Effects of UV-B radiation on terrestrial plants and ecosystems: interaction with CO2 enrichment

UV-B radiation is just one of the environmental factors, that affect plant growth. It is now widely accepted that realistic assessment of plant responses to enhanced UV-B should be performed at sufficiently high Photosynthetically Active Radiation (PAR), preferably under field conditions. This will often imply, that responses of plants to enhanced UV-B in the field will be assessed under simultaneous water shortage, nutrient deficiency and variation of temperature. Since atmospheric CO₂ enrichment, global warming and increasing UV-B radiation represent components of global climatic change, interactions of UV-B with CO₂ enrichment and temperature are particularly relevant. Only few relevant UV-B× CO₂ interaction studies have been published. Most of these studies refer to greenhouse experiments. We report a significant CO₂ × UV-B interaction for the total plant dry weight and root dry weight of the C₃-grass Elymus athericus. At elevated CO₂ (720 mol mol^-1, plant growth was much less reduced by enhanced UV-B than at ambient atmospheric CO₂ although there were significant (positive) CO₂ effects and (negative) UV-B effects on plant growth. Most other CO₂ × UV-B studies do not report significant interactions on total plant biomass. This lack of CO₂ × UV-B interactions may result from the fact that primary metabolic targets for CO₂ and UVB are different. UV-B and CO₂ may differentially affect plant morphogenetic parameters: biomass allocation, branching, flowering, leaf thickness, emergence and senescence. Such more subtle interactions between CO₂ and UV-B need careful and long term experimentation to be detected. In the case of no significant CO₂× UV-B interactions, combined CO₂ and UV-B effects will be additive. Plants differ in their response to CO₂ and UV-B, they respond in general positively to elevated CO₂ and negatively to enhanced UV-B. Moreover, plant species differ in their responsiveness to CO₂ and UV-B. Therefore, even in case of additive CO₂ and UV-B effects, plant competitive relationships may change markedly under current climatic change with simultaneous enhanced atmospheric CO₂ and solar UV-B radiation.     

Plant responses to atmospheric carbon dioxide enrichment: interactions with some soil and atmospheric conditions

In general, C₃ plant species are more responsive to atmospheric carbon dioxide (CO₂) enrichment than C₄-plants. Increased relative growth rate at elevated CO₂ primarily relates to increased Net Assimilation Rate (NAR), and enhancement of net photosynthesis and reduced photorespiration. Transpiration and stomatal conductance decrease with elevated CO₂, water use efficiency and shoot water potential increase, particularly in plants grown at high soil salinity. Leaf area per plant and leaf area per leaf may increase in an early growth stage with increased CO₂, after a period of time Leaf Area Ratio (LAR) and Specific Leaf Area (SLA) generally decrease. Starch may accumulate with time in leaves grown at elevated CO₂. Plants grown under salt stress with increased (dark) respiration as a sink for photosynthates, may not show such acclimation to increased atmospheric CO₂ levels. Plant growth may be stimulated by atmospheric carbon dioxide enrichment and reduced by enhanced UV-B radiation but the limited data available on the effect of combined elevated CO₂ and ultraviolet B (280–320 nm) (UV-B) radiation allow no general conclusion. CO₂-induced increase of growth rate can be markedly modified at elevated UV-B radiation. Plant responses to elevated atmospheric CO₂ and other environmental factors such as soil salinity and UV-B tend to be species-specific, because plant species differ in sensitivity to salinity and UV-B radiation, as well as to other environmental stress factors (drought, nutrient deficiency). Therefore, the effects of joint elevated atmospheric CO₂ and increased soil salinity or elevated CO₂ and enhanced UV-B to plants are physiologically complex.     

