Free radical generation and antioxidant content in chloroplasts from soybean leaves exposed to ultraviolet-B

The aim of this work was to study the effect of ultraviolet-B (UV-B) exposure on oxidative status in chloroplasts isolated from soybean ( Glycine max cv . Hood). Chloroplasts were isolated from soybean leaves excised from either control seedlings or those exposed to 30 and 60 kJ m⁻² day⁻¹ of UV-B radiation for 4 days. Chloroplastic oxidative conditions were assessed as carbon-centered radical, carbonyl groups and ascorbyl radical content. Treatment with UV-B increased the carbon-centered radical-dependent EPR signal significantly by 55 and 100% in chloroplasts from leaves exposed to 30 and 60 kJ m⁻² day⁻¹ UV-B, respectively, compared to radical content in chloroplasts from control leaves. The content of carbonyl groups increased by 37 and 62% in chloroplasts isolated from soybean leaves irradiated for 4 days with 30 and 60 kJ m⁻² day⁻¹ UV-B, respectively. The content of soluble metabolites in isolated chloroplasts should not be taken as absolute in vivo values; however, these data are valuable for comparative studies. UV-B exposure did not significantly affect ascorbyl radical content compared to controls. The content of ascorbic acid and thiols in chloroplasts isolated from leaves exposed to 60 kJ m⁻² day⁻¹ UV-B was increased by 117 and 20.8%, respectively, compared to controls. Neither the content of total carotene nor that of β -carotene or α -tocopherol was affected by the irradiation. The results presented here suggest that the increased content of lipid radicals and oxidized proteins in the chloroplasts isolated from leaves exposed to UV-B could be ascribed to both the lack of antioxidant response in the lipid soluble fraction and the modest increase in the soluble antioxidant content.     

Oxidative stress and antioxidant content in Chlorella vulgaris after exposure to ultraviolet-B radiation

Growth of Chlorella vulgaris was measured in cultures irradiated with 0, 0.8, 2.0 and 4.4 kJ m² UV-B. Growth expressed as chlorophyll content, declined significantly with increased UV-B dose. Ultraviolet-B irradiated cultures in log phase of growth showed a 284% increase in oxygen radical generation and a 145% increase in lipid peroxidation compared with unirradiated cultures, whereas cultures in the stationary growth phase showed no significant changes in these parameters. The activities of superoxide dismutase and catalase increased by 40 and 500%, respectively, after exposure to a UV-B dose of 4.4 kJ m⁻². Contents of the lipophilic antioxidants α-tocopherol and β-carotene increased by 180 and 63 amol cell⁻¹ respectively, between log and stationary phases in unirradiated cultures; but in UV-B-irradiated cultures these increases were significantly depressed. Photoreducing capacities of chloroplasts were decreased following UV-B irradiation of both isolated chloroplasts and those isolated from irradiated algae. Cells exposed to UV-B exhibited increased size and starch accumulation. These results suggest that oxidative stress conditions related to UV-B exposure trigger an antioxidant response that includes an increase in the activity of the antioxidant enzymes (superoxide dismutase and catalase).     

Influence of UV-B radiation on developmental changes, ethylene, CO2 flux and polyamines in cv. Doyenne d'Hiver pear shoots grown in vitro

In vitro shoots of cv. Doyenne ďHiver pear ( Pyrus communis L.) were irradiated under controlled environments for 6 h per day at 5 different levels of biologically effective UV-B radiation (UV-B_BE). UV-B exposure caused a progressive increase in apical necrosis above background levels and stimulated leaf abscission. Shoots grown for 2 weeks at 7. 8 mol m⁻² day ⁻¹ of photosynthetic photon flux (PPF) and treated with 8. 4 or 12. 0 kJ m⁻² day ⁻¹ UV-B_BE produced up to 4 times more ethylene than those given 2. 2 or 5. 1 kJ m⁻² day⁻¹ UV-B_BE or untreated controls. Exposure of shoots to 12 kJ m⁻² day ⁻¹ of UV-B_BE caused an increase in free putreseine content after 4 to 14 days of irradiation. Shoots showed a decrease in CO₂ uptake after 3 days of UV-B: thereafter, they appeared to recover their photosynthetic capacity. Under typical PPF conditions used in micropropagation (90 μmol m⁻² S⁻¹). 8. 4 kJ m⁻² day ⁻¹ of UV-B radiation was injurious to realatively tender tissues of in vitro pear shoots: increasing the level of UV-B_BE to 12 kJ m⁻² day⁻¹ produced even more adverse effects.     

