Chlorophyll fluorescence yield changes as a tool in plant physiology I. The measuring system

Different responses of two Chinese cabbage (Brassica rapa L. ssp. pekinensis) cultivars in photosynthetic characteristics and chloroplast ultrastructure to salt and alkali stress

Salt and alkali stress affected the photosynthetic characteristics of Chinese cabbages. A salt-tolerant cultivar maintained its tolerance by ensuring the high ability of photosynthesis. The synthesis of organic acids and carbohydrates in leaves played important roles in improving the photosynthetic capacity of alkali-tolerant plants. Soil salinization has become an increasingly serious ecological problem, which limits the quality and yield of crops. As an important economic vegetable in winter, however, little is known about the response of Chinese cabbage to salt, alkali and salt-alkali stress in photosynthetic characteristics and chloroplast ultrastructure. Thus, two Chinese cabbage cultivars, 'Qinghua' (salt-tolerant-alkali-sensitive) and 'Biyu' (salt-sensitive-alkali-tolerant) were investigated under stresses to clarify the similarities and differences between salt tolerance and alkali tolerance pathways in Chinese cabbage. We found that the root of Qinghua, the leaf ultrastructure and net photosynthetic rate (Pn), stomatal conductance (Gs), water use efficiency (WUE), maximum photochemical quantum yield of PSII (Fv/Fm) and nonphotochemical quenching (NPQ) were not affected by salt stress. However, Biyu was seriously affected under salt stress. Its growth indexes decreased by between 60 and 30% compared with the control and the photosynthetic indexes were also seriously affected under salt stress. This indicated that the salt-tolerant cultivar Qinghua improved the photosynthetic fluorescence ability to promote the synthesis of organic matter resulting in salt tolerance. In contrast, under alkali treatment, the root of Biyu was affected by alkali stress, but could still maintain good growth, and root and leaf structure were not seriously affected and could maintain the normal operations. Biyu improved its tolerance by improving the water use efficiency, regulating the synthesis of organic acids and carbohydrates, ensuring the synthesis of organic matter and ensured the normal growth of the plant.

Patterns of spatio-temporal distribution of winter chronic photoinhibition in leaves of three evergreen Mediterranean species with contrasting acclimation responses

High irradiance and relatively low temperature, which characterize Mediterranean winters, cause chilling stress in plants. Downregulation of photosynthetic efficiency is a mechanism that allows plants to survive these conditions. This study aims to address whether this process shows a regular spatial pattern across leaf surface or not. Three species (Buxus sempervirens, Cistus albidus and Arctostaphylos uva-ursi) with contrasting responses to winter stress were studied. During 7 days, macro and micro Fv/Fm spatial patterns were monitored by the use of chlorophyll fluorescence imaging techniques. In the field, the strongest photoinhibition was found in B. sempervirens, while there was almost no chronic photoinhibition in C. albidus. In leaves of the first species, Fv/Fm decreased from base to tip while in C. albidus it was uniform over the leaf lamina. An intermediate behavior is shown by A. uva-ursi leaves. Spatial heterogeneity distribution of Fv/Fm was found inside the leaves, resulting in greater Fv/Fm values in the inner layers than in the outer ones. Neither xanthophyll-linked downregulation of Fv/Fm nor protein remobilization were the reasons for such spatial patterns since pigment composition and nitrogen content did not reveal tip-base differences. During recovery from winter, photoinhibition changes occurred in Fv/Fm, pigments and chloroplast ultrastructure. This work shows for the first time that irrespective of physiological mechanisms responsible for development of winter photoinhibition, there is an acclimation response with strong spatio-temporal variability at leaf level in some species. This observation should be taken into account when modeling or scaling up photosynthetic responses.

