Acclimation and Compensating Metabolite Responses to UV-B Radiation in Natural and Transgenic Populus spp. Defective in Lignin Biosynthesis.

Plants have evolved to protect leaf mesophyll tissue from damage caused by UV-B radiation by producing an array of UV-absorbing secondary metabolites. Flavonoids (phenolic glycosides) and sinapate esters (hydroxycinnamates) have been implicated as UV-B protective compounds because of the accumulation in the leaf epidermis and the strong absorption in the wavelengths corresponding to UV. Environmental adaptations by plants also generate a suite of responses for protection against damage caused by UV-B radiation, with plants from high elevations or low latitudes generally displaying greater adaptation or tolerance to UV-B radiation. In an effort to explore the relationships between plant lignin levels and composition, the origin of growth elevation, and the hierarchical synthesis of UV-screening compounds, a collection of natural variants as well as transgenic Populus spp. were examined for sensitivity or acclimation to UV-B radiation under greenhouse and laboratory conditions. Noninvasive, ecophysiological measurements using epidermal transmittance and chlorophyll fluorescence as well as metabolite measurements using UPLC-MS generally revealed that the synthesis of anthocyanins, flavonoids, and lignin precursors are increased in Populus upon moderate to high UV-B treatment. However, poplar plants with genetic modifications that affect lignin biosynthesis, or natural variants with altered lignin levels and compositions, displayed complex changes in phenylpropanoid metabolites. A balance between elevated metabolic precursors to protective phenylpropanoids and increased biosynthesis of these anthocyanins, flavonoids, and lignin is proposed to play a role in the acclimation of Populus to UV-B radiation and may provide a useful tool in engineering plants as improved bioenergy feedstocks.

Photosynthesis, fluorescence, and biomass responses of white oak seedlings to urban soil and air temperature effects.

Urban forest patches can provide critical ecosystem services and their ability to regenerate native tree species is critical to their sustainability. Little is known about native tree seedling establishment and physiological function in urban ecosystems. This growth chamber study examined the effects of urban soil and air temperatures on white oak (Quercus alba L.) germination, seedling growth, and leaf-level physiology. A split-plot design tested effects of field collected soils from urban and reference forest sites in Baltimore, Maryland, and warm (urban) versus cool (rural) growth chamber temperature regimes. Seedlings were harvested at the end of the 23-week experiment to assess foliar chemistry and biomass allocation. Seed germination was unaffected by treatments and was high in both soil types and temperature regimes. Urban soils supported significantly higher total seedling biomass and had a significant effect on leaf-level physiological parameters, with seedlings grown in urban soils having greater Anet , Vcmax , ETRmax , Jmax , PNUE, gs , Anet /Rd , and PIabs (an integrated chlorophyll fluorescence parameter). PIabs measurements taken throughout the experiment revealed a significant time × temperature interaction effect. Baltimore urban forest patch soils were higher in nutrients than reference soils, but also higher in heavy metals. Despite higher levels of heavy metals, these results demonstrate that urban forest patch soils are able to support robust white oak seedling growth and enhanced seedling physiological parameters. However, interactions with temperature suggest that warming air temperatures may cause seedling stress and reduced growth.

Chlorophyll fluorescence parameters, leaf traits and foliar chemistry of white oak and red maple trees in urban forest patches.

The provisioning of critical ecosystem services to cities of the eastern USA depends on the health and physiological function of trees in urban areas. Although we know that the urban environment may be stressful for trees planted in highly developed areas, it is not clear that trees in urban forest patches experience the same stressful environmental impacts. In this study, we examine chlorophyll fluorescence parameters, leaf traits, foliar nutrients and stable isotope signatures of urban forest patch trees compared with trees growing at reference forest sites, in order to characterize physiological response of these native tree species to the urban environment of three major cities arranged along a latitudinal gradient (New York, NY; Philadelphia, PA; Baltimore, MD). Overall, white oaks (Quercus alba L.) show more differences in chlorophyll fluorescence parameters and leaf traits by city and site type (urban vs reference) than red maples (Acer rubrum L.). The exceptions were δ13C and δ15N, which did not vary in white oak foliage but were significantly depleted (δ13C) and enriched (δ15N) in urban red maple foliage. Across all sites, red maples had higher thermal tolerance of photosynthesis (Tcrit) than white oaks, suggesting a greater ability to withstand temperature stress from the urban heat island effect and climate change. However, the highest average values of Tcrit were found in the Baltimore urban white oaks, suggesting that species suitability and response to the urban environment varies across a latitudinal gradient. Stomatal pore index (SPI) showed inter-specific differences, with red maple SPI being higher in urban trees, whereas white oak SPI was lower in urban trees. These results demonstrate that differences in native tree physiology occur between urban and reference forest patches, but they are site- and species-specific. Data on local site characteristics and tree species performance over time remain necessary to gain insight about urban woodland ecosystem function.

