Effects of UV-B radiation on photosynthesis and growth of terrestrial plants.

The photosynthetic apparatus of some plant species appears to be well-protected from direct damage from UV-B radiation. Leaf optical properties of these species apparently minimizes exposure of sensitive targets to UV-B radiation. However, damage by UV-B radiation to Photosystem II and Rubisco has also been reported. Secondary effects of this damage may include reductions in photosynthetic capacity, RuBP regeneration and quantum yield. Furthermore, UV-B radiation may decrease the penetration of PAR, reduce photosynthetic and accessory pigments, impair stomatal function and alter canopy morphology, and thus indirectly retard photosynthetic carbon assimilation. Subsequently, UV-B radiation may limit productivity in many plant species. In addition to variability in sensitivity to UV-B radiation, the effects of UV-B radiation are further confounded by other environmental factors such as CO2, temperature, light and water or nutrient availability. Therefore, we need a better understanding of the mechanisms of tolerance to UV-B radiation and of the interaction between UV-B and other environmental factors in order to adequately assess the probable consequences of a change in solar radiation.

The Role of Flavonol Glycosides and Carotenoids in Protecting Soybean from Ultraviolet-B Damage.

The increase in ultraviolet-B (UV-B; 0.290-0.320 [mu]m) radiation received by plants due to stratospheric ozone depletion heightens the importance of understanding UV-B tolerance. Photosynthetic tissue is believed to be protected from UV-B radiation by UV-B-absorbing compounds (e.g. flavonoids). Although synthesis of flavonoids is induced by UV-B radiation, its protective role on photosynthetic pigments has not been clearly demonstrated. This results in part from the design of UV-B experiments in which experimental UV-A irradiance has not been carefully controlled, since blue/UV-A radiation is involved in the biosynthesis of the photosynthetic pigments. The relationship of flavonoids to photosynthetic performance, photosynthetic pigments, and growth measures was examined in an experiment where UV-A control groups were included at two biologically effective daily UV-B irradiances, 14.1 and 10.7 kJ m-2. Normal, chlorophyll-deficient, and flavonoid-deficient pigment isolines of two soybean (Glycine max) cultivars that produced different flavonol glycosides (Harosoy produced kaempferol, Clark produced quercetin and kaempferol) were examined. Plants with higher levels of total flavonoids, not specific flavonol glycosides, were more UV-B tolerant as determined by growth, pigment, and gas-exchange variables. Regression analyses indicated no direct relationship between photosynthesis and leaf levels of UV-B-absorbing compounds. UV-B radiation increased photosynthetic pigment content, along with UV-B-absorbing compounds, but only the former (especially carotenoids) was related to total biomass (r2 = 0.61, linear) and to photosynthetic efficiency (negative, exponential relationship, r2 = 0.82). A reduction in photosynthesis was associated primarily with a stomatal limitation rather than photosystem II damage. This study suggests that both carotenoids and flavonoids may be involved in plant UV-B photoprotection, but only carotenoids are directly linked to photoprotection of photosynthetic function. These results additionally show the importance of UV-A control in UV-B experiments conducted using artificial lamps and filters.

The effects of ultraviolet-B radiation on photosynthesis of different aged needles in field-grown loblolly pine.