Photomorphogenic regulation of increases in UV-absorbing pigments in cucumber (Cucumis sativus) and Arabidopsis thaliana seedlings induced by different UV-B and UV-C wavebands

Brief (1–100 min) irradiations with three different ultraviolet-B (UV-B) and ultraviolet-C (UV-C) wave bands induced increases the UV-absorbing pigments extracted from cucumber ( Cucumis sativus L.) and Arabidopsis . Spectra of methanol/1% HCl extracts from cucumber hypocotyl segments spanning 250–400 nm showed a single defined peak at 317 nm. When seedlings were irradiated with 5 kJ m⁻² UV-B radiation containing proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm; full-spectrum UV-B, FS-UVB), tissue extracts taken 24 h after irradiation showed an overall increase in absorption (91% increase at 317 nm) with a second defined peak at 263 nm. Irradiation with 1.1 kJ m⁻² UV-C (254 nm) caused similar changes. In contrast, seedlings irradiated with 5 kJ m⁻² UV-B including only wavelengths longer than 290 nm (8% of UV-B between 290 and 300 nm; long-wavelength UV-B, LW-UVB) resulted only in a general increase in absorption (80% at 317 nm). The increases in absorption were detectable as early as 3 h after irradiation with FS-UVB and UV-C, while the response to LW-UVB was first detectable at 6 h after irradiation. In extracts from whole Arabidopsis seedlings, 5 kJ m⁻² LW-UVB caused only a 20% increase in total absorption. Irradiation with 5 kJ m⁻² FS-UVB caused the appearance of a new peak at 270 nm and a concomitant increase in absorption of 72%. The induction of this new peak was observed in seedlings carrying the fah 1 mutation which disrupts the pathway for sinapate synthesis. The results are in agreement with previously published data on stem elongation indicating the existence of two response pathways within the UV-B, one operating at longer wavelengths (>300 nm) and another specifically activated by short wavelength UV-B (<300 nm and also by UV-C).     

Effects of UV-C and UV-B on cytomorphology and water permeability of inner epidermal cells of Allium cepa

Changes of cytomorphology and water permeability of inner epidermal cells of Allium cepa L. cvs Spartan and Keep Well were investigated with the light microscope after irradiation with UV-C (254 nm) and UV-B (280 and 310 nm). The sequence of the investigated changes of viscosity, protoplasmic streaming, organelle shape and water permeability was the same with 254 as with 280 nm. although a higher dose of 280 nm was needed to produce the same biologically equivalent effect. However, when calculated as a percentage of the lethal dose, the initial doses that produced the effects were the same for 254 and 280 nm. No changes could he observed at the cellular level after the cells were irradiated with 310 nm. The lethal dose depended upon the time between irradiation and observation. At 254 nm it was 1.1 kJ m⁻² up to 1 h after irradiation and dropped to 860 J m⁻² when measured 24 and 48 h later. At 280 nm a dose of 2.8 kJ m⁻² killed the cells within I h while the dose needed after 24 and 48S h was 1.99 kJ m⁻². The minimum doses which caused the different cytomorphological effects did not depend upon the observation time. Normal cell structure and functions that were altered immediately alter irradiation did not recover. Doses that were not immediately effective alter irradiation caused no later damage. Doses which increased water permeability were much higher than doses which influenced cytomorphological parameters of the cell.     

