Evaluation of the role of damage to photosystem II in the inhibition of CO2 assimilation in pea leaves on exposure to UV-B radiation

Mature pea (Pisum sativum L., cv. Meteor) leaves were exposed to two levels of UV-B radiation, with and without supplementary UV-C radiation, during 15 h photoperiods. Simultaneous measurements of CO₂ assimilation and modulated chlorophyll fluorescence parameters demonstrated that irradiation with UV-B resulted in decreases in CO₂ assimilation that are not accompanied by decreases in the maximum quantum efficiency of photosystem II (PSII) primary photochemistry. Increased exposure to UV-B resulted in a further loss of CO₂ assimilation and decreases in the maximum quantum efficiency of PSII primary photochemistry, which were accompanied by a loss of the capacity of thylakoids isolated from the leaves to bind atrazine, thus demonstrating that photodamage to PSII reaction centres had occurred. Addition of UV-C to the UV-B treatments increased markedly the rate of inhibition of photosynthesis, but the relationships between CO₂ assimilation and PSII characteristics remained the same, indicating that UV-B and UV-C inhibit leaf photosynthesis by a similar mechanism. It is concluded that PSII is not the primary target site involved in the onset of the inhibition of photosynthesis in pea leaves induced by irradiation with UV-B.     

Ultraviolet-B-induced DNA lesions and their removal in wheat (Triticum aestivum L.) leaves

Monoclonal antibodies were used in an enzyme-linked immunosorbent assay (ELISA) to detect the induction and removal of cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts in DNA isolated from ultraviolet B (UV-B)-exposed primary wheat (Triticum aestivum L. cv. Chinese Spring) leaf tissue. The accumulation of lesions in the primary leaves of 6-d-old wheat seedlings was followed during the exposure of the leaf to an approximate dose of 3.6×10^?1 W m^?2 UV-B (Caldwell weighting). Significant increases in the levels of both CPDs and (6-4) photoproducts were detected in wheat leaves exposed to UV-B in the absence of other light However, only an increase in (6-4) photoproduct levels could be measured in wheat leaves exposed to the same UV-B source in the presence of supplemental white light. The removal of CPD antibody binding sites in the DNA after irradiation was rapid under conditions of high light intensity in contrast to the removal of (6-4) photoproduct antibody binding sites, which was significantly slower. The removal of CPDs appeared to be light dependent, this rate of removal decreasing with decreasing light fluences. The removal of (6-4) photoproducts also appeared light dependent, but to a lesser extent than the removal of CPDs, under the conditions studied here. Gene expression in the primary wheat leaf was measured and showed an up-regulation of chalcone synthase expression and a reduction in expression of chlorophyll a/b-binding protein (cab) in response to supplementary UV-B. No effect was seen on the expression of the other photosynthetic genes studied (the genes coding for the enzymes sedoheptu-lose 1,7-bisphosphatase and fructose 1,6-bisphosphatase). Measurement of the levels of DNA lesions in this same tissue showed that the observed changes in gene expression accompanied the appearance of UV-B induced lesions in the form of (6-4) photoproducts in the wheat leaf genome.     

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.     

INFLUENCE OF ULTRAVIOLET RADIATION ON GROWTH AND PHOTOSYNTHESIS OF TWO COLD OCEAN DIATOMS1

The influence of chronic exposure to UV-B and UV-A radiation on growth and photosynthesis of two polar marine diatoms (Pseudonitzschia seriata and Nitzschia sp.) was investigated in cultures exposed to moderate photon fluences for 3–7 days. Population growth rates were diminished 50% by UV-B. Fluorescence induction kinetics of photo-system II (PSII) revealed that UV-B caused lower F_v/F_m ratios and half-rise times, indicating damage to the reaction center of PSII and to related elements of the photosynthetic electron transport chain. Carbon assimilation rates per cell and per chlorophyll a were nonetheless highest for UV-B—exposed populations, which also had the highest chlorophyll a content per cell. The UV-B—exposed cells were, however, more vulnerable to visible light-induced photoinhibition. Exposure to UV-A in the absence of UV-B had little effect on growth, fluorescence induction of PSII, or chlorophyll a contents but did have some inhibitory effects on carbon assimilation per chlorophyll a and per cell. The increased photosynthetic capacity of UV-B-exposed cells suggested some ability to compensate for damage to the photosynthetic apparatus.     

