Stephan Thiel

A Phytotron for Plant Stress Research: How Far Can Artificial Lighting Compare to Natural Sunlight?

Summary Plants have adapted very efficiently to their natural light habitat. Artificial plant illumination, therefore, requires careful design. Not only the quantity of radiation per area or volume (intensity) but also the spectral quality has to match seasonal and diurnal variations of natural global radiation as close as possible. The GSF Research Center has developed a phytotron system especially devoted to plant stress research, where these requirements are of particular importance. The phytotron consists of seven closed chambers (4 walk-in size chambers, two medium and one small sun simulator). Our contribution oudines the basic design of the lighting and presents spectral data. A good approximation of terrestrial global radiation is achieved if several commercially available lamp types are combined and adequate filters are applied to reject unwanted infrared and harmful ultraviolet radiation. A programmable switch control for the individual lamp banks allows a variation of both spectrum and intensity of the illumination. Spectroradiometric measurements show that the maximum level of illumination in the small and in the medium size chambers can compete both in spectral distribution and in intensity with outdoor global radiation for solar elevations up to 60°. The maximum light level available inside the large walk-in chambers reaches an irradiance corresponding to solar elevation of 50°. The UV-B: UV-A: PAR ratio, which mirrors the spectral balance of plant lighting, can be adjusted to values following the diurnal variation of natural global radiation.

MODIFICATION OF GLOBAL ERYTHEMALLY EFFECTIVE IRRADIANCE BY CLOUDS

The role clouds play in the modification of global radiation is still a major uncertainty in the risk assessment of UV effects on ecological systems and human health. This study presents cloud transmission data obtained from measurements with Robertson‐Berger meters and simultaneous cloud observations. The global transmission of erythemally weighted irradiance depends strongly on cloud amount and can be described by a cubic function. The comparison with results derived from long‐term records of total global irradiance indicates no statistically significant difference between the attenuation of ery‐themal and total global radiation. The large variance of data results from lumping together data from different cloud types. Classification of data according to cloud forms yields a more statisfactory fit. The coefficient of the cubic term characterizes the ability of various cloud forms to attenuate UV radiation. It varies between 0.4 for high clouds and approximately 1.0 for cumulonimbus. This attenuation parameter allows a quantitative description of the cloud influence on irradiance and therefore a more accurate risk assessment.

Spectral shaping of artificial UV-B irradiation for vegetation stress research

Summary Ecological plant experiments using artificial light sources require careful shaping of the spectral irradiance. This includes the steep UV-absorption characteristics resulting from the filtering of solar radiation by atmosphefic ozone. Borosilicate and soda-lime glass filters screen radiation very similarly to ozone. They have a high mechanical stability and are available in large filter sheets and are, therefore, suited for the simulation of future scenarios of enhanced solar UV-B radiation in large scale vegetation stress experiments. Although such filters meet many requirements of light engineering, there are limitations due to the slope of the UV-edge and due to solarisation effects. Thus, the interpretation of the artifical radiation spectra and their comparison to UV scenarios of decreasing stratospheric ozone need careful discussion. Different methods to classify spectra of artificial UV-radiation are presented, and a new classification by a cut-off wavelength of the UV-edge and its slope is introduced.

Ozone and Ultraviolet B Effects on the Defense-related Proteins ß-1,3-Glucanase and Chitinase in Tobacco

The air pollutant ozone is a potent abiotic inducer of defense-related enzymes such as pathogenesis-related proteins. Here we report on the accumulation of beta-1,3-glucanase and chitinase in Nicotiana tabacum L. treated with ozone and ultraviolet B radiation, singly and in combination, under a simulated sunlight spectrum. Ozone (0.16 mu L . L(-1), 2 x 5 h) induced the basic isoforms of beta-1,3-glucanase in both, ozone-sensitive (Eel W3) and -tolerant (Bel B) cultivars, while chitinase was only affected in cv. Bel W3. Ultraviolet B radiation (7.5 MED) alone did not lead to beta-1,3-glucanase or chitinase induction. In combined treatments ultraviolet B increased the ozone-dependent lesion formation and reduced chitinase accumulation in the sensitive cv. Bel W3. Analysis of the intercellular washing fluid of ozone-treated plants revealed the accumulation of a major ozone-related protein (O(3)R-1) of 28 kDa within 32 h. Microsequence analysis of two tryptic peptides showed 100 % homology to acidic chitinase PR-3b. These results indicate that basic beta-1,3-glucanase and chitinase are distinctly regulated in ozone and ultraviolet B treated tobacco, and that ultraviolet B radiation with a similar UV edge as the solar spectrum does not lead to an accumulation of basic pathogenesis-related proteins.

INTERCOMPARISON OF SPECTRAL-UV-RADIATION MEASUREMENT SYSTEMS

The results of what is to our knowledge the first intercomparison of seven independent spectroradiometers measuring solar UV irradiances are presented. The intercomparison was carried out in the GSF-Forschungszentrum für Umwelt und Gesundheit, Neuherberg (near Munich, Germany), on 13 July 1990. The spectroradiometric measurements were supplemented by other meteorological, optical, and chemical measurements at the same time. As this day was cloudless, the data can be compared with the measurements taken by Bener in Switzerland in the 1960s and with the results of radiative transfer models. The measured irradiances at noon differed by factors of up to 100. These large differences demonstrate the great difficulties with this type of measurement. Some instrument systems, however, ranged within tolerances of ±10%, thus allowing us to make recommendations for the spectroradiometry of solar UV irradiances.

