Differential effects of elevated ultraviolet-B radiation on the photochemical and reproductive performances of dicotyledonous and monocotyledonous arid-environment ephemerals

Four dicotyledonous Astcraceac and four monocotyledonous Iridaceac species were grown from seed under three different effective ultraviolet-B (UV-B, 280–320 nm) fluenccs, which approximated those received daily over the natural growing periods of these species at their southerly and northerly distribution limits under ambient stratospheric ozone, and at the northerly limit assuming a 20% ozone depletion. Diminished photochemical efficiencies, delayed flowering, decreased flower production, pollen germination and tube growth, and reduced numbers of seeds set, accompanied by corresponding increases in mass per seed, were observed mainly in dicotyledons under elevated UV-B radiation, despite their higher foliar and pollen polyphenolic contents, their possession of trinucleatc pollen and their natural occurrence at lower latitudes than monocotyledons with binucleate pollen. The results demonstrate the difficulties in determining plant sensitivity to UV-B radiation from polyphenolic content, pollen type and natural geographic distribution, and suggest that monocotyledons possess other attributes that confer greater resilience to UV-B radiation.     

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.     

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.     

Effects of artificial ultraviolet-B radiation on growth and fatty acid composition of duckweed (Lemna minor)

1. Duckweed (Lemna minor), collected either in summer or early fall was exposed under laboratory conditions to control (photosynthetically active and UV‐A radiation) or experimental (control plus UV‐B radiation) conditions. 2. Growth and survival were determined by counting the number of green, and brown/white fronds following 1–5 or 11 days of irradiation. Growth of duckweed was impaired by exposure to UV‐B radiation in the fall experiment but not in the summer. 3. Fatty acid compositions were analysed following 5 or 11 days of irradiation and a recovery period of 0, 5, 29 or 40 h. Concentrations of the major fatty acids, palmitic, linoleic (LA) and α‐linolenic (ALA) acids were similar in the summer and fall duckweed collections, but the summer samples had higher concentrations of the desaturation products of LA and ALA. 4. UV‐B exposure had small, but significant, and contrasting effects on duckweed fatty acid concentrations. In the summer experiment, duckweed exposed to UV‐B had slightly lower concentrations of major fatty acids than control duckweed, while the reverse was true in the fall experiments. 5. These minor effects of UV‐B on concentrations of LA and ALA would be unlikely to have a major impact on the supply of these essential fatty acids from duckweed to freshwater food webs.     

Moisture availability influences the effect of ultraviolet-B radiation on leaf litter decomposition

Altered surface ultraviolet‐B (UV‐B) radiation resulting from a combination of factors that include changes in stratospheric ozone concentrations, cloud cover, and aerosol conditions may affect litter decomposition and, thus, terrestrial nutrient cycling on a global scale. Although litter decomposition rates vary across biomes, patterns of decomposition suggest that UV‐B radiation accelerates litter decay in xeric environments where precipitation is infrequent. However, under more frequent precipitation regimes where litter decay rates are characteristically high, the effect of UV‐B radiation on litter decomposition has not been fully elucidated. To evaluate this association between moisture regime and UV‐B exposure, a litter decomposition experiment was designed for aspen (Populus tremuloides) leaf litter, where conditions that influence both abiotic (photodegradation) and biotic (microbial) processes could be manipulated quantitatively. We found that experimentally increasing UV‐B exposure (0, 7.4, and 11.2 kJ m^?2 day^?1, respectively) did not consistently increase litter decomposition rates across simulated precipitation frequencies of 4, 12, and 24 days. Instead, a UV‐B exposure of 11.2 kJ m^?2 day^?1 resulted in a 13% decrease in decomposition rates under the 4‐day precipitation frequency, but an increase of 80% under the 24‐day frequency. Furthermore, the same UV‐B dose increased litter decomposition rates under the 24‐day precipitation frequency by 78% even in conditions where microbial activity was suppressed. Therefore, under more xeric conditions, greater exposure to UV‐B radiation increased decomposition rates, presumably through photodegradation. In contrast, when decomposition was not moisture‐limited, greater UV‐B exposure slowed decomposition rates, most likely from the resulting inhibition of microbial activity. Ultimately, these experimental results highlight UV‐B radiation as a potential driver of decomposition, as well as indicate that both the direction and magnitude of the UV‐B effect is dependent on moisture availability, a factor that may change according to future patterns in global precipitation.     

