Losanka P. Popova

Exogenous treatment with salicylic acid attenuates cadmium toxicity in pea seedlings

The present study investigated the possible mediatory role of salicylic acid (SA) in protecting plants from cadmium (Cd) toxicity. The exposure of pea plants to increasing Cd concentrations (0.5, 1.0, 2.0 and 5.0 microM) during early stages of their establishment, caused a gradual decrease in shoot and root fresh weight accumulation, the rate of CO2 fixation and the activity of ribulose-1,5-bisphosphate carboxylase (RuBPC, E.C. 4.1.1.39), the effect being most expressed at higher Cd concentrations. In vivo the excess of Cd-induced alterations in the redox cycling of oxygen-evolving centers and the assimilatory capacity of the pea leaves as revealed by changes in thermoluminescence emission after flash illumination. The levels of some important parameters associated with oxidative stress, namely lipid peroxidation, electrolyte leakage and proline production were increased. Seed pretreatment with SA alleviated the negative effect of Cd on growth, photosynthesis, carboxylation reactions, thermoluminescence characteristics and chlorophyll content, and led to decrease in oxidative injuries caused by Cd. The data suggest that the beneficial effect of SA during an earlier growth period could be related to avoidance of cumulative damage upon exposure to cadmium thus reducing the negative consequences of oxidative stress caused by heavy metal toxicity. In addition, the observed high endogenous levels of SA after treatment with Cd suggests that SA may act directly as an antioxidant to scavenge the reactive oxygen species and/or indirectly modulate redox balance through activation of antioxidant responses. Taken together these evidences could explain at some extend the protective role of SA on photochemical activity of chloroplast membranes and photosynthetic carboxylation reactions in Cd-stressed pea plants.

Effects of salicylic acid on growth and photosynthesis in barley plants

Summary Hordeum vulgare L./cv. Alfa /was grown for 7 days over a range of salicylic acid (SA) concentrations (100 μM – 1 mM), and the effects on growth and photosynthetic performance were examined. Leaf and root growth, and chlorophyll and protein contents were reduced by SA. Gas exchange measurements and analysis of carboxylating enzyme activities were used to study the reactions of photosynthesis to SA. Longterm treatment of barley seedlings with SA led to a noticeable decrease in both the initial slope of the curves representing net photosynthetic rate versus intercellular CO 2 concentration and the maximal rate of photosynthesis. The calculated values of the intercellular CO 2 concentration, CO 2 -compensation point and the maximal carboxylating efficiency of ribulose-l,5-bisphosphate carboxylase (RuBP carboxylase, EC 4.1.1.39) support the suggestion that biochemical factors are involved in the response of photosynthesis to SA. RuBP carboxylase activity decreased with increasing SA concentrations, while the activity of phosphoenolpyruvate carboxylase (PEP carboxylase, EC 4.1.1.31) increased. When 7-day-old barley seedlings were supplied with SA through the transpiration stream for 2 h no changes in photosynthetic reactions were observed. Six and 24 h after treatment of barley seedlings with SA the rate of photosynthesis declined in a concentration-dependent manner. Possible reasons for the responses of photosynthesis to SA are discussed.

Exogenous treatment with Salicylic acid leads to increased antioxidant capacity in leaves of barley plants exposed to Paraquat

Our previous study suggests that salicylic acid mediates tolerance in barley plants to paraquat (Ananieva et al. 2002). To further define the role of SA in paraquat induced responses, we analysed the capacity of the antioxidative defence system by measuring the activities of several antioxidative enzymes: superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), dehydroascorbate reductase (DHAR, EC 1.8.5.1), catalase (CAT, EC 1.11.1.6), and guaiacol peroxidase (POX, EC 1.11.1.7). Twelve-day-old barley seedlings were supplied with 500 micromol/L SA or 10 micromol/L Pq via the transpiration stream and kept in the dark for 24 h. Then they were exposed to 100 micromol m(-2) s(-1) PAR and samples were taken 6 h after the light exposure. Treatment of seedlings with 10 micromol/L Pq reduced the activity of APX and GR, did not affect the activity of POX and DHAR but caused over a 40% increase in the activity of CAT. Pre-treatment with 500 micromol/L SA for 24 h in the dark before Pq application increased the activities of the studied enzymes in both the chloroplasts (SOD activity) and the other compartments of the cell (POX, CAT activity). The effect of SA pre-treatment was highly expressed on DHAR and POX activity. The data suggest that SA antagonizes Pq effects, via elicitation of an antioxidative response in barley plants.