Measuring Spectral Actinic Flux and Irradiance: Experimental Results from the Actinic Flux Determination from Measurements of Irradiance (ADMIRA) Project

Abstract Results are presented from the Actinic Flux Determination from Measurements of Irradiance (ADMIRA) campaign to measure spectral global UV irradiance and actinic flux at the ground, beneath an atmosphere well defined by supporting measurements. Actinic flux is required to calculate photolysis rates for atmospheric chemistry, yet most spectral UV measurements are of irradiance. This work represents the first part of a project to provide algorithms for converting irradiances to actinic fluxes with specified uncertainties. The campaign took place in northern Greece in August 2000 and provided an intercomparison of UV spectroradiometers measuring different radiation parameters, as well as a comprehensive radiation and atmospheric dataset. The independently calibrated spectroradiometers measuring irradiance and actinic flux agreed to within 5%, while measurements of spectral direct irradiance differed by 9%. Relative agreement for all parameters proved to be very stable during the campaign. A polarizat...

An empirical method for the conversion of spectral UV irradiance measurements to actinic flux data

Routinely monitored radiation parameters, including those at UV wavelengths, most commonly refer to irradiance, that is the radiation incident on a flat horizontal plate. However, in the study of atmospheric chemistry, where UV radiation is a prime photochemical driver, the target molecules are approximately spherical and the actinic flux (radiation on the surface of a sphere) is a better measure of the effective radiation. Unfortunately actinic flux measurements (also known as scalar irradiance) are uncommon research measurements. The ability to convert irradiance data to actinic flux data within a reasonable degree of uncertainty would provide an actinic flux database mapped from existing irradiance databases, thus vastly increasing knowledge of actinic flux variability and climatology. Synchronised spectral UV irradiance and actinic flux measurements have been made during the ADMIRA project, and used to develop an empirical method for converting irradiance to actinic flux. With some prior knowledge of the sky conditions during the irradiance measurements the actinic flux can be estimated to within a few percent. If no knowledge of the sky conditions is available then the empirical method still returns actinic fluxes within 10% of those measured at the site for which the conversion was developed.

Solar radiation at the Earth's surface

Abstract The risk assessment of detrimental effects of solar UV radiation requires a detailed knowledge of the intensity and the spectral composition of global radiation reaching the Earths surface. Both quantities exhibit large variations in time and location. Some of the variations are very regular because they are directly related to the Earths position on her orbit around the sun, to the angle the Earths axis makes with the orbital plane and of course, to the daily rotation around the axis. After entering the atmosphere radiation is modified by various absorption and scattering processes. The changes in atmospheric constituents which are involved in these processes also contribute to the variability of global radiation. The stratospheric ozone layer is surely the most important absorber for shortwave UV radiation. Therefore, its changes — mainly due to human activities — have a direct impact on the amount of UV radiation responsible for sunburn or even skin cancer. In the lower atmosphere, clouds are the main factor which modulate global radiation. The temporal and spatial changes in atmospheric parameters are much less predictable than astronomical and geometrical ones. Local parameters like altitude or surface albedo further modify the ambient UV climate. Our knowledge of the long-term and the short-term dynamics of UV radiation has tremendously increased by recent progress made both in the precision of UV measurements and the mathematical modeling of radiation transport through the atmosphere. This chapter reviews various physical aspects of solar radiation reaching the Earths surface.

Empirical approach to converting spectral UV measurements to actinic flux data

The vast majority of radiation measurements, including UV, refer to the radiation incident on a flat horizontal plate. However, this may not be the most appropriate way to specify radiation for bodies affected by UV, since they are rarely flat or horizontal. In particular the target molecules involved in atmospheric chemistry are approximately spherical and the actinic flux would be a better measure of the incident radiation. The ADMIRA project is addressing the issue of converting spectral UV irradiances to spectral actinic fluxes that can then be weighted with any required cross-section or action spectrum to give photolysis rates or biologically effective radiation incident on a sphere. The success with which this conversion can be made will depend on the prevailing atmospheric conditions and the knowledge of such at the time the irradiance measurements were made. Several different approaches to the conversion are being assessed, together with their associated uncertainties. These range from the simple empirical method to more complex radiative-transfer model based algorithms. Here we report on a coordinated campaign of simultaneous irradiance and actinic flux measurements supported by a wide range of ancillary measurements and their application to a simple empirical approach to converting irradiances to actinic fluxes.

Monitoring the solar UV-B radiation in the North of Munich: A comparison of two sites

Since 2008, measurements of the downwelling solar spectral irradiance from 290 to 400 nm were compared at two field sites at Neuherberg, north of Munich, Germany, 11.6 E, 48.22 N, 490 m above sea level: (1) Research Unit Environmental Simulation (EUS) at the Helmholtz Zentrum Munchen (former GSF National Reasearch Center for Environment and Health), (2) Federal Office for Radiation Protection (BfS). The spectral measurements of two double monochromator systems (TDM300, Bentham, Reading, UK) at 11:00 GMT were analyzed every day. Spectral comparison showed no misalignment of the wavelength calibration. The mean deviation of 1090 measurements at three different wavelengths in the UV-B range (280 – 315 nm) showed coefficients of determination better than 0.94 and a 10% higher value of spectral irradiance of the EUS system, mainly due to different entrance optics and spectral resolution.