紫外线-B照射对小白菜生长、产量及品质的影响

在温室条件下,采用辐照度为0.4、0．5和0．6W·m^-2的紫外线-B（UV—B）对小白菜（Brassi cacampestrisssp．chinensis Makino）照射2、3和4h·d^-1,连续照射7d后,研究了UV—B照射对小白菜生长、产量及品质的影响。结果表明,停止照射后35d,各处理组小白菜的鲜质量、株高、叶片数、干物质含量、叶绿素含量、可溶性糖含量及硝酸盐含量均与对照无显著差异,但Vc含量增加,可溶性蛋白质含量降低。经UV—B照射后,下胚轴长度大于2cm的小白菜植株所占的百分率显著低于对照,说明UV—B照射可有效控制小白菜植株徒长。从子叶充分展开时用0,4—0．6W·m^-2 UV—B照射2h·d^-1,连续照射7d,既能有效控制小白菜植株徒长、提高Vc含量,又不会造成产量和品质的下降,为最佳的补充UV—B处理条件。     

Effect of springtime solar ultraviolet-B radiation on growth of Colobanthus quitensis at Palmer Station, Antarctica

We examined the influence of solar ultraviolet‐B radiation (UV‐B; 280–315 nm) on the growth of Colobanthus quitensis plants by placing them under contrasting UV‐B filters at Palmer Station, along the Antarctic Peninsula. The filters reduced diurnal biologically effective UV‐B (UV‐B_BE) either by 83% (‘reduced UV‐B’) or by 12% (‘near‐ambient UV‐B’) over the 63 day experiment (7 November 1998–8 January 1999). Ozone column depletion averaged 17% during the experiment. Relative growth and net assimilation rates of plants exposed to near‐ambient UV‐B were 30 and 20% lower, respectively, than those of plants exposed to reduced UV‐B. The former plants produced 29% less total biomass, as a result of containing 54% less aboveground biomass. These reductions in aboveground biomass were mainly the result of a 45% reduction in shoot biomass, and a 31% reduction in reproductive biomass. Reductions in shoot biomass were owing to an 18% reduction in branch production by main shoots, while reductions in reproductive biomass were the result of a 19% reduction in individual capsule mass. Total plant leaf area was reduced by 19% under near‐ambient UV‐B, although total leaf biomass was unaffected because leaves had a greater specific leaf mass. The reduction in plant leaf area under near‐ambient UV‐B was attributable to: (1) production of 11% fewer leaves per main shoot system and plant, which resulted from an 18% reduction in branch production by main shoots. Leaf production per individual main shoot or branch was not affected; (2) shorter leaf longevity—main shoots contained 14% fewer green leaves at a given time; and (3) smaller individual leaves—leaf elongation rates were 14% slower and mature leaves were 13% shorter.     

Evidence for the ultraviolet-B (280–320 nm) radiation induced structural reorganization and damage of photosystem II polypeptides in isolated chloroplasts

Direct evidence for the possible loss of photosystem II (PS II) activity in chloroplasts of Vigna sinensis L. cv. Walp after ultraviolet-B (UV-B, 280–320 nm) radiation treatment was provided by polyacrylamide gel electrophoretic analysis of PS II polypeptides. A 30 min UV-B treatment of chloroplasts caused a 50% loss of PS II activity. The artificial electron donor. Mn²⁺ failed to restore UV-B radiation induced loss of PS II activity, while diphenyl carbazide (DPC) and NH₂OH only partially restored activity. Such a loss in PS II activity was found to be primarily due to a loss of 23 and 33 kDa extrinsic polypeptides. UV-B treatment induced the synthesis of a few polypeptides and a 29 kDa light-harvesting chlorophyll protein.     

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.     

Effects of elevated ultraviolet-B radiation on native and cultivated plants of southern Africa

Seventeen herb, shrub and tree species of commercial and ecological importance in southern Africa were exposed at one location to ultraviolet-B (UV-B, 280-315 nm) radiation approx. 35 % above clear-sky background (control). The aims were to assess how UV-B affects canopy area, dry mass, and some biochemical and morphological properties of leaves, and to investigate whether differences between species are related to growth form of the plants. There was no pattern of response to UV-B related to growth form. Leaves of trees had altered chlorophyll a and b, carotenoid and flavonoid concentrations, but those of shrubs or herbs did not. Non-structural carbohydrates were unaffected. Smaller canopy areas and dry masses were observed under enhanced UV-B, but these were not statistically different among growth forms. There was a general insensitivity of species to elevated UV-B. Only five species had significantly altered leaf biochemical and morphological properties, canopy area and dry mass, the changes differing in magnitude. There was no consistent pattern of change in leaf thickness or biochemical composition with increased UV-B. Correlation analyses did not support the view that growth is less negatively affected in species with thick leaves or in those where leaf thickness increases, or in species with naturally high leaf flavonoid contents or that are able to synthesize additional flavonoids in response to UV-B enhancement. The analyses did not support the hypothesis that growth was inhibited by starch accumulation in leaves under elevated UV-B. However, changes in leaf shape did correlate with canopy area and dry mass, showing the importance of photomorphogenetic changes caused by UV-B which affect species' performance. We conclude that generalizations on plant sensitivity to UV-B based on growth form and functional type could be misleading, and that the great majority of economically important species of the region are likely to be insensitive to future UV-B increases. Notable exceptions include the Colophospermum mopane tree ecotypes chota and leslie and the arable annual Vigna unguiculata, both of which are traditional sources of livelihood to rural African populations and of importance to African industry and agriculture.     

