Artificial Photosynthesis

Photosynthesis is one of the most important reactions on Earth, and it is a scientific field that is intrinsically interdisciplinary, with many research groups examining it. We could learn many strategies from photosynthesis and can apply these strategies in artificial photosynthesis. Artificial photosynthesis is a research field that attempts to replicate the natural process of photosynthesis. The goal of artificial photosynthesis is to use the energy of the sun to make different useful material or high-energy chemicals for energy production. This book is aimed at providing fundamental and applied aspects of artificial photosynthesis. In each section, important topics in the subject are discussed and reviewed by experts

Photodamage to Oxygen Evolving Complex - An Initial Event in Photoinhibition of Photosystem II.

Photosystem II (PSII) of oxygenic photosynthesis is susceptible to photoinhibition. Photoinhibition is defined as light induced damage resulting in turnover of the D1 protein subunit of the reaction center of PSII. Both visible and ultraviolet (UV) light cause photoinhibition. Photoinhibition induced by UV light damages the oxygen evolving complex (OEC) via absorption of UV photons by the Mn ion(s) of OEC. Under visible light, most of the earlier hypotheses assume that photoinhibition occurs when the rate of photon absorption by PSII antenna exceeds the use of the absorbed energy in photosynthesis. However, photoinhibition occurs at all light intensities with the same efficiency per photon. The aim of my thesis work was to build a model of photoinhibition that fits the experimental features of photoinhibition. I studied the role of electron transfer reactions of PSII in photoinhibition and found that changing the electron transfer rate had only minor influence on photoinhibition if light intensity was kept constant. Furthermore, quenching of antenna excitations protected less efficiently than it would protect if antenna chlorophylls were the only photoreceptors of photoinhibition. To identify photoreceptors of photoinhibition, I measured the action spectrum of photoinhibition. The action spectrum showed resemblance to the absorption spectra of Mn model compounds suggesting that the Mn cluster of OEC acts as a photoreceptor of photoinhibition under visible light, too. The role of Mn in photoinhibition was further supported by experiments showing that during photoinhibition OEC is damaged before electron transfer activity at the acceptor side of PSII is lost. Mn enzymes were found to be photosensitive under visible and UV light indicating that Mn-containing compounds, including OEC, are capable of functioning as photosensitizers both in visible and UV light. The experimental results above led to the Mn hypothesis of the mechanism of continuous-light-induced photoinhibition. According to the Mn hypothesis, excitation of Mn of OEC results in inhibition of electron donation from OEC to the oxidized primary donor P680+ both under UV and visible light. P680 is oxidized by photons absorbed by chlorophyll, and if not reduced by OEC, P680+ may cause harmful oxidation of other PSII components. Photoinhibition was also induced with intense laser pulses and it was found that the photoinhibitory efficiency increased in proportion to the square of pulse intensity suggesting that laser-pulse-induced photoinhibition is a two-photon reaction. I further developed the Mn hypothesis suggesting that the initial event in photoinhibition under both continuous and pulsed light is the same: Mn excitation that leads to the inhibition of electron donation from OEC to P680+. Under laser-pulse-illumination, another Mn-mediated inhibitory photoreaction occurs within the duration of the same pulse, whereas under continuous light, secondary damage is chlorophyll mediated. A mathematical model based on the Mn hypothesis was found to explain photoinhibition under continuous light, under flash illumination and under the combination of these two.Siirretty Doriast

Approaches in Enhancing Antioxidant Defense in Plants

This Special Issue, “Approaches in Enhancing Antioxidant Defense in Plants” published 13 original research works and a couple of review articles that discuss the various aspects of plant oxidative stress biology and ROS metabolism, as well as the physiological mechanisms and approaches to enhancing antioxidant defense and mitigating oxidative stress. These papers will serve as a foundation for plant oxidative stress tolerance and, in the long term, provide further research directions in the development of crop plants’ tolerance to abiotic stress in the era of climate change

Evaluation of the photoprotective role of quercetin to selected light-adapted and shade-tolerant plant species

