Ivan Yordanov

Oxidative stress and some antioxidant systems in acid rain-treated bean plants Protective role of exogenous polyamines

The effect of simulated acid rain (AR) (pH 1.8) on H2O2 and malonyldialdehyde (MDA) levels and activities of peroxidase and catalase in bean plants were investigated. The influence of exogenous polyamines spermidine and spermine on these parameters was also studied. AR treatment induced lipid peroxidation and increased level of H2O2 in leaves. Pretreatment with spermidine and spermine prevented these changes. The protective effect of spermine was higher than that of spermidine. The impact of polyamines could be attributed to their acid neutralizing and antioxidant effects, as well as to their ability to stabilize membranes by associating with negatively charged phospholipids. It was also found that AR significantly increased peroxidase and decreased catalase activities at the first hours after treatment. Later, both enzyme activities were enhanced that could contribute to the scavenging and detoxification of active oxygen species.

Plant responses to drought, acclimation, and stress tolerance

At the whole plant level, the effect of stress is usually perceived as a decrease in photosynthesis and growth. That is why this review is focused mainly on the effect of drought on photosynthesis, its injury, and mechanisms of adaptation. The analysed literature shows that plants have evolved a number of adaptive mechanisms that allow the photochemical and biochemical systems to cope with negative changes in environment, including increased water deficit. In addition, the acquisition of tolerance to drought includes both phenotypic and genotypic changes. The approaches were made to identify those metabolic steps that are most sensitive to drought. Some studies also examined the mechanisms controlling gene expression and putative regulatory pathways.

Effects of Water Stress and High-Temperature Stress on the Structure and Activity of Photosynthetic Apparatus of Zea Mays and Helianthus Annuus

Effects of high-temperature stress (HTS) and PEG-induced water stress (WS), applied separately or in combination, on the functional activity and ultrastructure of the photosynthetic apparatus (PSA) of maize (Zea mays L.) and sunflower (Helianthus annuus L.) plants were investigated. In maize plant tissues WS provoked the decrease in RWC by 10.9 %, HTS by 7.0 %, and after simultaneous application of the both treatments the decrease was 32.7 % in comparison with control plants. Similar but more expressed changes were observed in sunflower plants. Sunflower was more sensitive to these stresses. Net photosynthetic rate decreased significantly after all treatments, more in sunflower. In mesophyll chloroplasts after separately applied WS and HTS the number of grana and thylakoids was reduced and electron-transparent spaces appeared. At combined stress (WS+HTS) granal and stromal thylakoids were considerably affected and chloroplast envelope in many of them was partially disrupted.

Simultaneous analysis of prompt and delayed chlorophyll a fluorescence in leaves during the induction period of dark to light adaptation.

An attempt is made to reveal the relation between the induction curves of delayed fluorescence (DF) registered at 0.35-5.5 ms and the prompt chlorophyll fluorescence (PF). A simple formulation was proposed to link the ratio of the transient values of delayed and variable fluorescence with the redox state of the primary electron acceptor of Photosystem II--QA, and the thylakoid membrane energization. The term luminescence potential (UL) was introduced, defined as the sum of the redox potential of QA and the transmembrane proton gradient. It was shown that UL is proportional to the ratio of DF to the variable part of PF. The theoretical model was verified and demonstrated by analysing induction courses of PF and millisecond DF, simultaneously registered from leaves of barley--wild-type and the chlorophyll b-less mutant chlorina f2. A definitive correlation between PF and DF was established. If the luminescence changes are strictly due to UL, the courses of DF and PF are reciprocal and the millisecond DF curve resembles the first derivative of the PFt function.

