Katya Georgieva

Photosynthetic activity of homoiochlorophyllous desiccation tolerant plant Haberlea rhodopensis during dehydration and rehydration

The functional state of the photosynthetic apparatus of flowering homoiochlorophyllous desiccation tolerant plant Haberlea rhodopensis during dehydration and subsequent rehydration was investigated in order to characterize some of the mechanisms by which resurrection plants survive drought stress. The changes in the CO2 assimilation rate, chlorophyll fluorescence parameters, thermoluminescence, fluorescence imaging and electrophoretic characteristics of the chloroplast proteins were measured in control, moderately dehydrated (50% water content), desiccated (5% water content) and rehydrated plants. During the first phase of desiccation the net CO2 assimilation decline was influenced by stomatal closure. Further lowering of net CO2 assimilation was caused by both the decrease in stomatal conductance and in the photochemical activity of photosystem II. Severe dehydration caused inhibition of quantum yield of PSII electron transport, disappearance of thermoluminescence B band and mainly charge recombination related to S2QA− takes place. The blue and green fluorescence emission in desiccated leaves strongly increased. It could be suggested that unchanged chlorophyll content and amounts of chlorophyll–proteins, reversible modifications in PSII electron transport and enhanced probability for non-radiative energy dissipation as well as increased polyphenolic synthesis during desiccation of Haberlea contribute to drought resistance and fast recovery after rehydration.

Long-term variations and interrelations of ENSO, NAO and solar activity

Abstract Much attention is recently paid to the global warming observed in the 20th century, and especially to the relative impact of natural and anthropogenic factors responsible for it. Many studies have revealed a good correlation, up to the last decades, between century-scale changes in global surface temperature and solar activity, though the mechanism is still controversial. Long-term anomalies of atmospheric parameters are often connected to large-scale changes in atmospheric circulation influencing, through teleconnections, distant and apparently unconnected areas. Such large-scale phenomena are El Nino/Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) affecting the climate over a great part of the globe on interannual to decadal and centennial time-scales. Little is known about the factors determining the long-term variability of these phenomena. In the present paper we compare the century-long variability of NAO and ENSO with the solar activity variations in the secular (Gleissberg) solar cycle and find a close relation between them. We suggest that the influence of solar activity on these large-scale phenomena is mediated by atmospheric centers of action which undergo changes in intensity and location in response to long-term variations of solar activity.

Response of barley seedlings to UV-B radiation as affected by NaCl

The response of barley seedlings, subjected to 150 mmol/L NaCl for 4 days at different light regimes (4 d in the light, 4 d in darkness and a 12 h light/dark cycle) before UV-B radiation was investigated. NaCl treatment resulted in a decrease of total chlorophyll content and an increase in H2O2, free proline and lipid peroxidation, as quantified by measurement of malondialdehyde. Significantly more proline was accumulated in the light than in darkness. The combination of UV-B and NaCl treatment produced an additive effect on most of the parameters studied. UV-B radiation reduced the chlorophyll/carotenoids ratio and photochemical efficiency of PSII as estimated by chlorophyll fluorescence. NaCl pre-exposure decreased H2O2 generation and lipid peroxidation and alleviated the inhibitory effect of UV-B on PSII activity. Proline accumulated under salt stress conditions might be one of the reasons for the observed tolerance of barley seedlings to UV-B radiation.

Light-Dark Changes in Proline Content of Barley Leaves under Salt Stress

Proline accumulation in leaves of barley (Hordeum vulgare L. cv. Alfa) seedlings treated with 150 mM NaCl was promoted in the light and suppressed in the dark. The light/dark changes of proline content was enhanced with each 12 h light/12 h dark cycle and the proline content increased steadily. Root and shoot concentrations of Na+ and Cl− in salt treated plants increased about 10 to 25 times as compared to the control. The content of these ions and the content of malondialdehyde were higher in the shoot of seedlings exposed to salt stress for 4 d in the light in comparison with the seedlings exposed to NaCl for 4 d in darkness. Light stimulated both ions and proline accumulation in the leaves and has no effect in the roots. Oxygen uptake was higher in the seedlings kept 4 d in the light which have higher endogenous free proline content. Chlorophyll fluorescence measurements showed that the photochemical activity of PS 2 slightly decreased as a result of salt stress and was not influenced by light regimes during plant growth.

