(1988). The Role of Chlorophyll Fluorescence in The Detection of Stress Conditions in Plants. C R C Critical Reviews in Analytical Chemistry: Vol. 19, No. sup1, pp. S29-S85.

The Role of Chlorophyll Fluorescence in The Detection of Stress Conditions in Plants

The potential impacts of an increase in solar UV-B radiation reaching the Earth's surface due to stratospheric ozone depletion have been investigated by several research groups during the last 15 years. Much of this research has centered on the effects of plant growth and physiology under artificial UV-B irradiation supplied to plants in growth chambers or greenhouses. Since these artificial sources do not precisely match the solar spectrum and due to the wavelength dependency of photobiol-ogical processes, weighting functions, based on action spectra for specific responses, have been developed to assess the biological effectiveness of the irradiation sources and of predicted ozone depletion. Recent experiments have also utilized artificially produced ozone cuvettes to filter natural solar radiation and simulate an environment of reduced UV-B for comparative purposes.

Overall, the effectiveness of UV-B varies both among species and among cultivars of a given species. Sensitive plants often exhibit reduced growth (plant height, dry weight, leaf area, etc.), photosynthetic activity and flowering. Competitive interactions may also be altered indirectly by differential growth responses. Photosynthetic activity may be reduced by direct effects on photosynthetic enzymes, metabolic pathways or indirectly through effects on photosynthetic pigments or stomatal function. The fluence response of these changes has yet to be clearly demonstrated in most cases. Plants sensitive to UV-B may also respond by accumulating UV-absorbing compounds in their outer tissue layers, which presumably protect sensitive targets from UV damage. Several key enzymes in the biosynthetic pathways of these compounds have been shown to be specifically induced by UV-B irradiation.

Few studies have documented the effects of UV-B on total plant yield under field conditions. One notable exception is a 6-yr study with soybean demonstrating harvestable yield reductions under a simulated 25% ozone depletion. These effects are further modified by prevailing microclimatic conditions. Plants tend to be less sensitive to UV-B radiation under drought or mineral deficiency, while sensitivity increases under low levels of visible light. Further studies are needed to understand the mechanisms of UV-B effects and the interactions with present stresses and future projected changes in the environment.

Uv-b effects on terrestrial plants

The influence of UV-B irradiation on photosystem II activities has been investigated using isolated photosystem II membrane fragments from spinach. It was found: (a) The average amount of DCIP reduced per flash declined drastically with increasing irradiation time in the absence of DPC but remained almost unaffected in its presence, (b) After UV-B irradiation, the maximum amplitude of laser flash induced 830 nm absorption changes decreases only slightly; whereas the relaxation kinetics exhibit marked effects: the (JLS components dominate the decay at the expense of ns components. The γ.s kinetics already arise after illumination with a single flash of dark adapted samples, (c) The manganese content decreases only partly at irradiation times where the oxygen evolution capacity is almost completely lost, (d) The polypeptide pattern is hardly affected; the number of atrazine binding sites markedly decreases. Based on the results of this study, UV-B irradiation is inferred to deteriorate primarily the function of water oxidation. The action spectrum of the UV-B effect does not reveal a specific target molecule. It is assumed that structural changes of the D-l/D-2 polypeptide matrix are responsible for the modification by UV-B irradiation of the capacity of water oxidation and atrazine binding.

On the mechanism of photosystem ii deterioration by uv-b irradiation

43 Adverse Effects of UV-B Light on the Structure and Function of the Photosynthetic Apparatus

As depletion of the stratospheric ozone layer continues, the biosphere will most likely be exposed to higher levels of ultraviolet-B (UV-B) irradiation (290–320nm). For plants, damage from UV-B can occur at several molecular targets with the photosynthetic apparatus being especially vulnerable. We are interested both in the mechanisms of UV-B-induced damage and identifying adaptation processes that can confer protection from UV-B. Toward this end, Brassica napus (oil seed rape) plants grown under visible light plus a low level of UV-B radiation (adapted plants) were compared to plants grown under visible light alone (control plants). Relative to the control plants, the adapted plants showed little evidence of damage at the levels of morphology or photosynthesis, indicating that B. napus has some tolerance of UV-B and that the plants may have protection mechanisms. Consistent with this, a strong UV-B adaptation process was observed in the plants-accumulation of flavonoids in the epidermis. These pigments seemed to screen a molecular target in the mesophyll. Namely, the D1 photosystem II reaction center protein, which is rapidly degraded in UV-B, was partially protected from degradation in UV-B in the adapted plants. Moreover, the extent that the half-life of the D1 protein increased in the adapted plants was on par with the elevation in total flavonoid concentrations. These experiments demonstrate that degradation of the D1 protein can be used as an in vivo assay of penetration of UV-B photons to the mesophyll.

PROTECTION OF THE D1 PHOTOSYSTEM II REACTION CENTER PROTEIN FROM DEGRADATION IN ULTRAVIOLET RADIATION FOLLOWING ADAPTATION OF Brassica napus L. TO GROWTH IN ULTRAVIOLET-B

Abstract The effect of UV-B-radiation on PSII activity of spinach chloroplasts was analyzed by measuring the integrity of the herbicide-binding protein (HBP 32), by measurement of fluorescence induction in the presence of Diuron (DCMU), and by mathematical analysis of the fluorescence induction curves. It was shown that UV-B inactivates the PSII α-centers but not PSII β-centers. However, the possibility cannot be excluded that in addition the donor site of PSII near the reaction center is attacked by UV-B-radiation.

M. Tevini

Papers

Inhibition of photosystem II by UV-B-radiation