Quantitative changes in secondary metabolites of dark-leaved willow (Salix myrsinifolia) exposed to enhanced ultraviolet-B radiation

This is a study of the impact of increased ultraviolet-B (UV-B) radiation on the secondary chemistry of Salix myrsinifolia (dark-leaved willow). For nearly two decades, the loss of stratospheric ozone above the high latitudes of the Northern Hemisphere has increased UV-B radiation (280–320 nm) over the long-term mean. Willows (Salicaceae) are widely distributed in these northern regions. To determine the effects of increased UV-B radiation on willows, the plantlets of three clones of S. myrsinifolia were grown under ambient (3.6 kJ m⁻² day⁻¹) or enhanced (7.18 kJ m⁻² day⁻¹) UV-B irradiance. After the 2-week indoor experiment, the concentrations of UV-B-screening phenolics (flavonoids and phenolic acids) and low-UV-B-screening phenolics (salicylates and condensed tannins) in fresh leaves were investigated and the biomass of leaves, stems and roots was determined. As expected, the total amount of flavonoids in willow leaves clearly increased when plantlets were exposed to higher UV-B irradiation. However, the degree of increase of individual compounds varied: luteolin-7-glucoside, monomethyl-monocoumaryl-luteolin-7-glucoside and one myricetin derivative increased significantly, while the apigenin-7-glucuronide increased only slightly. The enhanced UV-B also increased the amount of p -hydroxycinnamic acid derivative. The UV-B effects on other phenolic acids and tannins were minor. In contrast to the other phenolics, the amounts of two salicylates, salicin and saligenin, decreased under enhanced UV-B irradiation. Our results indicate that the concentrations of both UV-B-screening and low-UV-B-screening phenolic compounds in leaves of S. myrsinifolia may vary in response to elevated UV-B radiation. However, while the UV-B protective flavonoids and phenolic acids accumulate during UV-B exposure, the concentrations of certain salicylates decrease.     

Intraspecific differences in Cucumis sativus sensitivity to ultraviolet-B radiation

Cucumber ( Cucumis sativus L.) cultivars Marketmore, Lama, XPH 1187, XPH 1484 and Sprint 440 (N) were grown in a greenhouse under two levels of biologically effective ultraviolet-B ( UV -B) radiation (daily dose: 0 and 11.6 kJ m⁻² UV-B_BE) for 31 days. Significant intraspecific differences were observed in plant height, number of leaves, leaf area and total dry weight. Based upon total biomass accumulation, Marketmore was found to be the most tolerant, and XPH 1484 the most sensitive to UV-B radiation. The dose response of accumulation of UV absorbing compounds (measured as absorbance of methanolic extracts) in leaf tissues showed an increase in UV absorbing compounds with UV-B dose in Marketmore, Sprint 440 (N) and XPH 1187. In Lama and XPH 1484, however, doses below 8.7 kJ m⁻² UV-B_BE produced no change in UV absorbing compounds. This study suggests that intraspecific differences in UV-B radiation sensitivity in cucumber may be related to inherent differences in the accumulation of UV absorbing compounds in leaves.     

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.     

Inhibition of hypocotyl elongation by ultraviolet-B radiation in de-etiolating tomato seedlings. I. The photoreceptor