Effects of enhanced UV-B radiation and elevated carbon dioxide concentrations on a sub-arctic forest heath ecosystem

An experiment is described which studies the effects of enhanced UV-B radiation (simulating a 15% reduction in the Ozone layer) and elevated atmospheric concentrations of CO₂ (600 ppm) on the dwarf shrub layer of a sub-arctic forest heath ecosystem at Abisko, North Sweden. The experimental treatments were first applied in 1993, and have covered most of the snow-free season (late May to early September) 1993–1995. Effects of the treatments on the four dwarf shrub species have been recorded largely using non-destructive measures (Vaccinium uliginosum, Vaccinium myrtillus – deciduous species and Vaccinium vitis-idaea and Empetrum hermaphroditum – evergreen species). Effects of the treatments on stem growth and leaf thickness have so far been small, although CO₂ treatments initially stimulated stem extension in Vaccinium myrtillus 1993 and depressed growth in V. vitis idaea in 1994 and E. hermaphroditum during 1995. UV-B treatments stimulated fruit production in V. myrtillus in both 1994 and 1995, but there was no effect on reproductive phenology. There were also marked effects of UV-B treatments on insect herbivory in the deciduous dwarf shrubs; with leaf area loss being greater than the control in the UV-B treatment in V. myrtillus and less in V. uliginosum. The results point to the possibility of important effects of the treatments on physiological and chemical processes within the plants. The ecological results of such effects may not be immediately apparent, but may be far reaching, pointing to the need for long-term in situ experimentation in predicting the effects of these global change variables.     

The combined effects of elevated CO2 levels and UV-B radiation on growth characteristics ofElymus athericus (= E. pycnanathus)

Elymus athericus (Link) Kerguélen, a C₃ grass, was grown in a greenhouse experiment to determine the effect of enhanced atmospheric CO₂ and elevated UV-B radiation levels on plant growth. Plants were subjected to the following treatments; a) ambient CO₂-control UV-B, b) ambient CO₂-elevated UV-B, c) double CO₂-control UV-B, d) double CO₂-elevated UV-B. Elevated CO₂ concentrations stimulated plant growth, biomass production was 67% higher than at ambient CO₂. Elevated UV-B radiation had a negative effect on growth, biomass production was depressed by 31%. Enhanced CO₂ combined with elevated UV-B levels caused a biomass depression of 8% when compared with the control plants. UV-B induced growth depression can be modified by a growth stimulus caused by high CO₂ concentrations. Growth analysis has been performed and possible physiological mechanisms behind changing growth parameters are discussed.     

Effects of increasing UV-B radiation and atmospheric CO2 on photosynthesis and growth: implications for terrestrial ecosystems

Increases in UV-B radiation reaching the earth as a result of stratospheric ozone depletion will most likely accompany increases in atmospheric CO₂ concentrations. Many studies have examined the effects of each factor independently, but few have evaluated the combined effects of both UV-B radiation and elevated CO₂. In general the results of such studies have shown independent effects on growth or seed yield. Although interspecific variation is large, high levels of UV-B radiation tends to reduce plant growth in sensitive species, while CO₂ enrichment tends to promote growth in most C₃ species. However, most previous studies have not looked at temporal effects or at the relationship between photosynthetic acclimation to CO₂ and possible photosynthetic limitations imposed by UV-B radiation. Elevated CO₂ may provide some protection against UV-B for some species. In contrast, UV-B radiation may limit the ability to exploit elevated CO₂ in other species. Interactions between the effects of CO₂ enrichment and UV-B radiation exposure have also been shown for biomass allocation. Effects on both biomass allocation and photosynthetic acclimation may be important to ecosystem structure in terms of seedling establishment, competition and reproductive output. Few studies have evaluated ecosystem processes such as decomposition or nutrient cycling. Interactive effects may be subtle and species specific but should not be ignored in the assessment of the potential impacts of increases in CO₂ and UV-B radiation on plants.     