Influence of illumination on settlement of diatom Navicula sp

Diatoms are responsible for biofouling, which causes many problems in various marine industries. This study examined the effects of different light conditions (intensity, incident direction, time of illumination) on the settling behavior of the marine diatom Navicula sp. on glass surfaces. The density of this diatom's settlement on glass was strongly influenced by light conditions. Moreover, very weak light emitted on the bottom of the culture dish could also rapidly inhibit diatom settlement. These phenomena were explained by spatial interference between chloroplast and holdfast-like structures inside the thecae. The holdfast-like structure is observed to be responsible for diatom locomotion and hence the settlement behavior. It was proposed that the interrelation of illumination and attachment of diatoms allowed them to better adapt to the habitat with higher efficiency of attachment and successive reproduction.

Multi-sensor plant imaging: Towards the development of a stress-catalogue

Agricultural production is limited by a wide range of abiotic (e.g. drought, waterlogging) and biotic (pests, diseases and weeds) stresses. The impact of these stresses can be minimized by appropriate management actions such as irrigation or chemical pesticide application. However, further optimization requires the ability to diagnose and quantify the different stresses at an early stage. Particularly valuable information of plant stress responses is provided by plant imaging, i.e. non-contact sensing with spatial resolving power: (i) thermal imaging, detecting changes in transpiration rate and (ii) fluorescence imaging monitoring alterations in photosynthesis and other physiological processes. These can be supplemented by conventional video imagery for study of growth. An efficient early warning system would need to discriminate between different stressors. Given the wide range of sensors, and the association of specific plant physiological responses with changes at particular wavelengths, this goal seems within reach. This is based on the organization of the individual sensor results in a matrix that identifies specific signatures for multiple stress types. In this report, we first review the diagnostic effectiveness of different individual imaging techniques and then extend this to the multi-sensor stress-identification approach.

Analysis of initial chlorophyll fluorescence induction kinetics in chloroplasts in terms of rate constants of donor side quenching release and electron trapping in photosystem II

The fluorescence induction F(t) of dark-adapted chloroplasts has been studied in multi-turnover 1 s light flashes (MTFs). A theoretical expression for the initial fluorescence rise is derived from a set of rate equations that describes the sequence of transfer steps associated with the reduction of the primary quinone acceptor Q (A) and the release of photochemical fluorescence quenching of photosystem II (PSII). The initial F(t) rise in the hundreds of mus time range is shown to follow the theoretical function dictated by the rate constants of light excitation (k (L)) and release of donor side quenching (k ( si )). The bi-exponential function shows sigmoidicity when one of the two rate constants differs by less than one order of magnitude from the other. It is shown, in agreement with the theory, that the sigmoidicity of the fluorescence rise is variable with light intensity and mainly, if not exclusively, determined by the ratio between rate of light excitation and the rate constant of donor side quenching release.

A two-photon excitation study on the role of carotenoid dark states in the regulation of plant photosynthesis

Plants are exposed to sun light intensities that vary rapidly over several orders of magnitude during a typical day. It is known that the regulation of photosynthetic activity under these circumstances is essential for the survival and fitness of natural and gene modified plants. A quick balancing between utilization and dissipation of absorbed light energy ensures optimized levels of CO(2) fixation and protection from photo damage by excessive light-irradiation. Despite intensive investigations the biophysical mechanisms of these regulation processes are still poorly understood. Potentially involved singlet states of carotenoids are optically "dark" and so far it was impossible to investigate their role directly in living plants by conventional absorption or fluorescence spectroscopy. Here, we show by selective two-photon excitation of the carotenoid dark states in plant that a dominant part of the regulation is correlated with a substantial change in the energy transfer between these states and the chlorophylls (Chl). The results support a considerable role of the molecular gear shift model in which a reversible and step-wise enzymatic modification of the electronic structure of xanthophyll carotenoids enables a switching between carotenoid-to-Chl light-harvesting and Chl-to-carotenoid quenching. The shifting can be observed in real time in any plant. Treatment with the xanthophyll cycle inhibitor dithiothreitol slowed down both the light adaptation and the carotenoid-Chl energy flow changes to the same extent. Based on these results, we propose a biophysical quenching model in which both carotenoid dark states and radical cations contribute to the dissipation of excessive excitation energy.