Plants respond to different light intensities depending on their genetic make-up, mutation and other environmental conditions. Mikania micrantha, Clidemia hirta, and Tetracera sarmentosa were selected so as to analyse how varying light intensity affects some gas exchange characteristics, pigment and production of total flavonoid content. Besides, the study was aimed at evaluating the photoprotective role of quercetin to the selected plants, in addition to correlating how the production of flavonoids affects the plants photosynthesis. The photosynthetic rates of the selected plants were determined using LI-6400. The chlorophyll, carotenoid, anthocyanin, flavonoid, antioxidant enzymes, malondialdehyde, soluble sugar and soluble protein contents were quantified using spectroscopic techniques. Quercetin was quantified using high-performance liquid chromatography (HPLC). Sun-exposed plants were having the maximum photosynthesis and quercetin content compared with semi-shaded or fully shaded plants. The highest quercetin content was recorded for sun-exposed C. hirta (0.950 ± 0.023 μg/ml) while the lowest was recorded for shaded T. sarmentosa (0.13 ± 0.007 μg/ml). The highest oxidative stress was recorded for sun-exposed T. sarmentosa (6.19 ± 019 μg/ml) which was also having the lowest quantum efficiency of photosystem II (0.509 ± 0.003). Superoxide dismutase activity was lowest under sun-exposed C. hirta (1.86 ± 0.06 U/mg protein), while peroxidase and catalase were lowest under Sun-exposed T. sarmentosa (59.59 ± 2.67 and 3.75 ± 0.17) U/mg protein respectively. The result obtained makes it possible to accept the generated hypothesis of the study because the quercetin content was higher when the antioxidant enzymes of the plants were low. This leads to a conclusion that increase in the production of secondary metabolites at high light intensity is not due to high CO2 assimilation rate, but rather due to the production of photoprotective metabolites to conquer the light stress

Evaluation of Antidiabetic Activity of Aqueous Extract of Bark of Pterocarpus Marsupium Silver Nanoparticles against Streptozotocin and Nicotinamide Induced Type 2 Diabetes in Rats

The main aim of this study is to formulate silver nanoparticles with the bark of aqueous extract of Pterocarpus marsupium by green synthesis method without the use of any chemicals to deliver drugs in a controlled manner and to evaluate the formulation against streptozotocin nicotinamide induced type 2 diabetes in wistar rats. The formulation has reduced side effects, thus minimized the dosing frequency when compared it with aqueous extract of Pterocarpus marsupium. There occurs a significant (P<0.01) decrease in the hyperglycemic state after the administration of Pterocarpus marsupium silver nanoparticle which reduce the severity of oxidative and acuity of hyperglycemia, a process that closely linked to glucose oxidation and formation of free radicals. Our results suggested that Pterocarpus marsupium silver nanoparticle has more favourable reduction in lipid level in STZ and nicotinamide - induced diabetic rats, compared with glibenclamide as well as regeneration of ß- cells of pancreas. The present study suggests that Pterocarpus marsupium silver nanoparticles can be successfully utilized for the management of diabetes due to their anti-hyperglycemic action

THE INTERPLAY BETWEEN PHOTORESPIRATION AND IRON DEFICIENCY.