Effect of UV-B Radiation on Leaf Optical Properties Measured with Fibre Optics

Changes in the internal light microenvironment in leaves ofplants of Brassica campestris L. cv. Emma, B. carinata L., andMedicago saliva L. cv. Armour in response to exposure to UV-B(UV-B, 280–320 nm) radiation were measured using a fibreopticmicroprobe. Plants were exposed for 2 weeks either to high visiblelight or to supplemental ultraviolet-B radiation. The spectral regime (400–700 nm; PAR) was measured eithermidway through the leaf palisade or the spongy mesophyll. Afterexposure to UV-B radiation leaves of Brassica campesiris attenuatedtransmitted light more than the controls. At the same time bothforward and back scattered light increased in the palisade andspongy mesophylls. In contrast, UV-treatment of Medicago salivaleaves increased light transmission into the palisade, whilethe back scattered component showed little change. Leaves ofcariiwla showed little change in response to UV. Other responsesto UV-B radiation included increases in leaf thickness, decreasedtotal chlorophyll content, and changes in UV-B screening pigmentsand chlorophyll fluorescence induction kinetics. Brassica campestriswas most sensitive to exposure to enhanced levels of UV-B radiation,whereas leaves of B. carinata were the least sensitive. Ourdata indicate that exposure to UV-B radiation altered the lightmicroenvironment within leaves of the species different ways.These changes appeared to be caused by alterations in pigmentcontent and leaf anatomy. In turn, the altered distributionof PAR within the leaf could influence photosynthesis. Key words: Brassica campestris, Brassica carinata, fibre optics, light scattering, Medicago saliva, optical properties, ozone depletion, photosynthesis, ultraviolet radiation     

Influence of UV-B radiation and Cd2+ on chlorophyll fluorescence, growth and nutrient content in Brassica napus

The possible interaction of two stresses, UV-B radiation and cadmium, applied simultaneously, was investigated in Brassica napus L. cv. Paroll with respect of chlorophyll fluorescence, growth and uptake of selected elements. Plants were grown in nutrient solution containing CdCl2, (0, 0.5, 2 or 5 M) and irradiated with photosynthetically active radiation (PAR, 400-700 nm, 800 mol m^-2 s^-1) with or without supplemental ultraviolet-B radiation (UV-B, 280-320 nm, 15 kJ m^-2 d^-1, weighted irradiance). After 14 d of treatment, the most pronounced effects were found at 2 and 5 M CdCl2 with and without supplemental UV-B radiation. Exposure to cadmium significantly increased the amount of Cd in both roots and shoots. In addition, increases occurred in the concentrations of Fe, Zn, Cu, and P in roots, while K was reduced. In shoots the S content rose significantly both in the presence and absence of UV-B radiation, while significant increases in Mg, Ca, P, Cu, and K occurred only in plants exposed to Cd and UV-B radiation. Manganese decreased significantly under the combined exposure treatment. The rise in S content may have been due to stimulated glutathione and phytochelatin synthesis. Cadmium exposure significantly decreased root dry weight, leaf area, total chlorophyll content, carotenoid content, and the photochemical quantum yield of photosynthesis. As an estimation of energy dissipation processes in photosynthesis, non-photochemical quenching (qNPQ) was measured using a pulse amplitude modulated fluorometer. The qNPQ increased with increasing Cd, while the combination of cadmium and UV-B reduced the qNPQ compared to that in plants exposed only to cadmium or UV-B radiation. The chlorophyll a:b ratio showed a reduction with UV-B at no or low Cd concentrations (0 M, 0.5 M CdCl2), but not at the higher Cd concentrations used (2 M, 5 M CdCl2). Thus in some instances there appeared to be a UV-B and Cd interaction, while in other plants response could be attributed to either treatment alone.Keywords: Brassica napus, cadmium, ultraviolet-B radiation.      

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.     

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

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

Responses to ultraviolet-B radiation (280–315 nm) of pea (Pisum sativum) lines differing in leaf surface wax

To test the hypothesis that leaf surface wax influences plant responses to UV-B, 6 lines of cultivated pea (Pisum sativum L.), selected as having more or less wax, were grown at 0 or 6.5 kJ m^-2 day^-1 plant-weighted UV-B against a background of 850–950 μmol m^-2 s^-1 photosynthetically active radiation. In the 4 lines with least leaf surface wax the amount of wax on adaxial and abaxial leaf surfaces was increased following exposure to 6.5 kJ m^-2 day^-1 UV-B, but UV-B decreased surface wax in Scout, which had the greatest wax deposits. On the adaxial leaf surface, UV-B radiation caused a shift in wax composition from alcohols to esters and hydrocarbons and the ratio of short to long chain length alkyl ester homologues was increased. There was no evidence of a shortening in carbon chain length of hydrocarbons, primary alcohols or fatty acids due to UV-B and no significant correlation between wax amount and UV reflectance from leaves. UV-B induced significant increases in UV-absorbing compounds in the expanded leaves and buds of most lines. UV-B reduced the growth of all lines. Foliage area (leaves plus stipules) declined by 5–30%, plant dry weight by 12–30%, and plant height by 24–38%. Reductions in growth occurred in the absence of any changes in chlorophyll fluorescence or photosynthetic rate. UV-B also had no major effect on carbon allocation patterns. The effects of UV-B on growth appeared to be due to changes in tissue extension and expansion. Indeed, many of the responses to UV-B observed in this study of pea appear more consistent with indirect effects being expressed in developing tissues rather than through the direct action of UV-B on mature tissues. There was no evidence that wax amount or biochemistry was associated with the sensitivity of the lines to UV-B radiation. Furthermore, induction of pigments was not correlated with changes in growth. However, lines with the greatest constitutive amounts of pigments in unexpanded bud tissues were most tolerant of elevated UV-B.     