Analysis of Chlorophyll Fluorescence Spectra in Some Tropical Plants

Laser-induced chlorophyll fluorescence (LICF) emission spectra of leaves of some tropical plants were measured using a compact fiber-optic fluorosensor with a continuous-wave blue diode laser as exciting source and an integrated digital spectrometer. Different chlorophyll-fluorescence signatures of light-green, fully-green, and yellow leaves were monitored at room temperature. Deconvolution procedure was used to determine fluorescence band position and width. Calibration of the fluorescence ratio F690/F730 relative to the 404 nm excitation is done from the curve-fitted parameter.

Ultraviolet Wavelength Dependence of Photomorphological and Photosynthetic Responses in Brassica napus and Arabidopsis thaliana

Among the photomorphological responses in plants induced by ultraviolet-B radiation (UVB; 290 nm-320 nm) are leaf asymmetry, leaf thickening and cotyledon curling. We constructed an action spectrum of cotyledon curling in light-grown Brassica napus to characterize the UVB photoreceptor that initiates this response. Cotyledon curling was also characterized in Arabidopsis thaliana. Peak efficiency for this response occurred between 285 and 290 nm. Additionally, UVB-induced changes in epidermal cells from A. thaliana cotyledons were assessed because they are the likely site of UVB photoreception that leads to curling. Investigation of cellular structure, chlorophyll a fluorescence and chlorophyll concentration indicated that cotyledon curling is not concomitant with gross cellular damage or inhibition of photosynthesis, which only occurred in response to wavelengths <280 nm. Many UVB effects are apparently an indirect consequence of UVB radiation, dependent on UVB-mediated increases in reactive oxygen species (ROS) that either act as a signal in the UVB transduction pathway or cause oxidative damage. The cotyledon curling response was impeded by ascorbate and cystine, ROS scavengers and was promoted by H(2)O(2), a ROS. We suggest that following absorption by a UVB chromophore, ROS are generated via photosensitization, ultimately leading to cotyledon curling.

In vivo monitoring of chlorophyll ?uorescence response to low-dose?-irradiation in pumpkin (cucurbita pepo) leaves

The effect of gamma-irradiation on the in vivo-measured chlorophyll fluorescence in a pumpkin leaves (Cucurbita pepo) has been investigated. Plants were grown in the same environment, then divided into several groups and irradiated at ambient conditions at small dose levels (up to 13.4 Gy) with (60)Co gamma-rays. The post-irradiation effect on chlorophyll status in the leaves was examined by measuring chlorophyll fluorescence 2 days (48 h) after exposure. It is undoubtedly found that the value of fluorescence intensity ratio (FIR) at 690 nm and 735 nm (F(690)/F(735)) depends upon the ionizing radiation dose. Even with the smallest dose of 3.35 Gy, ionizing radiation notably altered the fluorescence spectra of leaves. The spectra difference was manifested by decrease of FIR due to changed chlorophyll luminescence, the possible reason for which could be increase of chlorophyll concentration during the recovery process of the plant. The potential implications of these results for plant physiological status monitoring, as well as for pollution detection and assessment, are discussed in brief.

The effect of chilling on the photosynthetic activity in coffee (Coffea arabica L.) seedlings: The protective action of chloroplastid pigments

Coffea arabica is considered to be sensitive to low temperatures, being affected throughout its entire life cycle. Injury caused by chilling (low temperatures above zero degree centigrade) is characterized primarily by inhibition of the photosynthetic process. The objective of this work was to evaluate the role of photosynthetic pigments in the tolerance of coffee (C. arabica L.) seedlings to chilling. The evaluation the photosynthetic activity was made by emission of Chl a fluorescence at room temperature (25 ºC) in vivo and in situ, using a portable fluorometer. The pigment content was obtained by extraction with 80 % acetone, while estimation of membrane lipid peroxidation was determined by measuring the MDA content in leaf tissue extracts. The results indicated a generalized reduction in the quantum yield of PSII when the seedlings were maintained in the dark. The reduction occurred in the seedlings submitted to chilling treatment as well as in the control ones. This demonstrates that not only chilling acts to cause an alteration in PSII. It is possible that the tissue storage reserves had been totally exhausted, with the respiratory rate exceeding the photosynthetic rate; the later was nil, since the seedlings were kept in the dark. The efficiency in the capture, transfer and utilization of light energy in PS II photochemical reactions requires a sequence of photochemical, biochemical and biophysical events which depend on the structural integrity of the photosynthetic apparatus. However, this efficiency was found to be related to the protective action of chloroplastid pigments, rather than to the concentration of these pigments.