Broad-band UV-B radiation inhibited hypocotyl elongation in etiolated tomato ( Lycopersicon esculentum Mill. cv. Alisa Craig) seedlings. This inhibition could be elicited by < 3 μmol m⁻² s⁻¹ of UV-B radiation provided against a background of white light (> 620 μmol m⁻² s⁻¹ between 320 and 800 nm), and was similar in wild-type and phytochrome-1-deficient aurea mutant seedlings. These observations suggest that the effect of UV-B radiation is not mediated by phytochrome. An activity spectrum obtained by delivering 1 μmol m⁻² s⁻¹ of monochromatic UV radiation against a while light background (63 μmol m⁻² s⁻¹ showed maximum effectiveness around 300 nm, which suggests that DNA or aromatic residues in proteins are not the chromophores mediating UV-B induced inhibition of elongation. Chemicals that affect the normal (photo)chemistry of flavins and possibly pterins (KI, NaN, and phenylacetic acid) largely abolished the inhibitor) effect of broad-hand UV-B radiation when applied to the root zone before irradiation. KI was effective at concentrations < 10⁻⁴ M , which have been shown in vitro to be effective in quenching the triplet excited stales of flavins but not fluorescence from pterine or singlet states of flavins. Elimination of blue light or reduction of UV-A, two sources of flavin excitation, promoted hypocotyl elongation, but did not affect the inhibition of elongation evened by UV-B. Kl applied after UV-B irradiation had no effect on the inhibition response. Taken together these findings suggest that the chromophore of the photoreceptor system invoked in UV-B perception by tomato seedlings during de-etiolation may be a flavin.     

Growth analysis of UV-B-irradiated cucumber seedlings as influenced by photosynthetic photon flux source and cultivar

A growth analysis was made of ultraviolet-B (UV-B)-sensitive (Poinsett) and insensitive (Ashley) cultivars of Cucuumis satives L. grown in growth chambers at 600 μmol m⁻² s⁻¹ of photosynthetic photon flux (PPF) provided by red- and far-red-deficient metal halide (MH) or blue- and UV-A-deficient high pressure sodium/deluxe f HPS/DX) lamps. Plants were irradiated 6 h daiiy with 0.2 f-UV-B) or 18.2 C+UV-B) kJ m⁻² day⁻¹ of biologically effective UV-B for 8 or 15 days from time of seeding. In general, plants given supplemental UV-B for 15 days showed lower leaf area ratio (LARs, and higher specific leaf mass (SLM) mean relative growth rate (MRGR) and net assimilation rate (NAR) than that of control plants, but they showed no difference in leaf mass ratio (LMR), Plants grown under HPS/DX lamps vs MH lamps showed higher SLM and NAR. lower LAR and LMR. hut no difference in MRGR. LMR was the only growth parameter affected by cultivar: at 15 days, it was slightly greater in Poinsett than in Ashley. There were no interactive effects of UV-B. PPF source or cultivar on any of the growth parameters determined, indicating that the choice of either HPS/DX or MH lamps should not affect growth response to UV-B radiation. This was true even though leaves of UV-B-irradiated plants grown under HPS/DX lamps have been shown to have greater chlorosis than those grown under MH lamps.     

Relative effectiveness and interaction of ultraviolet-B, red and blue light in anthocyanin synthesis of apple fruit

The effect of light on anthocyanin production in apple ( Malus pumila Mill. cv. Jonathan) skin disks was investigated, with prolonged irradiation from different light sources. High fluence rates of white light provided from a xenon lamp were unable to produce large amounts of anthocyanin, and anthocyanin production became saturated at about 30 W m⁻². When UV-B light, provided by a fluorescent lamp which had an emission peak at 312 nm, was combined with the white light, anthocyanin production was synergistically stimulated and increased up to the highest fluence rates of white light tested (44 W m⁻²). This UV-B light was more effective than red and blue light provided from fluorescent lamps, but anthocyanin production became saturated at about 1.7 W m⁻². However, simultaneous irradiation with red and UV-B light had a synergistic effect. UV-B light was also effective in increasing anthocyanin production in whole fruit. Therefore this synergism seemed to have an important role in the development of the desirable red skin color under field light conditions. The results of aminoethoxyvinylglycine treatment suggested that ethylene was not involved in the stimulative effect of UV-B light.     