Phenylalanine is required to promote specific developmental responses and prevents cellular damage in response to ultraviolet light in soybean (Glycine max) during the seed-to-seedling transition.

UV-radiation elicits a suite of developmental (photomorphogenic) and protective responses in plants, but responses early post-germination have received little attention, particularly in intensively bred plants of economic importance. We examined germination, hypocotyl elongation, leaf pubescence and subcellular responses of germinating and/or etiolated soybean (Glycine max (L.) Merr.) seedlings in response to treatment with discrete wavelengths of UV-A or UV-B radiation. We demonstrate differential responses of germinating/young soybean seedlings to a range of UV wavelengths that indicate unique signal transduction mechanisms regulate UV-initiated responses. We have investigated how phenylalanine, a key substrate in the phenylpropanoid pathway, may be involved in these responses. Pubescence may be a key location for phenylalanine-derived protective compounds, as UV-B irradiation increased pubescence and accumulation of UV-absorbing compounds within primary leaf pubescence, visualized by microscopy and absorbance spectra. Mass spectrometry analysis of pubescence indicated that sinapic esters accumulate in the UV-irradiated hairs compared to unirradiated primary leaf tissue. Deleterious effects of some UV-B wavelengths on germination and seedling responses were reduced or entirely prevented by inclusion of phenylalanine in the growth media. Key effects of phenylalanine were not duplicated by tyrosine or tryptophan or sucrose, nor is the specificity of response due to the absorbance of phenylalanine itself. These results suggest that in the seed-to-seedling transition, phenylalanine may be a limiting factor in the development of initial mechanisms of UV protection in the developing leaf.

Reviewing the technical designs for experiments with ultraviolet-B radiation and impact on photosynthesis, DNA and secondary metabolism.

The ultraviolet-B (UV-B) portion of sunlight has received much attention in the last three decades, because radiation from this spectral region increases due to the stratospheric ozone depletion, which results from increases of chlorofluorocarbons in the atmosphere. Plant responses to UV-B exposure vary greatly and the interpretation of and comparison between studies is hindered, mainly by the contrasting experimental conditions used and interactive factors such as low light levels and possible artifacts due to the artificial experimental conditions. It seems likely that increases in solar UV-B radiation of the magnitude anticipated under current stratospheric ozone projections will not significantly inhibit photosynthesis and cause DNA damage in plants. This is in part due to the well-evolved protection mechanisms present in most plant species. One of the significant plant responses to UV-B is changes in foliar secondary chemistry, which could be translated into significant effects at higher trophic levels through plant-herbivore interactions and decomposition. Enhanced UV-B radiation due to stratospheric ozone depletion could also cause morphological changes that would affect competitive interactions, especially if contrasting UV-B sensitivity exists among the competitors.

Impact of solar ultraviolet-B on the proteome in soybean lines differing in flavonoid contents.

Two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) was used to systematically investigate the impact of solar ultraviolet-B (UV-B) radiation on the soybean leaf proteome. In order to investigate the protective role of flavonoids against UV-B, two isolines of the Clark cultivar (the standard line with moderate levels of flavonoids and the magenta line with reduced flavonoids) were grown in the field with or without natural levels of UV-B. The 12-day-old first trifoliates were harvested for proteomic analysis. More than 300 protein spots were reproducibly resolved and detected on each gel. Statistical analysis showed that 67 protein spots were significantly (P<0.05) affected by solar UV-B. Many more spots were altered by UV-B in the magenta line than in the standard line. Another 12 protein spots were not altered by UV-B but showed significantly (P<0.05) different accumulations between the two lines, and for most spots the line-specific differences were also observed under UV-B exclusion. Most of the differentially accumulated spots were identified by mass spectrometry. The proteins were quite diverse, and were involved in metabolism, energy, protein destination/storage, protein synthesis, disease/defense, transcription, and secondary metabolism. The results suggest that high levels of flavonoids lead to a reduction in UV-B sensitivity at the proteomic level.