Leaf anatomical changes in Populus trichocarpa, Quercus rubra, Pseudotsuga menziesii and Pinus ponderosa exposed to enhanced ultraviolet‐B radiation

Leaf anatomical characteristics are important in determining the degree of injury sustained when plants are exposed to natural and enhanced levels of ultraviolet-B (UV-B) radiation (280–320 nm). The degree to which leaf anatomy can adapt to the increasing levels of UV-B radiation reaching the earth's surface is poorly understood in most tree species. We examined four tree species, representing a wide range of leaf anatomical characteristics, to determine responses of leaf area, specific leaf weight, and leaf tissue parameters after exposure to ambient and enhanced levels of UV-B radiation. Seedlings were grown in a greenhouse with photosynthetically active radiation of 39 mol m^?2 day^?1 and under one of three daily irradiances of biologically effective UV-B radiation (UV-B_BE) supplied for 10 h per day: (1) approximate ambient level received at Pullman, Washington on June 21 (1 x ); two times ambient (2 x ), or three times ambient (3 x ). We hypothesized the response of each species to UV-B radiation would be related to inherent anatomical differences. We found that the conifers responded anatomically to nearly an equal degree as the broad-leaved trees, but that different tissues were involved. Populus trichocarpa, an indeterminate broadleaf species, showed significantly thicker palisade parenchyma in recently mature leaves at the 3 x level and in older leaves under the 2 x level. In addition, individual leaf area was generally greater with increased UV-B irradiance. Quercus rubra, a semi-determinate broadleaf species, exhibited significantly thicker palisade parenchyma at the 2 x and 3 x levels as compared to controls. Psuedotsuga menziesii, an evergreen coniferous species with bifacially flattened needles, and Pinus ponderosa, an evergreen coniferous species with a complete hypodermis, showed no significant change in leaf area or specific leaf weight under enhanced UV-B radiation. Epidermal thickness was unchanged in P. menziesii. However, P. ponderosa increased the thickness and number of hypodermal layers produced, presumably decreasing penetration of UV-B radiation into the leaf. We concluded that differences in inherent leaf anatomy of the four species examined are important in the responses to enhanced levels of UV-B radiation.     

Physiological and growth responses of two African species, Acacia karroo and Themeda triandra, to combined increases in CO2 and UV-B radiation

The interactive effects of increased carbon dioxide (CO₂) concentration and ultraviolet-B (UV-B, 280–320 nm) radiation on Acacia karroo Hayne, a C₃ tree, and Themeda triandra Forsk., a C₄ grass, were investigated. We tested the hypothesis that A. karroo would show greater CO₂-induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C₄ grasslands, but that increased UV-B could mitigate this advantage. Seedlings were grown in open-top chambers in a greenhouse in ambient (360 μmol mol^-1) and elevated (650 μmol mol^-1) CO₂, combined with ambient (1.56 to 8.66 kJ m^-2 day^-1) or increased (2.22 to 11.93 kJ m^-2 day^-1) biologically effective (weighted) UV-B irradiances. After 30 weeks, elevated CO₂ had no effect on biomass of A. karroo, despite increased net CO₂ assimilation rates. Interaction between UV-B and CO₂ on stomatal conductance was found, with conductances decreasing only where elevated CO₂ and UV-B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO₂. Elevated UV-B caused only an increase in foliar carbon concentrations. In T. triandra, net CO₂ assimilation rates were unaffected in elevated CO₂, but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO₂. and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO₂. Significantly moderating effects of elevated UV-B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO₂-induced growth responses of the species therefore do not support the hypothesis of CO₂-driven woody encroachment of C₄ grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV-B radiation is unlikely to substantially alter the CO₂ response of these species.     

The combined effects of CO2 concentration and enhanced UV-B radiation on faba bean. 3. Leaf optical properties,pigments, stomatal index and epidermal cell density