UV-B induced stress responses in three rice cultivars

UV-B responses of three rice (Oryza sativa L.) cultivars (Sasanishiki, Norin 1 and Surjamkhi) with different photolyase activity were investigated. Carbon dioxide assimilation data support that Sasanishiki was less sensitive to UV-B than Norin 1 and Surjamkhi. UV-B radiation sharply decreased the content of Rubisco protein in Surjamkhi and has no effect in Sasanishiki. The photochemical activities of photosystem (PS) 1 and PS 2 was slightly affected by UV-B treatment. The content of H2O2 and the activities of antioxidant enzymes, catalase (CAT), peroxides (POX) and superoxide dismutase (SOD) were enhanced after UV-B treatment. The activities of CAT and POX isoenzymes in Sasanishiki were more enhanced by UV-B radiation than those in Norin 1 and Surjamkhi.

Protection of thylakoids against combined light and drought by a lumenal substance in the resurrection plant Haberlea rhodopensis

BACKGROUND AND AIMS Haberlea rhodopensis is a perennial, herbaceous, saxicolous, poikilohydric flowering plant that is able to survive desiccation to air-dried state under irradiance below 30 micromol m-2 s-1. However, desiccation at irradiance of 350 micromol m-2 s-1 induced irreversible changes in the photosynthetic apparatus, and mature leaves did not recover after rehydration. The aim here was to establish the causes and mechanisms of irreversible damage of the photosynthetic apparatus due to dehydration at high irradiance, and to elucidate the mechanisms determining recovery. METHODS Changes in chloroplast structure, CO2 assimilation, chlorophyll fluorescence parameters, fluorescence imaging and the polypeptide patterns during desiccation of Haberlea under medium (100 micromol m-2 s-1; ML) irradiance were compared with those under low (30 micromol m-2 s-1; LL) irradiance. KEY RESULTS Well-watered plants (control) at 100 micromol m-2 s-1 were not damaged. Plants desiccated at LL or ML had similar rates of water loss. Dehydration at ML decreased the quantum efficiency of photosystem II photochemistry, and particularly the CO2 assimilation rate, more rapidly than at LL. Dehydration induced accumulation of stress proteins in leaves under both LL and ML. Photosynthetic activity and polypeptide composition were completely restored in LL plants after 1 week of rehydration, but changes persisted under ML conditions. Electron microscopy of structural changes in the chloroplast showed that the thylakoid lumen is filled with an electron-dense substance (dense luminal substance, DLS), while the thylakoid membranes are lightly stained. Upon dehydration and rehydration the DLS thinned and disappeared, the time course largely depending on the illumination: whereas DLS persisted during desiccation and started to disappear during late recovery under LL, it disappeared from the onset of dehydration and later was completely lost under ML. CONCLUSIONS Accumulation of DLS (possibly phenolics) in the thylakoid lumen is demonstrated and is proposed as a mechanism protecting the thylakoid membranes of H. rhodopensis during desiccation and recovery under LL. Disappearance of DLS during desiccation in ML could leave the thylakoid membranes without protection, allowing oxidative damage during dehydration and the initial rehydration, thus preventing recovery of photosynthesis.

Changes in some thylakoid membrane proteins and pigments upon desiccation of the resurrection plant Haberlea rhodopensis

The changes in some proteins involved in the light reactions of photosynthesis of the resurrection plant Haberlea rhodopensis were examined in connection with desiccation. Fully hydrated (control) and completely desiccated plants (relative water content (RWC) 6.5%) were used for thylakoid preparations. The chlorophyll (Chl) a to Chl b ratios of thylakoids isolated from control and desiccated leaves were very similar, which was also confirmed by measuring their absorption spectra. HPLC analysis revealed that beta-carotene content was only slightly enhanced in desiccated leaves compared with the control, but the zeaxanthin level was strongly increased. Desiccation of H. rhodopensis to an air-dried state at very low light irradiance led to a little decrease in the level of D1, D2, PsbS and PsaA/B proteins in thylakoids, but a relative increase in LHC polypeptides. To further elucidate whether the composition of the protein complexes of the thylakoid membranes had changed, we performed a separation of solubilized thylakoids on sucrose density gradients. In contrast to spinach, Haberlea thylakoids appeared to be much more resistant to the same solubilization procedure, i.e. complexes were not separated completely and complexes of higher density were found. However, the fractions analyzed provided clear evidence for a move of part of the antenna complexes from PSII to PSI when plants became desiccated. This move was also confirmed by low temperature emission spectra of thylakoids. Overall, the photosynthetic proteins remained comparatively stable in dried Haberlea leaves when plants were desiccated under conditions similar to their natural habitat. Low light during desiccation was enough to induce a rise in the xanthophyll zeaxanthin and beta-carotene. Together with the extensive leaf shrinkage and some leaf folding, increased zeaxanthin content and the observed shift in antenna proteins from PSII to PSI during desiccation of Haberlea contributed to the integrity of the photosynthetic apparatus, which is important for rapid recovery after rehydration.