Response of sugar beet plants to ultraviolet-B (280–320 nm) radiation and Cercospom leaf spot disease

Sugar beet ( Beta vulgaris L.) plants injected with Cercospora beticota Sace. as well as non-infected plants were grown under visible light with or without ultraviolet-B (UV-B, 280–320 nm) radiation for 40 days. An interaction between UV-B radiation and Cercospora leaf spot disease was observed, resulting in a large reduction in leaf chlorophyll content, dry weight of leaf laminae, petioles and storage roots. Lipid peraxidation in leaves also increased the most under the combined treatments. This was also true for ultraweak luminescence from both adaxial and abaxial leaf surfaces. However, no correlation between lipid peroxidation and ultraweak luminescence was observed. Ultraviolet-B radiation given alone appeared to have either a stimulating effect, giving an increase in dry weight of laminae and reducing lipid peroxidation, or no effect. This lack of effect was seen in the absence of change in dry weight of storage roots and chlorophyll content relative to controls. The :study demonstrated a harmful interaction between UV-B radiation and Cercospom leaf spot disease on sugar beet.     

Differential effects of elevated ultraviolet-B radiation on plant species of a dune grassland ecosystem

In a greenhouse study, plants of three monocotyledonous and five dicotyledonous species, which occur in a Dutch dune grassland, were exposed to four levels of ultraviolet-B (UV-B) radiation. UV-B levels simulated up to 30% reduction of the stratospheric ozone column during summertime in The Netherlands. Six of the plant species studied in the greenhouse were also exposed to enhanced UV-B irradiance in an experimental field study. In the field experiment plants either received the ambient UV-B irradiance (control) or an enhanced UV-B level simulating 15–20% ozone depletion during summertime in The Netherlands. The purpose of both experiments was to determine the response of the plant species to UV-B radiation and to compare results obtained in the greenhouse with results of the field experiment. Large intraspecific differences in UV-B sensitivity were observed in the greenhouse study. Total dry matter accumulation of monocotyledons was increased, while dry matter accumulation of dicotyledons remained unaffected or decreased. The increase in biomass production of monocotyledons at elevated UV-B was not related to the rate of photosynthesis but to alterations in leaf orientation. In the greenhouse study, UV-B radiation also affected morphological characteristics. Shoot height or maximum leaf length of five out of eight species was reduced. In the field study only one species showed a significantly decreased maximum leaf length at enhanced UV-B. Possible reasons for this discrepancy are discussed. The absorbance of methanolic leaf extracts also differed between species. UV absorbance of field-grown plants was higher than greenhouse-grown plants. In the greenhouse study, the highest UV-B level increased UV-B absorbance of some species. In the field study however, this stimulation of UV absorbance was not observed. In general, results obtained in the greenhouse study were similar to results obtained in the field study. Difficulties in extrapolating results of UV-B experiments conducted in the greenhouse to the field situation are discussed.     

The effect of elevated UV-B radiation on herbivory of pea byAutographa gamma

Leaves exposed to above-ambient fluxes of ultraviolet-B (UV-B) radiation commonly contain increased concentrations of phenolic compounds which may influence herbivores. However, the hypothesis that elevated UV-B modifies herbivory, whether mediated by phenolics or other plant constituents, has rarely been studied experimentally. We investigated the responses of the mothAutographa gamma L. (Lepidoptera: Noctuidae) to pea (Pisum sativum L.) grown at a range of plant-effective UV-B fluxes. Although total phenolics did increase significantly with increasing UV-B, this change had little deleterious effect on the 5th instar larvae ofA. gamma. However, tissue nitrogen also increased with increasing UV-B. Increased nitrogen was correlated with an increase in the efficiency with which larvae utilized their food and in larval growth rate, but in a reduction in the amount of plant material consumed. The apparently major role of nitrogen in determining herbivore responses to changing UV-B demonstrates the risks in predicting such responses soley on the basis of changes in phenolics and other secondary metabolites.     