Seedlings of Vicia faba L. (cv. Minica) were grown in a factorial experiment in a greenhouse. The purpose of the study was to determine whether CO₂ enrichment and supplemental UV-B radiation affect leaf optical properties and whether the combined effects differ from single factor effects. Seedlings were grown at either 380 mol mol^-1 or 750 mol mol^-1 CO₂ and at four levels of UV-B radiation. After 20 and 40 days of treatment, absorptance, transmittance and reflectance of photosynthetically active radiation (PAR) were measured on the youngest fully developed leaf. On the same leaf, the specific leaf area on a fresh weight basis (SLA_fw), chlorophyll content, UV-B absorbance, transmittance of UV light and stomatal index were measured. UV-B radiation significantly increased PAR absorptance and decreased PAR transmittance. The increased PAR absorptance can be explained by an increased chlorophyll content in response to UV-B radiation. Leaf transmittance of UV radiation decreased with increasing UV-B levels mainly caused by increased absorbance of UV absorbing compounds. UV-B radiation decreased both the stomatal density and epidermal cell density of the abaxial leaf surface, leaving the stomatal index unchanged. Effects of CO₂ enrichment were less pronounced than those of UV-B radiation. The most important CO₂ effect was an increase in stomatal density and epidermal cell density of the adaxial leaf surface. The stomatal index was not affected. No interaction between CO₂ and UV-B radiation was found. The results are discussed in relation to the internal light environment of the leaf.     

Effects of an enhanced ultraviolet-B irradiation on photosynthetic apparatus of several forest coniferous tree species from different locations

The effect of UV-B on the photosynthetic apparatus of coniferous trees: Picea abies (L.) Karst., Picea pungens (Engelm.), Pinus sylvestris (L.), Pinus cembra (L.) and Abies alba (Mill.) was investigated. Three and four-year-old plantlets coming from different latitudes, longitudes and altitudes were used. The experiment was carried out in greenhouse. Two doses of ultraviolet-B irradiation were applied: control=0, low dose=11.32 and high dose=22.64 kJ·m⁻²·d⁻¹ UV-B_BE (biologically effective irradiance of UV-B). Measurements of chlorophyll fluorescence, gas exchange, chlorophyll and flavonoids content were carried out. Response of forest trees to an increased UV-B radiation depends on species, location of place of pantalets collecting and UV-B dose. Pinus cembra, Picea abies and Pinus sylvestris from high altitude (1000 m a.s.l.) were less sensitive to UV-B than these from plain location. The altitude determined adaptation of forest coniferous trees to an enhanced UV-B radiation much more than the latitudinal gradient. Permanent discoloration was observed only on the young needles of the fir plantlets that were grown in light limiting conditions. Photosynthetic parameters were affected by the UV-B radiation. Both maximal and the steady state fluorescence of chlorophyll were reduced as a consequence of elevated UV-B in case of some species. The chlorophyll content was enhanced, increased or was not affected according to species and to locations. The flavonoids content in the needles increased with chlorophyll content at both UV-B treatments. An opposite trend was found in the control. The increased content of screening pigments in the needles of all the tested coniferous trees was detected. Picea abies and Picea pungens photosynthesis response curves to the light and to the intercellular CO₂ concentration did not change significantly under increased UV-B because of higher concentration in screening pigments in leaves. The increased concentration of flavonoids in forest litter may lead to changes in the biogeochemical cycle in the forest ecosystem.     

Combined effects of CO2 concentration and enhanced UV-B radiation on faba bean

Due to anthropogenic influences, both solar UV-B irradiance at the earth's surface and atmospheric [CO₂] are increasing. To determine whether effects of CO₂ enrichment on faba bean (cv. Minica) growth are modified by UV-B radiation, the effects of enhanced [CO₂] on growth and photosynthetic characteristics, were studied at four UV-B levels. Faba bean was sensitive to enhanced UV-B radiation as indicated by decreases in total biomass production. Growth stimulation by CO₂ enrichment was greatly reduced at the highest UV-B level. [CO₂] by UV-B interactions on biomass accumulation were related to loss of apical dominance. Both [CO₂] and UV-B radiation affected biomass partitioning, UV-B effects being most pronounced. Effects of [CO₂] and UV-B on faba bean growth were time-dependent, indicating differential sensitivity of developmental stages. [CO₂] and UV-B effects on photosynthetic characteristics were rather small and restricted to the third week of treatment. CO₂ enrichment induced photosynthetic acclimation, while UV-B radiation decreased light-saturated photosynthetic rate. It is concluded that the reduction in biomass production cannot be explained by UV-B-induced effects on photosynthesis.     