Leaf expansion and development of PHOTOSYNTHETIC CAPACITY AND PIGMENTS IN Liquidambar styraciflua (Hamamelidaceae)—EFFECTS OF UV-B RADIATION

In order to perform their functions as photosynthetic organs, leaves must cope with excess heat and potentially damaging ultraviolet radiation. Possible increases in the UV-B portion of the solar spectrum may place an additional burden on leaves, and this could be particularly important for young expanding leaves with poorly developed UV-B defense mechanisms. We evaluated the effects of supplemental UV-B radiation on leaf expansion and the development of photosynthetic capacity and pigments in sweetgum (Liquidambar styraciflua L.) seedlings. Seedlings were grown in the field under either ambient or ambient plus 3 or 5.0 kJ of biologically effective supplemental UV-B radiation. Although final leaf size was unaffected, the rate of leaf elongation and accumulation of leaf area was slower in leaves exposed to the lower supplemental UV-B irradiance. In contrast, chlorophyll accumulation and the development of photosynthetic capacity was more rapid in plants exposed to the higher, compared to the lower supplemental UV-B irradiance. The accumulation of anthocyanins and other putative flavonoids or UV-absorbing compounds was scarcely affected by exposure to supplemental UV-B radiation. These results suggest that the UV-B portion of the solar spectrum may, in the absence of gross affects on biomass, exert subtle influences on leaf ontogeny and the development of photosynthetic pigments and capacity in sweetgum.     

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

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

Giant clams in shallow reefs: UV-resistance mechanisms of Tridacninae in the Red Sea

The photosymbiosis of tropical giant clams (subfamily Tridacninae) with unicellular algae (Symbiodiniaceae) restricts their distribution to the sunlit, shallow waters of the euphotic zone where organisms are additionally exposed to potentially damaging levels of solar UV radiation. Metabolic and physiological responses of Red Sea Tridacna maxima clams, including net calcification and primary production, as well as valvometry (i.e., shell gaping behavior) were assessed when exposed to simulated high radiation levels received at 3 and 5 m underwater. The two levels of radiation included exposure treatments to photosynthetically active radiation (PAR; 400–700 nm) alone and to both, PAR and ultraviolet-B radiation (UV-B; 280–315 nm). The valvometry data obtained using flexible magnetic sensors indicated that specimens under PAR + UV-B exposure significantly reduced the proportion of their exposed mantle area, a potential photo-protective mechanism which, however, reduces the overall amount of PAR received by the algal symbionts. Consequently, specimens under PAR + UV-B displayed a slight, although non-significant, reduction in primary production rates but no signs of additional oxidative stress, changes in symbiont densities, chlorophyll content, or levels of mycosporine-like amino acids. Net calcification rates of T. maxima were not affected by exposure to UV-B; however, calcification was positively correlated with incident PAR levels. UV-B exposure changes the valvometry, reducing the exposed mantle area which consequently diminishes the available PAR for the photosymbionts. Still, T. maxima maintains high rates of primary production and net calcification, even under high levels of UV-B. This provides experimental support for a recently described, effective UV-defensive mechanism in Tridacninae, in which the photonic cooperation of the associated algal symbionts and giant clam iridocytes is assumed to establish optimal conditions for the photosynthetic performance of the clams’ symbionts.

     

The response of five tropical dicotyledon species to solar ultraviolet-B radiation

Tropical regions currently receive the highest levels of global solar ultraviolet-B radiation (UV-B, 280–320 nm) even without ozone depletion. The influence of natural, present-day UV-B irradiance in the tropics was examined for five tropical species including three native rain forest tree species (Cecropia obtusifolia, Tetragastris panamensis, Calophyilum longifolium) and two economically important species (Swietenia macrophylla, Manihot esculenta). Solar UV-B radiation conditions in a small clearing on Barro Colorado Island, Panama (9° N), were obtained using either a UV-B-excluding plastic film or a film that transmits most of the solar UV-B. Significant differences between UV-B-excluded and near-ambient UV-B plants were often exhibited as increased foliar UV-B absorbing compounds and, in several cases, as reduced plant height with exposure to solar UV-B. Increases in leaf mass per area and reductions in leaf blade length under solar UV-B occurred less frequently. Biomass and photosystem II function using chlorophyll a fluorescence were generally unaffected. The results of this study provide evidence that tropical vegetation, including native rain forest species, responds to the present level of natural solar UV-B radiation. This suggests that even minor ozone depletion in the tropics may have biological implications.