(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

Predictions based on evolutionary theory suggest that the adaptive value of evolved herbicide resistance alleles may be compromised by the existence of fitness costs. There have been many studies quantifying the fitness costs associated with novel herbicide resistance alleles, reflecting the importance of fitness costs in determining the evolutionary dynamics of resistance. However, many of these studies have incorrectly defined resistance or used inappropriate plant material and methods to measure fitness. This review has two major objectives. First, to propose a methodological framework that establishes experimental criteria to unequivocally evaluate fitness costs. Second, to present a comprehensive analysis of the literature on fitness costs associated with herbicide resistance alleles. This analysis reveals unquestionable evidence that some herbicide resistance alleles are associated with pleiotropic effects that result in plant fitness costs. Observed costs are evident from herbicide resistance-endowing amino acid substitutions in proteins involved in amino acid, fatty acid, auxin and cellulose biosynthesis, as well as enzymes involved in herbicide metabolism. However, these resistance fitness costs are not universal and their expression depends on particular plant alleles and mutations. The findings of this review are discussed within the context of the plant defence trade-off theory and herbicide resistance evolution.

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Fitness costs associated with evolved herbicide resistance alleles in plants.

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

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.

Inhibition of photosystem II by UV-B-radiation

The decay of chlorophyll variable fluorescence after a “single turnover” flash is generally assumed to represent the reoxidation of the reduced quinone Qa. We have observed that the kinetics of this decay are very similar in a wide variety of species. Comparing 28 different species, we found an average half decay time of 314 ± 46 μsec. No systematic correlations were found between the decay rate and biochemical or physiological specializations such as C2, C4 or CAM. This indicates that structural as well as functional factors controlling photosystem II electron transfer between Qa and Qb are highly conserved. Apparently, the freedom for natural structural variations in this region is very limited. Triazine resistant plants, characterized by an altered amino acid sequence of the D1 protein, have clearly decreased rates of Qa/Qb electron transfer. We found an average half decay time of 946 ± 100 (isec (5 species). However, this three-fold decrease is much less than previously reported. Therefore, if alterations of photosystem II electron transfer efficiency contributes to an often reported reduction of “ecological fitness”, this contribution is smaller than was hitherto assumed.

Conserved kinetics at the reducing side of reaction-center II in photosynthetic organisms; changed kinetics in triazine-resistant weeds.

Quinone derivates of the formula I below are capable of protecting cultivated plants against the phytotoxic effect of herbicides and are therefore used together with herbicides for selective weed control in crops. The quinone derivatives are those of the formula I: where n is a number from 1-8, R is hydrogen, or in each case two adjacent R radicals together are the bridge members required for forming an aromatic, mono- or bicyclic ring which can also contain a nitrogen atom instead of a -CH= group, while each R is hydroxyl, halogen, cyano, C1-C18-alkyl, C3-C18-alkenyl, C3-C18- alkynyl, C1-C18-alkoxy, C3-C18-alkenyloxy, C3-C18-alkynyloxy, C1-C18- alkylthio, C3-C18-alkenylthio, C1-C18-alkanoyloxy, C3-C18-alkenoyloxy, C1-C18-alkoxycarbonyl, C3-C18-alkenyloxycarbonyl, carboxy-C1-C18-alkyl, C1-C18-alkoxycarbonyl-C1-C18-alkyl, C3-C18-alkenylcarbonyl-C1-C18-alkyl, amino, C1-C18-alkylamino, di-(C1-C18-alkyl)-amino, carbamoyl, C1-C18- alkylcarbamoyl, di-(C1-C18-alkyl)-phosphonylamido, or a phenyl, phenoxy, phenylthio, benzoyl or benzoyloxy radical which is unsubstituted or substituted by halogen, cyano, nitro, amino, carboxyl, C1-C4-alkylthio or C1-C4-alkoxycarbonyl.

Klaus Dr. Pfister

Papers

Inhibition of photosystem II by UV-B-radiation

Conserved kinetics at the reducing side of reaction-center II in photosynthetic organisms; changed kinetics in triazine-resistant weeds.

Use of quinone derivatives for the protection of cultivated plants against the phytotoxic effects of herbicides