Lipid peroxidation in and photo-damage to a light-sensitive chlorescence pea mutant

Leaves of 10- to 12-day-old chlorescence lethal Pisum sativum L. mutant are similar to control plants with respect to the content of chlorophylls, carotenoids, fatty acids and α-tocopherol. Subsequent development of the mutant under high irradiation resulted in th destruction of the photosynthetic pigments, polyunsaturated fatty acids, α-tocopherol, and also in the accumulation of liposoluble fluorescent products. No increase in the level of malondialdehyde was observed. In chloroplasts isolated from mutant plants the contents of chlorophyll a and β-carotene were decreased to a greater extent than the more oxidized pigments (xanthophylls and chlorophyll b ). The data obtained are discussed with special reference to the role of lipid peroxidation in the injury of plant cells under the action of visible light and to the antioxidative mechanisms stabilizing photosynthetic membranes.     

Effects of ultraviolet-B irradiance on soybean. VI. Influence of phosphorus nutrition on growth and flavonoid content

Soybeans Glycine max (L.) cv. Essex were hydroponically grown in a greenhouse at 2 levels of ultraviolet-B (UV-B) radiation (0 and 2 500 J m⁻¹ day⁻¹ biologically effective UV-B radiation) and 4 levels of P (6.5, 13, 26 and 52 μ M ). Plants were grown in each treatment combination to the complete expansion of the 4th trifoliolate leaf. UV-B radiation and reduced P supply generally decreased plant height, leaf area and total biomass, but increased specific leaf weight and flavonoid content (measured as absorbance of methanolic extracts). Although both UV-B radiation and low P supply produced deleterious effects on plant biomass, the effects were non-additive. The combination of UV-B and the lowest P level (6.5 μ M ) had no effect on total biomass or leaf area. This was at least partially due to the accumulation of flavonoids and leaf thickening. The results show that the sensitivity of soybean to UV-B radiation is dependent upon plant P supply. Plants experiencing P deficiency are less sensitive to UV-B than plants at optimum P levels.     

Salt tolerance in the halophyte Suaeda maritima (L.) Dum. The influence of the salinity of the culture solution on leaf starch and phosphate content

Abstract. The starch concentration in mature leaves of the halophyte Suaeda maritima increased from 4.7 to 7.3 mg mg⁻¹ chlorophyll when sodium chloride (680 mol M⁻³) was added to the solution in which the plants were grown. This effect of salinity on the starch: chlorophyll ratio was greater in young than in old leaves. Electron micrographs showed the starch to be in the chloroplasts and this was confirmed by measurements on isolated chloroplasts. Total phosphorus concentration (mg mg⁻¹ chlorophyll) in leaves of all ages from plants of S. maritima decreased on salinization of the growth medium suggesting an inverse relationship between phosphorus and starch concentrations. However, although leaf starch concentration varied with leaf age, phosphorus concentration did not. The cause of starch accumulation in chloroplasts at salinities which are optimal for growth (340 mol m⁻³) remains unclear.     

The effect of exposure to enhanced UV-B radiation on the penetration of monochromatic and polychromatic UV-B radiation in leaves of Brassica napus

Using quartz optical fibres, penetration of both monochromatic (310 nm) and polychromatic UV-B (280–320 nm) radiation in leaves of Brassica napus L. (cv. Ceres) was measured. Plants were grown under either visible light (750 μmol m⁻² s⁻¹ photosynthetically active radiation) or with the addition of 8. 9 KJ m⁻² day⁻¹ biologically effective UV-B (UV-B_BE) radiation. Results showed that of the 310 nm radiation that penetreated the leaf, 90% was within the intial one third of the leaf with high attenuation in the leaf epidermis, especially in UV-treated plants. Polychromatic UV-B radiation, relative to incident radiation, showed a relatively uniform spectral distribution within the leaf, except for collimated radiation. Over 30% of the UV-screening pigments in the leaf, including flavonoids, were found in the adaxial epidermal layer, making this layer less transparent to UV-B radiation than the abaxial epidermis, which contained less than 12% of the UV-screening pigments. UV-screening pigments increased by 20% in UV-treated leaves relative to control leaves. Densely arranged epicuticular wax on the adaxial leaf surface of UV-treated plants may have further decreased penetration of UV-B radiation by reflectance. An increased leaf thickness, and decreases in leaf area and leaf dry weight were also found for UV-treated plants.     

