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Oxidative stress inside cells occurs when the production of reactive oxygen species (ROS) is no longer efficiently counterbalanced by the generation of antioxidants. In this study, we measured the intracellular production of ROS, including hydrogen peroxide (H2O2), superoxide (O2−), and singlet oxygen (1O2), in cultured dinoflagellates of the genus Symbiodinium under thermal and oxidative stress. ROS tagged with fluorescent probes were measured by flow cytometry. Dissimilar Symbiodinium internal transcribed spacer 2 (ITS2) clades or phylotypes (A1, B2, E, F1) produced ROS in different quantities in response to stress. For example, when comparing the control (26 °C) to the high-temperature treatment (35 °C), Symbiodinium E showed no change in the intracellular concentrations of any of the ROS; but phylotype A1 displayed a 10-fold increase in the overall ROS concentration and a 4-fold increase in O2−. Under oxidative stress, when 8 mmol l−1 H2O2 was added to the cells, these same two Symbiodinium phylotypes increased their overall concentrations of ROS, but only Symbiodinium E showed an increase in the concentrations of O2− (2×) and 1O2 (3×). Therefore, not only were the stress responses of the various Symbiodinium phylotypes different but also the responses of individual phylotypes to thermal and oxidative stress were different in terms of ROS production. Variation in the quality and quantity of ROS generation and its implications for subsequent antioxidant production suggest that different stress mechanisms are at play. While our experiments were done under laboratory conditions that did not necessarily mirror ecological ones, these results provide new insight into processes inside Symbiodinium cells during stress events and add new explanations for a phylotype's susceptibility to stress. [ABSTRACT FROM AUTHOR]

Silicon (Si)-mediated alleviation of Cr(VI) toxicity was examined in wheat seedlings using an in vivo approach that involves chlorophyll fluorescence, laser induced breakdown spectroscopy (LIBS) and anatomical changes. Exposure to Cr(VI) significantly reduced the growth and photosynthetic activities (chlorophyll fluorescence) in wheat which was accompanied by remarkable accumulation of this element in tissues. However, addition of Si to the growth medium alleviated the effects of Cr(VI). The LIBS spectra were used as a fingerprint of the elemental compositions in wheat seedlings, which showed a reduction in Cr accumulation following Si addition. Nutrient element levels (Ca, Mg, K and Na) declined in wheat following the addition of Cr (VI), as recorded by LIBS and inductively coupled plasma atomic emission spectroscopy (ICAP-AES). However, addition of Si along with Cr(VI) increased the contents of nutrient elements in wheat. LIBS, ICAP-AES and AAS showed a similar distribution pattern of elements measured in wheat. Anatomical observations of leaf and root revealed that Cr(VI) affected internal structures while Si played a role in protection from toxic effects. The results showed the suitability of chlorophyll fluorescence as a parameter and appropriateness of LIBS technique and anatomical procedures to elucidate Si-mediated alleviation of Cr(VI) toxicity. Furthermore, our results suggest that the measured parameters and techniques can be used non-invasively for monitoring the growth of crops under different environmental conditions [ABSTRACT FROM AUTHOR]

The chlorophyll-protein CP43′ (isiA gene) induced by stress conditions in cyanobacteria is shown to serve as an antenna for Photosystem II (PSII), in addition to its known role as an antenna for Photosystem I (PSI). At high light intensity, this antenna is converted to an efficient trap for chlorophyll excitations that protects system II from photo-inhibition. In contrast to the ‘energy-dependent non-photochemical quenching’ (NPQ) in chloroplasts, this photoprotective energy dissipation in cyanobacteria is triggered by blue light. The induction is proportional to light intensity. Induction and decay of the quenching exhibit the same large temperature-dependence. [Copyright &y& Elsevier]