Carbon dioxide and methane fluxes in boreal peatland microcosms with different vegetation cover—effects of ozone or ultraviolet-B exposure

O₃ concentrations in the troposphere are rising and those in the stratosphere decreasing, the latter resulting in higher fluxes of solar ultraviolet-B (UV-B) radiation to the earth's surface. We assessed whether the fluxes of CO₂ and CH₄ are altered by enhanced UV-B radiation or elevated tropospheric O₃ concentrations in boreal peatland microcosms (core depth 40 cm, diameter 10.5 cm) with different vegetation cover. At the end of the UV-B experiment which lasted for a growing season, net CO₂ exchange (NEE) and dark ecosystem respiration (R _TOT) were sevenfold higher, and CH₄ efflux 12-fold higher, in microcosms with intact vegetation dominated by Eriophorum vaginatum L. and Sphagnum spp., compared to microcosms from which we removed E. vaginatum. Vegetation treatment had minor effects on CH₄ production and consumption potentials in the peat, suggesting that the large difference in CH₄ efflux is mainly due to efficient CH₄ transport via the aerenchyma of E. vaginatum. Ambient UV-B supplemented with 30% and elevated O₃ concentrations (100 and 200 ppb, for 7 weeks) significantly increased R _TOT in both vegetation treatments. Elevated O₃ concentrations reduced NEE over time, while UV-B had no clear effects on the fluxes of CO₂ or CH₄ in the cloudy summer of the study. Field experiments are needed to assess the significance of increasing UV-B radiation and elevated tropospheric O₃ concentration on peatland gas exchange in the long-term.     

Influence of elevated CO2 and ultraviolet-B radiation levels on floral nectar production: a nectary-morphological perspective

 Investigations of the effects of two global events – elevated CO₂ levels and enhanced ultraviolet-B (UV-B) radiation – on floral nectar production are reviewed from twelve dicotyledonous families. Furthermore, to allow comparisons between nectary morphology and nectar production in treated plants of these fifteen species, new data on floral nectary structure are provided for Malcolmia maritima (L.) R. Br. (Brassicaceae) and Scabiosa columbaria L. (Dipsacaceae). All but the last taxon possessed mesenchymatic floral nectaries with surface stomata. Few clear relationships existed between nectary morphology and various physiological responses to CO₂ or UV-B enrichment, indicating that species responded notwithstanding nectary structure itself. Overall, nectar-solute concentration was least affected by elevated CO₂ or UV-B radiation; consequently, changes in nectar volume were responsible for differences in nectar-sugar production per flower. Three species of Fabaceae experienced no change in floral nectar production upon exposure to elevated CO₂. To date, no study of enhanced UV-B radiation reported a consistent reduction in floral nectar production; three species of Brassicaceae responded differently, but various levels of ozone depletion were simulated. Experimentation with more taxa – including those possessing nectary types such as septal (gynopleural) nectaries (e.g. many monocotyledons) or aggregations of glandular trichomes – and expanding such physiological studies to species possessing extrafloral nectaries, are recommended. Received August 8, 2002; accepted November 23, 2002 Published online: June 2, 2003     

Interaction of Elevated Ultraviolet-B Radiation and CO(2) on Productivity and Photosynthetic Characteristics in Wheat, Rice, and Soybean