Effect of salicylic acid on the synthesis of ribulose-1,5-bisphosphate carboxylase/oxygenase in barley leaves

Summary Hordeum vulgare L. (cv. Alfa ) was grown for 7 days over a range of salicylic acid (SA) concentrations (100 μmol/L-1 mmol/L), and the effect on the level of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) was examined. The treated plants showed a decrease in the level of total soluble protein, in particular in the level of RubisCO. In the presence of 1 mmol/L SA the level of total soluble protein on a fresh weight basis was about 68% of the control, whereas the level of RubisCO was about 50% that of control plants. The percentage of inhibition on the small subunit was higher, as a result of which the small/large subunit ratio was lower for the experimental variants. When 7-day-old barley seedlings were supplied with SA through the transpiration stream for 24 h no changes in the levels of total soluble protein and RubisCO were observed. The influence of SA on the synthesis of RubisCO was followed by in vivo labelling with a 14 C-amino acid mixure ( 14 C-AAM). Results demonstrated that SA inhibited both the synthesis of total soluble protein and, more pronounced, the synthesis of RubisCO.

Treatment with salicylic acid decreases the effects of paraquat on photosynthesis

Summary The present study investigated the possible mediatory role of salicylic acid (SA) in paraquat (Pq) toxicity on photosynthesis. Twelve-day-old barley seedlings were supplied with 500 μmol/L salicylic acid (SA) or 10 μmol/L paraquat (Pq) via the transpiration stream and kept in the dark for 24 h. Then they were exposed to 100 μmol m−2 s−1 PAR and samples were taken 1,2,3, and 6 h after the light exposure. Leaf gas exchange parameters, the activity of ribulose-1,5 bisphosphate carboxylase (RUBPC, EC 4.1.1.39) and of the photorespiratory enzymes phosphoglycolate phosphatase (PGP, EC 3.1.3.18), glycolate oxidase (GO, EC 1.1.3.1), and catalase (EC 1.11.1.6) were determined. Treatment of seedlings with SA alone resulted in decreased levels of chlorophyll, CO2 assimilation and transpiration rates. Pq treatment led to a decrease in chlorophyll content and to a very strong inhibition of photosynthesis, the effects were manifested by 1 h of illumination. Neither SA nor Pq caused changes in RUBPC activity, but instead greatly increased the activity of the photorespiratory enzymes. Pre-treatment of seedlings with SA provided protection against subsequent Pq-induced stress. This observation was confirmed by gas exchange parameters, chlorophyll content and by changes in some important parameters associated with oxidative stress, namely H2O2 production, lipid peroxidation and electrolyte leakage. Pre-treatment with SA fully blocked the inhibitory effect of Pq on photosynthesis and reduced Pq-induced H2O2 production, lipid peroxidation, and electrolyte leakage. The data suggest that SA might protect cells against oxidative damage and prevent photosynthesis upon paraquat treatment.