ABSTRACT Iron (Fe) is an essential micronutrient for plants as it takes part in major metabolic pathways such as photosynthesis and respiration and is linked to many enzymes that accomplish many other cellular functions (DNA synthesis, nitrogen fixation, hormone production). Fe deficiency reduces crop yields worldwide but particularly in plants grown on calcareous soils, which represent almost the 30% of the earth land surface. In the near future to cope with the increasing demand of food caused by a strong increase in world\u2019s population (FAO estimates in 9 billion people by 2050), agriculture must be extended to marginal areas, many of which include calcareous soils. The most evident effect of Fe deficiency in plant leaves is a marked chlorosis caused by a decrease in chlorophyll biosynthesis, which may result in a reduction in CO2 assimilation rate. In these conditions leaves have low photo-synthetic activity but they absorb more light energy per chlorophyll mol\uacecule than required for photosynthesis, especially under high radiation. This results in a high risk for photoinhibitory and photooxidative dam\uacages in Fe-deficient leaves. The photorespiratory cycle can be considered in these circumstances as an energy dissipating cycle, operating between chloroplasts, peroxisomes, mitochondria and cytosol, which helps to protect chloroplasts from photoinhibition and plants from excessive accumulation of reactive oxygen species. We suggest that Fe deficiency leads to a strong impairment of the photosynthetic apparatus at different levels: an increase in the rate of CO2 assimilation in many biological repetition (+29%) was observed, suggesting a possible induction of photorespiratory metabolism. However, the variation was not significative and so further analysis must be required in order to reduce the variability among the repetition to get more reliable results. In addition, the reduction of CO2 assimilation can be also attributable to a reduced stomatal conductance or to a mesophyll-reduced utilization of CO2. Iron deficiency affects also amminoacid (aa) metabolism since the concentration of Ser and Gly, two aa involved in the photorespiratory metabolism, increased in leaves (+94% and +160%, respectively). Resupply of iron to Fe-deficient plants led to an increase in the concentration of some divalent cations other than Fe like Ca and Mn, whilst Na, Mg, Cu, Zn decrease as Fe sufficient condition are restored. On the other hand, as Fe deficiency proceeds during time, we observed a significant increase in Na, Mg, Zn, Mn content. This alterations suggest that Fe deficiency induces a metabolic imbalance in which other divalent cations are absorbed by unspecific transporter, due to their similar characteristics to Fe. Under our experimental conditions, ROS accumulation detected in cucumber plants grown in the absence of Fe could be attributable to an increase in the activity of enzymes involved in their formation or to a reduced detoxification. We observed a slight induction in the activity of Cu/Zn-SOD isoform whereas a reduction in Fe- and also in Mn-SOD isoforms activity was also recorded. At the same time, the concentration of H2O2 in the leaves of Fe-deficient plants was significantly higher (+40%). This overproduction could lead to an onset of oxidative stress which can lead to further cell damage at different levels also with the involvement of the photosynthetic apparatus. Fe deficiency also induces alterations in peroxisomes at different levels indicating modifications in the photorespiratory metabolism. The complete lack of Fe results in a strong inhibition of catalase activity (-35%). Nevertheless, we detect higher levels of catalase in Fe-deficient plants compared to the control condition. In Fesufficient condition the total activity of hydroxypyruvate reductase was fully attributable to the peroxisomal isoform (HPR1), while we recorded an equal distribution of the activity between the two isoforms, peroxisomal and cytosolic (HPR2) in plants grown under conditions of Fe deficiency. Moreover, the characterization of rice mutant plants defective in mitochondrial Fe importer allow us to investigate the involvement of this organelle in the photorespiratory metabolism during Fe deficiency. The partial loss of function of MIT (mit-2) affects the mitochondrial functionality by decreasing the respiratory chain activity. Furthermore, the transcriptome and the metabolome strongly change in rice mutant plants, in a different way in roots and shoot. Biochemical characterization of purified mitochondria from rice roots showed alteration in the respiratory chain of mit-2 compared to wild type plants. In particular, proteins belonging to the type II alternative NAD(P)H dehydrogenases strongly accumulated in mit-2 plants, indicating that mit-2 mitochondria activate alternative pathways to keep the respiratory chain working. The data obtained and exposed in this doctorate thesis, in agreement with what widely previously reported in literature, allow us to state that the absence or the low Fe bioavailability during the growth of the plants results in several alterations more or less reversible at different levels of the overall metabolic plant system

Responses of Plants to Environmental Stresses

Environmental abiotic stresses, such as extreme temperatures, drought, excess light, salinity, and nutrient deficiency, have detrimental effects on plant growth, development, and yield. Plants are equipped with various adaptation mechanisms to cope with such unfavorable conditions. Our understanding of plants’ abiotic stress responses is crucial to maintaining efficient plant productivity. This book on the responses of plants to environmental stresses is an attempt to find answers to several basic questions related to their adaptation and protective mechanisms against abiotic stresses. The following chapters of the book describe examples of plants’ protective strategies, which cover physiological, cellular, biochemical, and genomic mechanisms. This book is aimed for use by advanced students and researchers in the area of stress biology, plant molecular biology and physiology, agriculture, biochemistry, as well as environmental sciences