Wheat (Triticum aestivum L. cv Bannock), rice (Oryza sativa L. cv IR-36), and soybean (Glycine max [L.] Merr cv Essex) were grown in a factorial greenhouse experiment to determine if CO₂-induced increases in photosynthesis, biomass, and yield are modified by increases in ultraviolet (UV)-B radiation corresponding to stratospheric ozone depletion. The experimental conditions simulated were: (a) an increase in CO₂ concentration from 350 to 650 microliters per liter; (b) an increase in UV-B radiation corresponding to a 10% ozone depletion at the equator; and (c) a and b in combination. Seed yield and total biomass increased significantly with elevated CO₂ in all three species when compared to the control. However, with concurrent increases in UV-B and CO₂, no increase in either seed yield (wheat and rice) or total biomass (rice) was observed with respect to the control. In contrast, CO₂-induced increases in seed yield and total plant biomass were maintained or increased in soybean within the elevated CO₂, UV-B environment. Whole leaf gas exchange indicated a significant increase in photosynthesis, apparent quantum efficiency (AQE) and water-use-efficiency (WUE) with elevated CO₂ in all 3 species. Including elevated UV-B radiation with high CO₂ eliminated the effect of high CO₂ on photosynthesis and WUE in rice and the increase in AQE associated with high CO₂ in all species. Elevated CO₂ did not change the apparent carboxylation efficiency (ACE) in the three species although the combination of elevated CO₂ and UV-B reduced ACE in wheat and rice. The results of this experiment illustrate that increased UV-B radiation may modify CO₂-induced increases in biomass, seed yield and photosynthetic parameters and suggest that available data may not adequately characterize the potential effect of future, simultaneous changes in CO₂ concentration and UV-B radiation.     

Decline in gypsy moth (Lymantria dispar) performance in an elevated CO2 atmosphere depends upon host plant species

Plant species differ broadly in their responses to an elevated CO₂ atmosphere, particularly in the extent of nitrogen dilution of leaf tissue. Insect herbivores are often limited by the availability of nutrients, such as nitrogen, in their host plant tissue and may therefore respond differentially on different plant species grown in CO₂-enriched environments. We reared gyspy moth larvae (Lymantria dispar) in situ on seedlings of yellow birch (Betula allegheniensis) and gray birch (B. populifolia) grown in an ambient (350 ppm) or elevated (700 ppm) CO₂ atmosphere to test whether larval responses in the elevated CO₂ atmosphere were species-dependent. We report that female gypsy moths (Lymantria dispar) reared on gray birch (Betula populifolia) achieved similar pupal masses on plants grown at an ambient or an elevated CO₂ concentration. However, on yellow birch (B. allegheniensis), female pupal mass was 38% smaller on plants in the elevated-CO₂ atmosphere. Larval mortality was significantly higher on yellow birch than gray birch, but did not differ between the CO₂ treatments. Relative growth rate declined more in the elevated CO₂ atmosphere for larvae on yellow birch than for those on gray birch. In preference tests, larvae preferred ambient over elevated CO₂-grown leaves of yellow birch, but showed no preference between gray birch leaves from the two CO₂ atmospheres. This differential response of gypsy moths to their host species corresponded to a greater decline in leaf nutritional quality in the elevated CO₂ atmosphere in yellow birch than in gray birch. Leaf nitrogen content of yellow birch dropped from 2.68% to 1.99% while that of gray birch leaves only declined from 3.23% to 2.63%. Meanwhile, leaf condensed tannin concentration increased from 8.92% to 11.45% in yellow birch leaves while gray birch leaves only increased from 10.72% to 12.34%. Thus the declines in larval performance in a future atmosphere may be substantial and host-species-specific.     

Rising CO2 from historical concentrations enhances the physiological performance of Brassica napus seedlings under optimal water supply but not under reduced water availability

The productivity of many important crops is significantly threatened by water shortage, and the elevated atmospheric CO₂ can significantly interact with physiological processes and crop responses to drought. We examined the effects of three different CO₂ concentrations (historical ~300 ppm, ambient ~400 ppm and elevated ~700 ppm) on physiological traits of oilseed rape (Brassica napus L.) seedlings subjected to well‐watered and reduced water availability. Our data show (1) that, as expected, increasing CO₂ level positively modulates leaf photosynthetic traits, leaf water‐use efficiency and growth under non‐stressed conditions, although a pronounced acclimation of photosynthesis to elevated CO₂ occurred; (2) that the predicted elevated CO₂ concentration does not reduce total evapotranspiration under drought when compared with present (400 ppm) and historical (300 ppm) concentrations because of a larger leaf area that does not buffer transpiration; and (3) that accordingly, the physiological traits analysed decreased similarly under stress for all CO₂ concentrations. Our data support the hypothesis that increasing CO₂ concentrations may not significantly counteract the negative effect of increasing drought intensity on Brassica napus performance.     