Tansley Review No. 36 Excited leaves

Photosynthesis is largely to do with energy transduction; the conversion of light energy into electrical energy into chemical energy. Precisely how much light energy is needed to bring about the reduction of one molecule of carbon dioxide and the release of one molecule of oxygen (the quantum requirement) is a matter of fundamental importance and one which has attracted much past controversy. This article concludes that a minimum quantum requirement of eight, as demanded by the Z-scheme, is obviously consistent with much contemporary work which puts the measured value for C₃ leaves close to nine. Moreover, while values of less than eight (obtained in some circumstances with micro-organisms), are a reminder that nothing is beyond challenge they are not, in the absence of confirmation and extension, sufficiently compelling to demand rejection of either the Z-scheme or current measuring procedures. This article also shows why, even if the underlying minimum requirement was now accepted beyond all reasonable doubt, there would still be very good reasons for continuing, indefinitely, to measure actual photosynthetic efficiency in the natural environment. It discusses some of the implications of the fact that all plants, if not stressed, appear to photosynthesize at the same rate in low light. It explains the role of fluorescence in its relation to quantum yield, the possibility that the rate of photosynthesis might be determined from fluorescence measurements alone, and that a combination of fluorescence and gas exchange measurements could provide new information about the manner in which 'dark respiration' is affected by light. It indicates how contemporary interest in all of these matters has focused attention on the necessity for safe dissipation of excitation energy by leaves and on the manner by which this might be achieved. CONTENTS Summary 325 I. Excitation 325 II. Quantum requirement 326 III. Learning from fluorescence 335 IV. Safely dissipated 340 Acknowledgements 342 References 342.

Differential response to the herbicidal activity of delta-aminolevulinic acid in plants with high and low SOD activity

delta-aminolevulinic acid (ALA) is the obligatory precursor for tetrapyrroles and for chlorophylls in plants. Under illumination, these photosensitizers generate singlet oxygen, thus causing bleaching and death of treated tissues. We have examined whether superoxide is involved in the mode of action of ALA and whether SOD provides protection. Bean genotypes with similar carotenoid content but differing in SOD activity and cucumber seedlings were used throughout. Cucumber plants treated with 10 mM ethanolamine (EA) prior to ALA, had higher levels of chlorophyll fluorescence and lower values of electrolyte leakage than control. Bean cultivars with high SOD activity were considerably more tolerant to membrane damage caused by ALA than those with low SOD activity. SOD activity was greatly reduced in cucumber leaves treated with diethyldithiocarbamate (DDTC). Electrolyte leakage was markedly increased and chlorophyll fluorescence values were significantly lower in DDTC and ALA treated tissues as compared with those treated with ALA only. The results indicate that superoxide is involved in the toxicity caused by ALA and that, by breeding for high SOD activity, resistance to ALA can be achieved, thus allowing the use of ALA as a selective herbicide in the field.

Application of chlorophyll fluorescence in ecophysiology

In vivo chlorophyll fluorescence measurements have become a valuable tool in ecophysiology. Fluorescence emission spectra are influenced by the reabsorption of the tissue and indicate the composition of the antenna system and are influenced by the chlorophyll content per leaf area. The fluorescence induction kinetics ("Kautsky effect") can be used to study photosynthetic activity. These rapid, non-destructive methods can be applied for ecophysiological field research to check the vitality of plants and to document stress effects on the photosynthetic apparatus. The Rfd-values (Rfd = fd/fs), the ratio of the fluorescence decrease (fd) to the steady state fluorescence (fs), can be taken as a rapid vitality index of the leaves and trees. We here describe fundamental chlorophyll fluorescence results of leaves which are needed for the interpretation of in vivo fluorescence signatures in stress physiology and in the forest dieback research.