Coupling short-term changes in ambient UV-B levels with induction of UV-screening compounds.

A substantial number of studies have been conducted over the last several decades to assess the potential impacts of long-term increases in ultraviolet-B radiation (UV-B between 280 and 320 nm) that will result from continued depletion of stratospheric ozone. However, seasonal changes, tropospheric chemistry and cloudiness are the dominant factors controlling ambient UV-B levels on a short-term or daily basis. The effects of short-term changes in UV-B on plant growth, phytochemistry and physiological processes have received relatively little attention. The USDA UV-B Monitoring and Research Program provides an excellent network of stations that provide an opportunity to monitor long-term changes in solar UV-B radiation and evaluate the responses of plants to short-term variation in UV-B levels on a near-real-time basis. In this study barley (Hordeum vulgare L.) and soybean (Glycine max [L] Merr.) were used as model systems. Emerging seedlings of these species were grown under either near-ambient levels of UV-B or under reduced levels (ca 90% reduction) in the field. Periodic measurements of foliar UV-screening compounds were made on separate groups of seedlings planted at intervals over the growing season during contrasting periods of ambient levels of UV radiation. The levels of UV-screening compounds correlated with UV-B levels in both species and with UV-A in soybean but the sensitivity of the response differed between the two species and among the soybean cultivars. Response differences among species may be related to unique secondary chemistry of each species, so one response estimate or action spectrum may not be appropriate for all species.

Possible impacts of changes in UV-B radiation on North American trees and forests.

Approximately 35 species representing 14 tree genera have been evaluated for responses to UV-B radiation in North America. The best representation has been in the conifers where some 20 species representing three genera have been studied. Overall, about 1/3 of these have demonstrated some deleterious response to UV-B. However, most negative impacts have been observed under controlled environment conditions where sensitivity may be enhanced. Therefore, it seems unlikely that expected levels of ozone depletion will result in direct losses in productivity. However, the role that ambient or enhanced levels of UV-B may play in forest ecosystem processes is more difficult to access. One possible indirect response of forests to changes in UV-B radiation levels could be via alterations in plant secondary metabolites. Increases in phenolics and flavonoids that enhance epidermal UV-screening effectiveness may also influence leaf development, water relations or ecosystem processes such as plant-herbivore interactions or decomposition.

Variability in leaf-level CO(2) and water fluxes in Pinus banksiana and Picea mariana in Saskatchewan.

We measured seasonal and canopy-level gas exchange in two stands of jack pine (Pinus banksiana Lamb.) and one stand of black spruce (Picea mariana (Mill.) B.S.P.) on relatively clear days from late May until mid-September 1994. Field measurements were made with a portable infrared gas analyzer, and laboratory measurements included photosynthetic oxygen evolution and needle chemical composition. Seasonally averaged light-saturated assimilation rates in the field were 4.0 micro mol m(-2) s(-1) in jack pine and 2.7 micro mol m(-2) s(-1) in black spruce. Rates of assimilation and transpiration were highest in midsummer. The seasonal pattern was especially pronounced for black spruce, probably because cold soil temperatures limited early season gas exchange rates in this species. Among stands, instantaneous water-use efficiency was highest in a young jack pine stand early in the season and higher in the upper canopy foliage than in the lower canopy foliage at all sites at the end of the season. Needles of young jack pine exhibited higher photosynthetic capacity, dark respiration and needle N concentrations than needles of trees at the old site. In both species, slight acclimation to shading was manifested by reductions in photosynthetic capacity in the lower canopy foliage. In both species, first-year needles had greater photosynthetic capacity than older needles but in situ rates of CO(2) assimilation in the field showed little difference among needle age classes. In both species, there was a strong correlation between assimilation and stomatal conductance, indicating that assimilation was highly stomatal limited and that environmental factors that alter conductance (e.g., VPD) have a strong influence on CO(2) and water fluxes, especially after early season thawing concludes.