We examined the effect of supplemental UV-B radiation (290-320 nm) on photosynthetic characteristics of different aged needles of 3-year-old, field-grown loblolly pine (Pinus taeda L.). Needles in four age classes were examined: I, most recently fully expanded, year 3; II, first flush, year 3; III, final flush, year 2; and IV, oldest needles still present, year 2. Enhanced UV-B radiation caused a statistically significant decrease (6%) in the ratio of variable to maximum fluorescence (F(v)/F(m)) following dark adaptation only in needles from the youngest age class, suggesting transient damage to photosynthesis. However, no effects of enhanced UV-B radiation on other instantaneous measures of photosynthesis, including maximum photosynthesis, apparent quantum yield and dark respiration, were seen for needles of any age. Foliar nitrogen concentration was unaffected by UV-B treatment. However, the (13)C/(12)C carbon isotope ratios (delta(13)C-a time integrated measure of photosynthetic function) of needles in age classes II and IV were 3% (P < 0.01) and 2% (P < 0.05) more negative, respectively, in treated plants than in control plants. Exposure to enhanced UV-B radiation caused a 20% decrease in total biomass and a 4% (P < 0.05), 25% (P < 0.01), and 9% (P < 0.01) decrease in needle length of needles in age classes I, II, and IV, respectively. The observed decreases in delta(13)C, and F(v)/F(m) of the needles in the youngest needle age class suggest subtle damage to photosynthesis, although overall growth reductions were probably a result of decreased total leaf surface rather than decreased photosynthetic capacity. Needles of age class IV had lower light- and CO(2)-saturated maximum photosynthetic rates (39%), lower dark respiration (34%), lower light saturation points (37%), lower foliar nitrogen concentration (28%), and lower delta(13)C (14%) values than needles of age class I. Apparent quantum yield and F(v)/F(m) did not change with needle age. The observed changes in photosynthesis and foliage chemical composition with needle age are consistent with previous studies of coniferous trees and may represent adaptations of older needles to shaded conditions within the canopy.

Effects of below- and aboveground competition from the vines Lonicera japonica and Parthenocissus quinquefolia on the growth of the tree host Liquidambar styraciflua.

Detrimental effects of vines on tree growth in successional environments have been frequently reported. Little is known, however, about the relative importance of below and aboveground competition from vines on tree growth. The objective of this study was to quantify and compare the growth responses of Liquidambar styraciflua saplings to below and/or aboveground competition with the exotic evergreen vine, Lonicera japonica (Japanese honeysuckle), and the native deciduous vine, Parthenocissus quinquefolia (Virginia creeper). Soil trenching and/or vine-trellising were used to control the type of vine competition experienced by trees. Comparisons among untrenched treatments tested for effects of belowground competition. Comparisons among trenched treatments tested for effects of aboveground competition. After two growing seasons, Lonicera japonica had a greater effect on the growth of L. styraciflua than did P. quinquefolia. This effect was largely due to root competition, as canopy competition only had a negative effect on tree growth when it occurred in combination with root competition. Leaf expansion was consistently and similarly affected by all treatments which involved belowground competition.

CO(2) Enhancement of Growth and Photosynthesis in Rice (Oryza sativa) : Modification by Increased Ultraviolet-B Radiation.

Two cultivars of rice (Oryza sativa L.) IR-36 and Fujiyama-5 were grown at ambient (360 microbars) and elevated CO(2) (660 microbars) from germination through reproduction in unshaded greenhouses at the Duke University Phytotron. Growth at elevated CO(2) resulted in significant decreases in nighttime respiration and increases in photosynthesis, total biomass, and yield for both cultivars. However, in plants exposed to simultaneous increases in CO(2) and ultraviolet-B (UV-B) radiation, CO(2) enhancement effects on respiration, photosynthesis, and biomass were eliminated in IR-36 and significantly reduced in Fujiyama-5. UV-B radiation simulated a 25% depletion in stratospheric ozone at Durham, North Carolina. Analysis of the response of CO(2) uptake to internal CO(2) concentration at light saturation suggested that, for IR-36, the predominant limitation to photosynthesis with increased UV-B radiation was the capacity for regeneration of ribulose bisphosphate (RuBP), whereas for Fujiyama-5 the primary photosynthetic decrease appeared to be related to a decline in apparent carboxylation efficiency. Changes in the RuBP regeneration limitation in IR-36 were consistent with damage to the photochemical efficiency of photosystem II as estimated from the ratio of variable to maximum chlorophyll fluorescence. Little change in RuBP regeneration and photochemistry was evident in cultivar Fujiyama-5, however. The degree of sensitivity of photochemical reactions with increased UV-B radiation appeared to be related to leaf production of UV-B-absorbing compounds. Fujiyama-5 had a higher concentration of these compounds than IR-36 in all environments, and the production of these compounds in Fujiyama-5 was stimulated by UV-B fluence. Results from this study suggest that in rice alterations in growth or photosynthesis as a result of enhanced CO(2) may be eliminated or reduced if UV-B radiation continues to increase.

