobion avenae (Fabricius)种群生命表参数和取食行为的影响。【方法】采用特定年龄生命表方法和刺探电位图谱（EPG）技术, 研究了不同强度UV-B胁迫处理小麦对麦长管蚜生命表参数和取食行为的影响。【结果】生命表结果表明, 取食高强度（0.75 mW/cm²） UV-B辐射小麦后, 麦长管蚜种群内禀增长率r_m、 净增殖率R₀、 繁殖力F下降, 平均世代周期T缩短, 且差异显著(P<0.05); 取食低强度（0.2 mW/cm²）UV-B辐射小麦后, 麦长管蚜种群生命表参数与对照组（正常光照处理小麦饲喂的蚜虫种群）无显著差异(P>0.05)。刺探电位图谱结果显示, 高强度UV-B处理组np波(即非取食)数量显著增加, pd波（刺探波）首次出现的时间延迟, 表明紫外处理小麦干扰了蚜虫的在叶表面的正常刺探活动, 延长了搜索和刺探时间; C波持续时间显著延长, 反映取食难度加大, 有效取食时间缩短, 从而影响种群生命表参数; 而低强度UV-B处理组小麦对蚜虫取食行为影响与对照组差异不显著, 但pd波首次出现时间显著延长。【结论】高强度UV-B胁迫小麦会严重影响麦长管蚜的生长发育、 繁殖与取食行为。     

Inhibition of photosynthesis by solar radiation in Dunaliella salina: relative efficiencies of UV-B, UV-A and PAR

Inhibition of photosynthesis after exposure to solar radiation was investigated in the marine green alga Dunaliella salina by monitoring photosynthetic optimal quantum yield F_v/F_m and efficiency of oxygen production. Samples were exposed to solar radiation in Ancient Korinth, Greece (37°58′ N, 23°0′ E) in August 1994. Within 30 min, F_v/F_m and efficiency of oxygen production decreased with similar kinetics with increasing exposure time. The inhibition, however, diminished when ultraviolet radiation was progressively excluded by means of colour filter glasses. Samples exposed for 3 h showed complete or partial recovery of photosynthesis, with almost the same rate under all irradition conditions. The fit of the experimental data with an analytical model describing inhibition of photosynthesis as a function of a linear combination of the photon fluence in the UV-B, UV-A and PAR allows one to estimate the relative mean effectiveness for inhibition by the three spectral ranges [about 2 × 10^?4, 4 × 10^?6 and 2 × 10^?7 (μmol photons m^?2)^?1 for UV-B, UV-A and PAR, respectively].     

A flavonoid mutant of barley (Hordeum vulgare L.) exhibits increased sensitivity to UV-B radiation in the primary leaf

The aim of the present investigation was to define the role of soluble flavonoids as UV-B protectants in the primary leaf of barley (Hordeum vulgare L.). For this purpose we used a mutant line (Ant 287) from the Carlsberg collection of proanthocyanidin-free barley containing only 7% of total extractable flavonoids in the primary leaf as compared to the mother variety (Hiege 550/75). Seven-day-old leaves from plants grown under high visible light with or without supplementary UV-B radiation were used for the determination of UV-B sensitivity. UV-B-induced changes were assessed from parameters of chlorophyll fluorescence of photosystem II, including initial and maximum fluorescence, apparent quantum yield, and photochemical and non-photochemical quenching. A quartz fibre-optic microprobe was used to evaluate the amount of potentially harmful UV-B (310 nm radiation) penetrating into the leaf as a direct consequence of flavonoid deficiency. Our data indicate an essential role of flavonoids in UV-B protection of barley primary leaves. In leaves of the mutant line grown under supplementary UV-B, an increase in 310nm radiation in the mesophyll and a strong decrease in the quantum yield of photosynthesis were observed as compared to the corresponding mother variety. Primary leaves of liege responded to supplementary UV-B radiation with a 30% increase in the major flavonoid saponarin and a 500% increase in the minor compound lutonarin. This is assumed to be an efficient protective response since no changes in variable chlorophyll fluorescence were apparent. In addition, a further reduction in UV-B penetration into the mesophyll was recorded in these leaves.