Elevated CO2 reduces field decomposition rates of Betula pendula (Roth.) leaf litter

The effect of elevated atmospheric CO₂ and nutrient supply on elemental composition and decomposition rates of tree leaf litter was studied using litters derived from birch (Betula pendula Roth.) plants grown under two levels of atmospheric CO₂ (ambient and ambient +250 ppm) and two nutrient regimes in solar domes. CO₂ and nutrient treatments affected the chemical composition of leaves, both independently and interactively. The elevated CO₂ and unfertilized soil regime significantly enhanced lignin/N and C/N ratios of birch leaves. Decomposition was studied using field litter-bags, and marked differences were observed in the decomposition rates of litters derived from the two treatments, with the highest weight remaining being associated with litter derived from the enhanced CO₂ and unfertilized regime. Highly significant correlations were shown between birch litter decomposition rates and lignin/N and C/N ratios. It can be concluded, from this study, that at levels of atmospheric CO₂ predicted for the middle of the next century a deterioration of litter quality will result in decreased decomposition rates, leading to reduction of nutrient mineralization and increased C storage in forest ecosystems. However, such conclusions are difficult to generalize, since tree responses to elevated CO₂ depend on soil nutritional status.     

The Influence of Increased Solar UV-B Radiation on Magnesium-Deficient Cowpea Seedlings: Changes in the Photosynthetic Characteristics

The influence of increased solar UV-B radiation on the photosynthetic characteristics in cowpea seedlings (Vigna unguiculata) grown at optimal (Mg_s) and low (Mg_d) Mg levels were studied. Both higher UV-B and Mg_d treatments caused significant drops of photochemical activities and net CO₂ uptake rates (P_N). Yet the UV-B-induced decrease in the photosynthetic efficiency was lesser in Mg_d seedlings. The leaf Chl a fluorescence measurements proved that after receiving an enhanced UV-B radiation these seedlings showed a significant enhancement in their variable parts. The PSM oscillation of slow fluorescence kinetics was remarkably altered by both treatments. The P_N also followed a typical inhibitory pattern as seen in photochemical activities. Concentrations of several chloroplast proteins in trifoliate leaves were significantly reduced by Mg_d treatment and unaffected by the other two treatments. Whereas the contents of 43-47 kDa polypeptides in primary leaves were markedly reduced with a maximal effect in Mg_d seedlings, no major difference was noted for combined stress. This revised version was published online in September 2006 with corrections to the Cover Date.     

Changes of epicuticular wax induced by enhanced UV-B radiation impact on gas exchange in Brassica napus

The epicuticular wax covering on plant surface plays important roles in protecting plants against UV radiation. However, the role of epicuticular wax in affecting leaf gas exchange under enhanced ultraviolet-B (UV-B) radiation remains obscure. In the present study, different aged leaves of Brassica napus were used to analyze the responses of crystal structure and chemical constituents of epicuticular wax to UV-B radiation and the effects of such responses on gas exchange indices. Enhanced UV-B radiation significantly decreased the amount of esters in all leaves except the first leaf, amount of secondary alcohols in the second, third and fourth leaves, and amount of primary alcohols in the second and third leaves, while increased the amounts of ketones and aldehydes in the first leaf. Enhanced UV-B level had no significant effect on the amounts of alkanes and total wax in all leaves. Exposure to UV-B radiation resulted in wax fusion on adaxial leaf and stomata opening on abaxial leaf. Fusions of plates and rods on adaxial leaf surface covered most of the stomata, thereby influencing the photosynthesis in the upper mesophyll of leaves. Enhanced UV-B level significantly reduced the net photosynthesis rate (P _N) but increased the stomata conductance (g _s), concentrations of intercellular CO₂ (C _i ), and transpiration rate (E) in all leaves. Both UV-B radiation and the wax fusion induced by enhanced UV-B radiation resulted in different stomata status on abaxial and adaxial leaf surface, causing decrease of P _N, and increase of g _s, C _i and E in leaves.     