Effects of UV-B radiation on growth and motility of four phytoplankton species

A depletion of the stratospheric ozone layer would result in an increased UV-B radiation, which could have harmful effects on marine organisms. The aim of this study was to determine the effects of an enhanced UV-B radiation (280–320 nin) on the motility and growth in four Swedish phytoplanklon species. The different plankton species were exposed to different doses of UV-B radiation during growth. The growth of the motile dinoflagellates, Gyronidium aureolum Hulburt (Ba 6), and Prorocentrum minimum (Pav.) P. Schiller (Ba 12), was more sensitive to UV-B radiation than the non-motile diatoms Dityhim brightwellii (P. West) Grun (Ba 15) and Phaeodactylum tricornutum Bohlin (Ba 16). One week of UV-B radiation 2 h daily (159 J m⁻² day⁻¹), had a dramatic effect on the growth of the dinoflagellates, while the diatoms were nearly unaffected. On the other hand, when given higher intensity of UV-B radiation (312, 468 and 624 J m⁻² day⁻¹) during the initial phase of growth, also the growth of the diatom, D. brightwellii, was inhibited. Not only the growth but also the swimming speed of the dinoflagellates C. aureolum and P. minimum were affected by UV-B radiation. The speed decreased rapidly after 1–2 h of UV-B radiation (312 J m⁻² day⁻¹), and after longer irradiation times the dinoflagellates lost their motility. G. aureolum exposed to UV-B radiation, regained normal speed after two weeks of visible light.     

Effects of UV-B on activities of enzymes of secondary phenolic metabolism in barley primary leaves

Barley ( Hordeum vulgare L.) was grown in a glasshouse with 13.56 or 8.84 kJ m⁻²: biologically effective UV-B (280–320 nm: UV-B_BE) simulating levels predicted to occur with 25 or 5% ozone depletion at 40°N latitude, with UV-A (320–400 mm), or with no supplemental irradiation. Activities of L-phenylalanine ammonia-lyase (PAL, EC 4.3.1.5). chalcone-flavanone isomerase (CFI, EC 5.5.1.6) and peroxidase (EC 1.11.1.7) were determined from the 5th through the 30th day after planting. PAL regulates diversion of L-phenylalanine into precursors for secondary phenolics. CFI regulates an early step of flavonoid biosynthesis, and peroxidase activates phenolic precursors for cross-linking and rigidifying cell walls. At all ages UV-B decreased soluble protein leaf⁻¹ but had little effect on fresh weight or CFI activity. Exposure to UV-B decreased peroxidase activity only slightly in early growth stages but decreased it about 40% by day 30. PAL activity was highest 5 days after planting under all treatments, decreased thereafter, and was not detectable in control plants after day 10. UV-B prolonged PAL activity through day 15 in plants given the highest level of UV-B. This UV-B prolongation of PAL activity is correlated with, and is a likely underlying mechanism to explain, the UV-B- enhanced accumulation of flavonoids and ferulic acid in barley primary leaves. The results are discussed in terms of barley leaf adaptation to UV-B as developmental response dependent on conditions of plant growth.     