Temperature Dependence of Chlorophyll Fluorescence Parameters of Pea Seedlings

Summary The thermosensitivity of the photosynthetic apparatus of 12 day-old seedlings of Pisum sativum cv. Ran was investigated within the temperature range 2–50°C at intervals of 2°C. The parameters of prompt chlorophyll fluorescence were used as a criteria of photosynthetic capacity, measured after 5 min temperature treatment (simultaneously with dark adaptation) at the respective temperature. It was established that in the temperature interval 2–42°C the ground fluorescence (Fo) did not change significantly, but above 42°C it increased dramatically. This gave us reason to suppose that temperatures higher than 44°C induced irreversible injuring of pea thylakoid membranes. The variable fluorescence (Fv) remained unchanged from 2°C to 20°C, after 22°C started to decrease first monotonously, then sharply and at 50°C it comprised only 6 % of its initial level. It was also shown that in the temperature range 2–30°C the ratio Fv/Fm, considered as a measure of PS2 activity, did not change significantly, but began to decrease at higher temperatures. After 42°C its level strongly decreased and at 50°C it was only about 10 % of the initial value of Fv/Fm. Similar changes were found for the Fv/Fo ratio. It is concluded that the physiological state and capacity of PS2 for electron transport do not changes significantly in a wide temperature range.

Preservation of photosynthetic electron transport from senescence-induced inactivation in primary leaves after decapitation and defoliation of bean plants.

The comparative effects of decapitation and defoliation on the senescence-induced inactivation of photosynthetic activity in primary leaves of bean plants were investigated. Decapitation was performed during different phases of bean plant ontogenesis, immediately after the appearance of the 1st, 2nd, 3rd and 4th composite leaf. In addition, we examined a variant with primary leaves and stem with an apical bud, but without composite leaves, i.e. defoliated plants. Analyses of chlorophyll fluorescence, millisecond delayed fluorescence and absorption at 830nm in primary leaves were undertaken to investigate the alterations in photosystems II and I electron transport during the decapitation-induced delayed senescence in the non-detached leaves. Analysis of the OKJIP transients using the JIP-test (see [Strasser R, Srivastava A, Tsimilli-Michael M. Analysis of the chlorophyll a fluorescence transient. In: Papageorgiou G, Govindjee, editors. Chlorophyll a fluorescence: a signature of photosynthesis. The Netherlands: Kluwer Academic Publishers, 2004; pp. 321-362]) showed an increase in several biophysical parameters of photosystem II in decapitated plants, specifically, the density of active reaction centers on a chlorophyll basis, the yields of trapping and electron transport, and the performance index. We also observed a decrease in the absorbed light energy per reaction center. Such a decrease in light absorption could be a result of the photosystem II down regulation that appeared as an increase in Q(B)-non-reducing photosystem II centers. The effect was identical when all leaves except the primary leaves were removed. The variant with a preserved apical bud, the defoliated plant, showed values similar to those of decapitated plants with primary leaves only. The changes in the induction curves of the delayed fluorescence also indicated an acceleration of electron transport beyond photosystem II in the decapitated and in defoliated plants. In these plants, the photosystem I-driven electron transport was accelerated, and the size of the plastoquinone pool was enhanced. It was established that decapitation can retard the senescence of primary leaves, can expand leaf life span and can cause activation of both photosystems I and II electron transport. The decapitation procedure shows similarities to the process of defoliation. The overcompensation effect that is developed after defoliation could initially be manifested as an acceleration of the linear photosynthetic electron flow in the rest of the leaves.

Temperature Dependence of Photochemical and Non-Photochemical Fluorescence Quenching in Intact Pea Leaves

Summary The thermosensitivity of the photosynthetic apparatus of 12-day-old seedlings of Pisum sativum cv. Ran was investigated within the temperature range of 2–50 °C. Fluorescence quenching was used as a criteria. The photochemical (qP) and non-photochemical (qN) fluorescence quenching were measured after a 5-min treatment at the respective temperature. It was established that low and high-temperatures strongly affect the pattern of the fluorescence induction curve. The results obtained show that qP increases with the temperature rise from 2 to 35 °C and at temperatures higher than 40 °C it decreases sharply. Temperatures enhancing from 2 to 25 °C led to a decrease of qN. The values of this parameter increased strongly at temperatures higher than 45 °C. The results showed that within the temperature range of 15–40 °C the photochemical quenching dominated over the non-photochemical one. The influence of temperature on the quantum efficiency of PS2 photochemistry ϕPS2 was very similar to that of qP. It is concluded that the physiological state and the activity of PS2 are preserved in a wide temperature interval. The unfavourable temperatures provoke the switching on of the protective mechanisms as a qN enhancement and stimulation of cyclic electron transport through PS1, which allows the plants to endure the stress conditions.