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 CO(2)-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 CO(2) 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 CO(2) in all three species when compared to the control. However, with concurrent increases in UV-B and CO(2), no increase in either seed yield (wheat and rice) or total biomass (rice) was observed with respect to the control. In contrast, CO(2)-induced increases in seed yield and total plant biomass were maintained or increased in soybean within the elevated CO(2), UV-B environment. Whole leaf gas exchange indicated a significant increase in photosynthesis, apparent quantum efficiency (AQE) and water-use-efficiency (WUE) with elevated CO(2) in all 3 species. Including elevated UV-B radiation with high CO(2) eliminated the effect of high CO(2) on photosynthesis and WUE in rice and the increase in AQE associated with high CO(2) in all species. Elevated CO(2) did not change the apparent carboxylation efficiency (ACE) in the three species although the combination of elevated CO(2) and UV-B reduced ACE in wheat and rice. The results of this experiment illustrate that increased UV-B radiation may modify CO(2)-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 CO(2) concentration and UV-B radiation.

Field Study of the Interaction between Solar Ultraviolet-B Radiation and Drought on Photosynthesis and Growth in Soybean.

Soybean, Glycine max (L.) Merr. cv Essex, plants were grown in the field in a 2 x 2 factorial design, under ambient and supplemental levels of ultraviolet-B (UV-B) radiation (supplemental daily dose of 5.1 effective kilojoules per square meter) and were either well-watered or subjected to drought. Soil water potentials were reduced to -2.0 megapascals by the exclusion of natural precipitation in the drought plots and were maintained at approximately -0.5 megapascal by supplemental irrigation in well-watered plots. Plant growth and gas exchange characteristics were affected under both drought and supplemental UV-B radiation. Whole-leaf gas exchange analysis indicated that stomatal limitations on photosynthesis were only significantly affected by the combination of UV-B radiation and drought but substrate (ribulose bisphosphate) regeneration limitations were observed under either stress. The combined effect of both drought and UV-B radiation on photosynthetic gas exchange was a reduction in apparent quantum efficiency and the rapid appearance of biochemical limitations to photosynthesis concomitant with reduced diffusional limitations. However, the combination of stresses did not result in additive effects on total plant growth or seed yield compared to reductions under either stress independently.

The changing solar ultraviolet climate and the ecological consequences for higher plants.

There is compelling evidence that a general erosion of the global ozone layer is occurring. Since ozone in the stratosphere absorbs much of the shortwave solar ultraviolet radiation (UV-B), diminished ozone means that more UV-B of a very specific wavelength composition will be received at the earth's surface. Evaluating the implications for vegetation involves consideration of the wavelength specificity of biological photochemical reactions and their sensitivity to the extant and future solar spectrum. Recent research suggests the occurrence of direct damaging reactions and of indirect morphological and chemical responses with implications at the community and ecosystem levels.

Field water relations of three temperate vines.

A two year comparative field water relations study was conducted in central Maryland on three sympatric temperate lianas, Lonicera japonica, Vitis vulpina and Parthenocissus quinquefolia. Seasonal physiological activity was longer by approximately 9 weeks in the evergreen L. japonica, while peak rates of stem elongation were 4-10 fold higher in the two deciduous species. There were marked differences in vascular anatomy and water use patterns among the three species, however all three evidenced varying degrees of stomatal closure in response to decreasing soil water availability and increasing atmospheric evaporative demand. The range of leaf water potentials measured in these species was quite narrow in comparison to other temperate woody species. Two of the species showed no alterations in their tissue water release properties in response to decreased soil water availability, while V. vulpina showed a limited capacity in this regard. Most significant among the species differences in water relations were the conservative water use patterns of P. quinquefolia, and the midday maxima of transpirational water loss measured in L. japonica compared to the morning peaks in traspiration for the two deciduous species. The differences found in anatomy, leaf phenology, climbing mechanics, water relations and canopy development among these three sympatric vines implies a spatial and temporal partitioning of light and water resources and emphasizes the diversity of morphological-physiological suites of characters present among species co-occurring in the same macrohabitat.