Atmospheric carbon dioxide and fertilizer nitrogen effects on radiation interception by rice 1

In order to predict the potential impacts of global change, it is important to understand the impact of increasing global atmospheric [CO₂] on the growth and yield of crop plants. The objectives of this study were to determine the interaction of N fertilization rates and atmospheric [CO₂] on radiation interception and radiation-use efficiency of rice (Oryza sativa L. cv. IR72) grown under tropical field conditions. Rice plants were grown inside open top chambers in a lowland rice field at the International Rice Research Institute in the Philippines at ambient (about 350 μmol mol^-1) or elevated (about 600 μmol mol^-1 during the 1993 wet season and 700 μmol mol^-1 during the 1994 dry season) in combination with three levels of applied N (0, 50 or 100 kg N ha^-1 in the wet season; 0, 90 or 200 kg N ha-1 in the dry season). Light interception was not directly affected by [CO₂], but elevated [CO₂] indirectly increased light interception through increasing total absorbed N. Plant N requirement for radiation interception was similar for rice grown under ambient [CO₂] or elevated [CO₂] treatments. The conversion efficiency of intercepted radiation to dry matter, radiation-use efficiency (RUE), was about 35% greater at elevated [CO₂] than at ambient [CO₂]. The relationship between leaf N and RUE was curvilinear. At ambient [CO₂], RUE was fairly stable across levels of leaf N, but leaf N less than about 2.5% resulted in lower RUE for plants grown with elevated [CO₂] than for plant grown at ambient [CO₂]. Decreased leaf N with increased [CO₂], therefore decreased RUE of rice plants grown at elevated [CO₂]. When predicting responses of rice to elevated [CO₂], RUE should be adjusted with a decrease in leaf N. This revised version was published online in June 2006 with corrections to the Cover Date.     

Exposures to elevated CO2, elevated temperature and enhanced UV-B radiation modify activities of polyphenol oxidase and guaiacol peroxidase and concentrations of chlorophylls,polyamines and soluble proteins in the leaves of Betula pendula seedlings

The activity of polyphenol oxidase (PPO) and guaiacol peroxidase (POD) and the concentrations of chlorophylls, free polyamines and soluble proteins were determined from the leaves of six genotypes of silver birch (Betula pendula Roth) seedlings exposed to short-term elevated carbon dioxide (CO₂), temperature (T), ultraviolet-B irradiation (UV-B, 280-315 nm) and their combinations. Results showed that the activity of PPO in the leaves was low but increased by elevated CO₂ and elevated T. The POD activity varied between the genotypes due to an interactive effect of CO₂ × UV-B. The soluble proteins were clearly decreased by elevated CO₂, but the level of response varied among the genotypes. The concentrations of chl a and total chlorophylls were lower in the leaves treated with elevated CO₂ than in leaves grown at ambient CO₂. An interactive effect of CO₂ × UV-B on the chl a/b ratio was found. Elevated T increased chl b concentration and decreased chl a/b ratio. Temperature treatments also caused variation in the concentrations of chl a, chl b and total chlorophylls among the genotypes. Polyamine analyses showed that the concentrations of putrescine were increased and spermine decreased in leaves treated with elevated T. However, the change in putrescine by elevated T was clearer at ambient CO₂ than in eCO₂ environment (significant effect of T × CO₂). In conclusion, the defensive enzymes, photosynthetic pigments, soluble proteins and growth-regulating polyamines in silver birch leaves were not susceptible to enhanced UV-B radiation. In contrast, all the variables responded to elevated T and/or elevated CO₂, reflecting the enhancive effects of climate change conditions not only on leaf productivity, but also on leaf turn-over rate. Most of these climate-driven changes were not regulated by UV-B radiation.