High-temperature damage and acclimation of the photosynthetic apparatus I. Temperature sensitivity of some photosynthetic parameters of chloroplasts isolated from acclimated and non-acclimated bean leaves

The thermosensitivity of delayed fluorescence, the relative values of variable chlorophyll fluorescence and the degree of quenching of 9-aminoacridine fluorescene were studied in the chloroplasts from heat-acclimated and non-acclimated (treated 6 h at 52,5°C) young bean plants. The temperature sensitivity of each parameter studied was defined by that temperature at which chloroplast activity decreased by 50% (T50) of its maximum value. There was appreciable increase in the thermostability of membrane energization in chloroplasts isolated from acclimated and non-acclimated plants compared with the controls. The photosynthetic parameters differed according to the suspending medium and the preacclimation treatment. When chloroplast were suspended in phosphate buffer with the addition of stabilizing compounds (2 M sucrose or 0.5% human serum albumin) the thermostability of the thylakoid membranes increased, as was evident by the increases in T50 of about 8–10° C (sucrose) and 2–5° C (human serum albumin) for all the parameters investigated. Photoinduced quenching of 9-aminoacridine fluorescence decreased to some extent in the presence of protective compounds, but in chloroplasts from acclimated plants the T50 was practically equal to that for their long-lived luminescence under the same conditions. At the thylakoid membrane level, acclimation was clearly manifested as an increased thermostability of photoinduced proton-gradient formation.

Photosynthetic activity during high temperature treatment of pea plants

Summary The functional activity of the photosynthetic apparatus of pea plants ( Pisum sativum , L.) under high temperature conditions was investigated. The rates of CO 2 uptake and O 2 evolution declined after 1 h exposure to 45 °C and their activity did not change significantly up to the 8 th h of treatment, which could be due to some acclimation to the unfavourable temperature. Photosynthetic activity sharply decreased after 24 hat 45 °C. Information about the functional activity of PSII was derived from analyses of in vivo chlorophyll fluorescence. It was recorded at 25 °C and also at 45 °C, immediately after the respective time of 45 °c treatment of pea plants. Our results showed that when whole plants were exposed to 45 °C up to 24 h the CO 2 assimilation and O 2 evolution rate were more inhibited than the yield of primary photochemistry (estimated by the ratio F v /F m ). These values started to decrease after 5 h of high temperature exposure and was reduced to a greater extent after 24 h of treatment. The higher PSII thermostability in vivo could be due to the protective effect of low light intensity and the low air humidity during the heat treatment.

Effects of exogenous polyamines applied separately and in combination with simulated acid rain on functional activity of photosynthetic apparatus

Summary Ten-day-old bean seedlings (Phaseolus vulgaris L.) were grown in a greenhouse and sprayed with spermidine or spermine. After 24 h they were treated with a cocktail of simulated acid rain (pH 5.6 and 1.8). Polyamines were applied separately or in combination with acid rain. Their protective effect on the functional activity of the photosynthetic apparatus in the acid rain-treated plants was investigated. It was found that acid rain with pH 1.8 applied separately decreased strongly the photosynthetic rate, the oxygen evolution, as well as the PS2 activity. Polyamines (spermidine and spermine) applied 24 h the before the acid rain treatment partially diminished the inhibitory effect of acid rain on the photosynthetic apparatus. The results showed also that the favourable effect of spermine was higher than that of spermidine. The protective action of polyamines could be explained by their polycationic nature, their ability to bind with photosynthetic membranes, resulting in conformational changes and stabilization, of the membrane and by their ability to act as free radical scavengers.