Field photosynthesis, microclimate and water relations of an exotic temperate liana, Pueraria lobata, kudzu.

Kudzu occurs in a variety of habitats in the southeastern United States. It is most common in exposed, forest edge sites and road cuts where it forms an extensive ground canopy as well as a canopy overtopping nearby trees, but it can also be found in completely open fields and deeply shaded sites within a forest. Microclimate, stomatal conductance, leaf water potential and photosynthetic responses to light, temperature and humidity were measured in two contrasting microhabitats on Pueraria lobata, kudzu. Midsummer leaf temperatures and leaf-to-air water vapor deficits for plants growing in an exposed site were significantly greater than for those in a shaded site, exceeding 35° C and 50 mmol mol-1, respectively. Maximum stomatal conductance exceeded 400 mmol m-2 s-1 in exposed leaves during peak vegetative growth. Stomatal conductance in shaded leaves was approximately half the value measured in exposed leaves on any particular dya. Maximum photosynthetic carbon uptake was also higher in leaves growing in exposed sites compared to leaves in shaded sites, exceeding 18.7 and 14.0 µmol m-2 s-1, respectively. Photosynthesis, stomatal conductance and intercellular CO2 concentration decreased dramatically in response to increasing water vapor deficit for leaves from both sites. However, transpiration showed an initial increase at intermediate water vapor deficits, leveling off or even decreasing at higher values. Leaf water potential demonstrated marked diurnal variation, but remained constant over a wide range of transpirational water fluxes. This latter feature, combined with microenvironmental modification through rapid leaf orientation and pronounced stomatal responses to water vapor deficits may represent important adaptive responses in the exploitation of a diverse array of habitats by kudzu.

Effects of Ultraviolet-B Irradiance on Soybean : V. The Dependence of Plant Sensitivity on the Photosynthetic Photon Flux Density during and after Leaf Expansion.

Soybeans (Glycine max [L.] Merr. cv Essex) were grown in a green-house, and the first trifoliate leaf was either allowed to expand under a high photosynthetic photon flux density (PPFD) (1.4 millimoles per square meter per second) or a low PPFD (0.8 millimoles per square meter per second). After full leaf expansion, plants from each treatment were placed into a factorial design experiment with two levels of ultraviolet-B (UV-B) radiation (0 and 80 milliwatts per square meter biologically effective UV-B) and two levels of concomitant PPFD (0.8 and 1.4 millimoles per square meter per second) resulting in a total of eight treatments. Measurements of net photosynthesis and the associated diffusion conductances, ribulose-1,5-bisphosphate carboxylase activity, chlorophyll and flavonoid concentrations, and leaf anatomy were examined for all treatments. Leaves expanded in the high PPFD were unaffected by UV-B radiation while those expanded in the low PPFD were sensitive to UV-B-induced damage. Likewise, plants which were UV-B irradiated concomitantly with the high PPFD were resistant to UV-B damage, while plants irradiated under the low PPFD were sensitive. The results of this study indicate that both anatomical/morphological and physiological/biochemical factors contribute toward plant sensitivity to UV-B radiation.

Effects of Ultraviolet-B Irradiances on Soybean: II. INTERACTION BETWEEN ULTRAVIOLET-B AND PHOTOSYNTHETICALLY ACTIVE RADIATION ON NET PHOTOSYNTHESIS, DARK RESPIRATION, AND TRANSPIRATION.

Soybean plants (cv. Hardee) were grown from seed under four ultraviolet-B radiation flux densities and four photosynthetically active radiation levels in a factorial design. Net photosynthesis, dark respiration, and transpiration were measured after 2 and 6 weeks of exposure. Effects of ultraviolet-B radiation were dependent upon photosynthetically active radiation levels. Ultraviolet-B radiation adversely affected net photosynthesis at low photosynthetically active radiation levels, but had little consequence at levels normally saturating photosynthesis in the field. Ultraviolet-B radiation affected both stomatal and nonstomatal resistances to carbon dioxide under low levels of photosynthetically active radiation. The present study demonstrates interactions between ultraviolet-B and photosynthetically active radiation.