The effects of salt stress (0–400 mM NaCl) on PSII photochemistry, photoinhibition and photosynthetic pigment composition were investigated in the halophyte Suaeda salsa grown under outdoor conditions and exposed to full sunlight. Salt stress resulted in a growth stimulation which was optimal at 200 mM NaCl. With increasing salt concentration, leaf water potential and evaporation rate decreased significantly while there were no changes in leaf relative water content. Salt stress also resulted in a significant accumulation of sodium and chloride in leaves. Salt stress induced neither effects on the maximal efficiency of PSII photochemistry measured either at predawn or at midday nor effects on the actual PSII efficiency (ΦPSII), the efficiency of excitation energy capture by open PSII reaction centres, photochemical quenching, and non-photochemical quenching (NPQ) at midday. No significant changes were observed in the contents of neoxanthin, lutein, β-carotene, violaxnthin, antheraxthin, zeaxanthin, chlorophyll a and b in salt-stressed plants. Our results suggest that S. salsa showed high resistance not only to salinity stress but also to photoinhibition even when treated with high salinity as high as 400 mM NaCl and exposed to full sunlight. The results indicate that tolerance of PSII to high salinity stress and high light stress can be viewed as an important strategy for S. salsa to grow in very high saline soil. [Copyright &y& Elsevier]

The functional organization of chloroplasts in adaxial guard cells (GCs) of Vicia faba leaves was investigated by the saturation pulse method of chlorophyll fluorescence induction using single GC pairs. Quantitative imaging analysis of chlorophyll fluorescence performed on adaxial GCs upon dark–light transition revealed a large chlorophyll fluorescence transient. Adaxial GCs displayed PS II quantum yields and relative electron transport rates (ETR) comparable to the abaxial GCs. The slow chlorophyll fluorescence transients in the two cells provided further evidence for the activation of the Calvin cycle in GC chloroplasts on both sides of the leaf. The ETR upon increasing light intensity was, however, differentially expressed in the two cell types. When the rates were normalized to their maximum ETR activity, half-saturation was achieved at 76 and 45 μmol m−2 s−1 in adaxial and abaxial GCs, respectively. These results provide unequivocal evidence that the chloroplasts of adaxial GCs have a highly expressive functional organization and that the photosynthetic apparatus of adaxial GCs is well adapted to elevated light intensities. [Copyright &y& Elsevier]

Systemic acquired acclimation (SAA) is an important light acclimatory mechanism that depends on the global adjustments of non-photochemical quenching and chloroplast retrograde signaling. As the exact regulation of these processes is not known, we measured time-resolved fluorescence of chlorophyll a in Arabidopsis thaliana leaves exposed to excess light, in leaves undergoing SAA, and in leaves after excess light episode. We compare the behavior induced in wild-type plants with null mutant of non-photochemical quenching (npq4-1). The wild type rosettes exhibit a small reduction of fluorescence decay times in leaves directly exposed to excess light and in leaves undergoing SAA in ambient low light. However in npq4-1 exposition to excess light results in much faster fluorescence decay, which is insensitive to excitation power. At the same time npq4-1 leaves undergoing SAA displayed intermediate fluorescence decay. The npq4-1 plants also lost the ability to optimize florescence decay, and thus chlorophyll a dynamics up to 2 h after excess light episode. The fluorescence decay dynamics in both WT and npq4-1 can be described by a set of 3 maximum decay times. Based on the results, we concluded that functional PsbS is required for optimization of absorbed photon fate and optimal light acclimatory responses such as SAA or after excess light stress.

Oxidative stress in plants causes ferredoxin down-regulation and NADP(+) shortage, over-reduction of the photosynthetic electron transport chain, electron leakage to oxygen and generation of reactive oxygen species (ROS). Expression of cyanobacterial flavodoxin in tobacco chloroplasts compensates for ferredoxin decline and restores electron delivery to productive routes, resulting in enhanced stress tolerance. We have designed an in vivo system to optimize flavodoxin reduction and NADP(+) regeneration under stress using a version of cyanobacterial ferredoxin-NADP(+) reductase without the thylakoid-binding domain. Co-expression of the two soluble flavoproteins in the chloroplast stroma resulted in lines displaying maximal tolerance to redox-cycling oxidants, lower damage and decreased ROS accumulation. The results underscore the importance of chloroplast redox homeostasis in plants exposed to adverse conditions, and provide a tool to improve crop tolerance toward environmental hardships.