Enhanced UV-B radiation has little effect on growth, δ13C values and pigments of pot-grown rice (Oryza sativa) in the field

Predicted increase in ultraviolet-B (UV-B: 280–320 mn) radiation may have adverse impacts on growth and yield of rice ( Oryza sativa L.), as has been found in studies hitherto. However, most of the studies were conducted in growth chambers or greenhouses where the plants are generally more sensitive to UV-B than in the field, presumably because of the distorted balance between UV-B and ultraviolet-A as well as PAR. This study was conducted to address the effects of enhanced UV-B on growth and yield of rice under a realistic spectral balance in the field. Three cultivars, "Koshihikari",'IR 45'and'IR 74'were pot-grown and irradiated with enhanced UV-B for most of the growing season in the field at Tsukuba, Japan (36°01'N, 140°07'E). The UV-B enhancement simulated ca 38% depletion of stratospheric ozone at Tsukuba. The results showed no UV-B effects on plant height, numbers of tillers and panicles, dry weight of the plant parts or the grain yield for any of the 3 cultivars. Natural abundance of ¹³C in the flag leaves was not altered by the UV-B enhancement either. While UV-absorbing compounds showed no response to the UV-B enhancement, chlorophyll contents decreased with enhanced UV-B. However, the decrease of chlorophyll was limited to an early growth stage with no effect later. We thus found no extraordinary impact of the nearly doubled UV-B radiation on rice in the field, and it would appear that a reliable prediction of the effects of UV-B will require experiments carried out over a number of years under various climatic and solar UV-B regimes.     

UV-B辐射对植物花粉萌发率和花粉管生长的累积效应

研究了19种植物花粉在不同UV－B辐射强度和辐照时间下其萌发率和花粉管伸长的变化,结果表明,UV－B辐射增加显著抑制大多数植物花粉的萌发率和花粉管生长;与对照相比,较高强度的UV－B对花粉的抑制作用大于较低强度;几个种的花粉萌发率及花粉管生长对UV－B增强不敏感,甚至被UV－B辐射所促进;辐射时间越长,对花粉抑制作用愈大,说明具有辐射累积效应,由此可知,植物花粉的萌发过程对UV－B的敏感性变化在自然条件下将会产生严重的生态学后果。     

Phenology and life cycle of the annual, <Emphasis Type=Italic>Chamaesyce maculata</Emphasis> (L.) Small (Euphorbiaceae), with multiple overlapping generations in Japan

The phenology of germination, vegetative growth and sexual reproduction in the annual Chamaesyce maculata (L.) Small (Euphorbiaceae) were investigated in a natural population in western Japan. Seedlings emerged from mid-June to early October, with three peaks: mid-June, late July and late August. Plants that emerged in June commenced sexual reproduction from late July, and thereafter both vegetative growth and sexual reproduction occurred together until early November, the plants showing no switching from vegetative growth to sexual reproduction. Seedlings that emerged in June and July suffered high mortality, but most seedlings that emerged from August onward survived until the reproductive stage. The minimum size for reproduction was largest for plants that emerged early in the season, and it decreased with a delay in seedling emergence. The late emergence of seedlings that resulted in low reproductive output may be to some extent compensated for by the increased probability of survival in the seedling stage. A transplant experiment clarified that C. maculata can repeat a maximum of three overlapping generations within a year. Multiple generations per year were attained by non-dormant seeds produced in the first and second generations and clearly resulted in an increased reproductive output per year. The life cycle with multiple overlapping generations may have been acquired in habitats where unpredictable disturbance results in temporally unsuitable conditions for germination, vegetative growth and sexual reproduction of annual plants, but where suitable conditions frequently continue over a period longer than the single generation time of annual plants.     

桔梗花粉萌发与花粉管生长研究

以2年生桔梗植株为材料,采用液体培养法研究了培养基种类、PEG、蔗糖、pH以及培养温度、培养时间对桔梗花粉离体萌发生长的影响,结果表明:(1)浓度为100~150 g.L-1的PEG可显著促进桔梗花粉萌发和花粉管的生长;200~250 g.L-1PEG显著促进花粉萌发,但对花粉管生长的作用不显著。(2)100 g.L-1的蔗糖有利于花粉萌发和花粉管生长,高浓度蔗糖(200 g.L-1)有明显抑制作用;(3)桔梗花粉离体萌发和花粉管生长的适宜培养基为ME3+BK+10%蔗糖+150 g.L-1PEG(pH5.8);(4)25~40℃条件下桔梗花粉均可较好萌发,以30℃培养1.5 h为最佳培养条件。