Involvement of abscisic acid in photosynthetic process in Hordeum vulgare L. during salinity stress

In Hordeum vulgare L. plants, NaCl stress imposed through the root medium for a period of 8 days decreased the rate of CO2 assimilation, the chlorophyll and protein leaf content, and the activity of ribulose-1,5-bisphosphate carboxylase. The activity of phosphoenolpyruvate carboxylase was twofold over the control. Pretreatment with abscisic acid (ABA) for 3 days before salinization diminished the inhibitory effect of NaCl on the rate of CO2 fixation. The leaf Na+ and Cl− content decreased in ABA-pretreated plants. Both ABA and NaCl treatments led to an increase in the endogenous level of ABA in the plant leaves. Patterns of total proteins extracted from the leaves of control or ABA- and salt-treated plants were compared. Both ABA and NaCl induced marked quantitative and qualitative changes in the polypeptide profiles concerning mainly the proteins with approximately equal mobility. The results are discussed in terms of a possible role of ABA in increasing the salt tolerance when ABA is applied to the plants for a short period before exposure to salinity stress, thus improving the invulnerability to unfavorable conditions.

A Possible Role for Jasmonic Acid in Adaptation of Barley Seedlings to Salinity Stress

Abstract. The changes caused by NaCl salinity and jasmonic acid (JA) treatment (8 days) on growth and photosynthesis of barley plants (Hordeum vulgare L., var. Alfa) have been studied. Gas exchange measurements and analysis of enzyme activities were used to study the reactions of photosynthesis to salinity and JA. Both 100 mm NaCl and 25 μm JA treatment led to a noticeable decrease in both the initial slope of the curves representing net photosynthetic rate vs intercellular CO2 concentration and the maximal rate of photosynthesis. The calculated values of the intercellular CO2 concentration, CO2 compensation point, and maximal carboxylating efficiency of ribulose-1,5-bisphosphate carboxylase support the suggestion that biochemical factors are involved in the response of photosynthesis to JA and salinity stress. The activities of phosphoenolpyruvate carboxylase and carbonic anhydrase increased more than twofold. Pretreatment with JA for 4 days before salinization diminished the inhibitory effect of high salt concentration on the growth and photosynthesis. The results are discussed in terms of a possible role of JA in increasing salinity tolerance of the barley plants.

Changes in some photosynthetic and photorespiratory properties in barley leaves after treatment with jasmonic acid

Summary The influence of jasmonic acid (JA) on photosynthesis, dark respiration, photorespiration, and the activities of ribulose bisphosphate carboxylase/oxygenase (RuBPCO) and phosphoenolpyruvate carboxylase in 7-day-old barley seedlings ( Hordeum vulgare L., var. Alfa) was investigated. Plants treated with JA showed a decrease of the rate of photosynthetic CO 2 fixation and the activity of RuBP carboxylase. The rate of dark respiration was increased two-fold, of photorespiration increased about 70 %, and the CO 2 -compensation point increased about 20 %. Stomatal resistance increased about 70 % over control plants. As a result of the inhibition of the carboxylase activity of RuBPCO, the carboxylase/oxygenase ratio decreased.

Effects of short-term soil flooding on stomata behaviour and leaf gas exchange in barley plants

Exposure of barley plants (Hordeum vulgare L.) to soil flooding for 2 to 24 h reduced the net photosynthetic rate and transpiration rate. Stomatal conductance also decreased in flooded plants. Stomatal closure started within 2 − 6 h and stomata remained closed up to 24 h of treatment.

Adaptation to salinity as monitored by PSII oxygen evolving reactions in barley thylakoids

Summary The effects of NaCl salinity and Abscisic acid pretreatment on the polypeptide composition, photochemical activity and kinetic characteristics of oxygen evolving reactions in isolated thylakoids from barley ( Hordeum vulgare L., var. Alfa ) seedlings were studied. It was shown that thylakoids isolated from plants previously subjected to a prolonged NaCl treatment were more resistant to high concentration of NaCl after in vitro incubation. Pretreatment with ABA 3 days before salinization diminished the inhibitory effect of the NaCl. Both ABA and NaCl treatment lead to an increase in the internal ABA level in the plant leaves. ABA seems to play a specific role in this adaptive process, promoting some structural changes in the chloroplast membranes, which in turn reflect on the oxygen evolving mechanisms. The obtained results are discussed in terms of an effect of salinity and ABA treatment leading to an increased participation of the stroma situated photosystem II centers, possessing higher resistance to damaging factors.