Long-term variations in the correlation between NAO and solar activity: The importance of north–south solar activity asymmetry for atmospheric circulation

Abstract General atmospheric circulation is the system of atmospheric motions over the Earth on the scale of the whole globe. Two main types of circulation have been identified: zonal – characterized by low amplitude waves in the troposphere moving quickly from west to east, and meridional with stationary high amplitude waves when the meridional transfer is intensified. The prevailing type of circulation is related to global climate. Based on many years of observations, certain “circulation epochs” have been defined when the same type of circulation prevails for years or decades. Here we study the relation between long-term changes in solar activity and prevailing type of atmospheric circulation, using NAO index reconstructed for the last four centuries as a proxy for large-scale atmospheric circulation. We find that when the southern solar hemisphere is more active, increasing solar activity in the secular solar cycle results in increasing zonality of the circulation, while when the northern solar hemisphere is more active, increasing solar activity increases meridional circulation. In an attempt to explain the observations, we compare the short-term reaction of NAO and NAM indices to different solar drivers: powerful solar flares, high speed solar wind streams, and magnetic clouds.

Effects of Succinate on Manganese Toxicity in Pea Plants

ABSTRACT Pea (Pisum sativum cv. Citrine) plants were grown in nutrient solution containing various manganese (Mn) concentrations in the presence or absence of succinate to evaluate the potential role of succinate in the plant tolerance to Mn excess. Supplying pea plants with excess Mn led to a reduction in the relative growth rate (RGR), chlorophyll a and b content, photosynthetic O2 evolution activity, and photosystem II (PSII) activity, as measured in the light-adapted state (φPSII) in comparison to the control. The primary photochemical efficiency of PSII, estimated by the Fv/Fm ratio, was less affected by increasing Mn concentration. Chloroplasts from Mn-treated leaves exhibited significant changes in their ultrastructure, depending on the strength of Mn toxicity. The concentration of Mn in roots, stem, and leaves increased with the increase of Mn in the nutrient solution. Addition of succinate before and after Mn treatment did not reduce the inhibitory effect of Mn on the plant growth, chlorophyll fluorescence parameters, photosynthetic O2 evolution activity, and chloroplast structure of the pea plants. It was found that supply of exogenous succinate at a high Mn concentration (over 1500 μM) in the nutrient solution led to an increase of Mn uptake in the roots accompanied by a decrease in a Mn translocation to the leaves and stems compared to Mn-treated pea-plants. However, differences in the toxicity effect of Mn in both Mn and Mn/Succinate-treated pea plants were not detected. Thus, such changes in Mn distribution within the Mn/succinate-treated plant did not confer tolerance of Mn excess to pea plants. These results suggest that succinate probably has an affinity for Mn and may function as a “terminal acceptor” of large amounts of Mn, decreasing Mn transport to the stem and leaves, but does not contribute to Mn tolerance.

Response of chlorina barley mutants to heat stress under low and high light

Barley plants (Hordeum vulgare L.) of wild type and two chlorina mutants, chlorina 126 and chlorina f2, were subjected to 42°C for 5 h at light intensities of 100 and 1000 μmol photons m-2 s-1. The exposure of plants to heat stress at a light intensity of 100 μmol m-2 s-1 induced enormous proline accumulation, indicating that the effect of heat stress was stronger when it was combined with low light intensity. The functional activity of PSII, O2evolution and flash-induced thermoluminescence B-band amplitude were strongly reduced when plants were exposed to heat at low light intensity. The results clearly showed that high light intensity had a protective effect on photosynthetic activity when barley plants were treated with high temperature. Comparison of the thermosensitivity of wild type plants and chlorina mutants revealed that O2 evolution in chlorina 126 and, especially, in chlorina f2 was more sensitive to heat than in wild type.