Effect of ultraviolet radiation on accumulation and leakage of 86Rb+ in guard cells of Vicia faba

The effects of UV-C (254 nm), UV-A (365 nm) and broad-band UV (280–380 nm) on guard cells of Vicia faba L. cv. Long Pod were investigated in the presence of white light (450 μmol m⁻² s⁻¹). UV-C (7 μmol m⁻² s⁻¹) was found to cause leakage of ⁸⁶Rb⁺ from guard cells, while UV-A (0.3 μmol m⁻² s⁻¹) stimulated increased uptake in these cells. A relatively small stimulatory effect was observed by broad-band UV (3 μmol m⁻² s⁻¹) during the first 30 min of irradiation with an apparent equilibration of influx and efflux thereafter. Leakage of ⁸⁶Rb⁺ from guard cells continued despite the removal of UV-C and an increase in the amount of white light from 450 to 1500 μmol m⁻² s⁻¹, suggesting that membranes were irreversibly damaged. Irradiation of guard cells with UV-C for 30, 45 and 90 min indicated that these cells began to be affected already by 30 min UV-C irradiation.     

Effect of UV-B radiation on the shoot dry matter production and stable carbon isotope composition of two Arabidopsis thaliana genotypes

An application of stable carbon isotope analysis to the mechanistic interpretation of ultraviolet-B (UV-B) effects on growth inhibition is described that is particularly useful for small plants such as Arabidopsis thaliana that are not well suited for gas exchange studies. Many investigators use tissue δ¹³C, relative abundance of ¹³C and ¹²C, as a proxy for water use efficiency and as an indicator of environmental effects on stomatal behaviour and on photosynthesis during growth. Discrimination against ¹³C is enhanced by both high stomatal conductance and damage to photosynthetic machinery. Because the thinning of the stratospheric ozone layer is permitting more UV-B to enter the biosphere, the mechanisms of action of UV-B radiation on plants are of particular current interest. Arabidopsis thaliana wild-type Landsberg erecta (L er ) and the UV-B-sensitive mutant fah I , deficient in UV-absorbing sinapate esters, were grown in a controlled environment and exposed to UV-B^BE doses of 0 or 6–7 kJ m⁻² day⁻¹. UV-B exposure decreased dry matter production and δ¹³C in both genotypes, but growth inhibition was generally greater in fah I than in L er . The fah I mutant also had less leaf greenness than L er . Changes in leaf tissue δ¹³C were detected before growth inhibition and were evident in treatments of both genotypes that did not cause marked growth effects. This suggests that the effects of UV-B contributing to increased carbon isotope discrimination in L er may have been primarily associated with high stomatal conductance, and in fah I with both high stomatal conductance and damage to photosynthetic machinery.     

A morphological and cytological study of Petunia hybrida exposed to UV-B radiation

The aim of this study was to investigate whether the cytoskeleton, and in particular the microtubular system, is affected by enhanced levels of ultraviolet-B (280–320 nm, 9 kJ m⁻² day⁻¹ biologically effective UV-B radiation) radiation in epidermal cells of Petunia x hybrida Vilm, isolated from leaves of plants grown under UV-B radiation and visible light. In addition, morphological changes during development were monitored. In a previous study microtubules were depolymerized and delays in the different stages of the cell cycle were found when protoplasts of Petunia were irradiated with UV-B radiation (Staxén et al. 1993. Protoplasma 173: 70–76). Thus it was of interest to ascertain whether the cytoskeleton would be similarly affected in an intact system. Assuming an effect of UV-B radiation on the microtubular system, we wished to determine whether this could be correlated to concomitant changes in leaf morphology. Plants of Petunia hybrida were grown in greenhouse conditions in the presence or absence of UV-B radiation. During the course of the experiment, samples were taken from young, expanding leaves and from older, fully expanded leaves and prepared for localization and analysis of microtubules from the adaxial epidermal cells. Morphology rather than the cytoskeleton was affected by UV radiation, despite the fact that the epidermal cytoskeleton would most likely be affected, since it is located in the cells which form the first intercepting layer for incident radiation.
Morphological changes under UV-B radiation, as compared to those under control conditions, were reflected in earlier flowering and an increase in leaf number. Cell division was thus stimulated as was also evidenced from the increased leaf area. Our results indicate that the number of stomata differentiated on a leaf area basis was not altered although the number of stomata per epidermal cell was reduced.