Hydroponically grown cucumber plants were exposed to 14-d period of salinity (0, 50, 100 mM NaCl). NaCl caused reduction in the relative water content in the leaves. The Na + content increased and the K + content decreased. The net photosynthetic rate, stomatal conductance and transpiration rate were markedly decreased by all of the salt treatments. Salinity decreased also the maximum quantum efficiency of photosystem 2 (PS 2) determined as the variable to maximum fluorescence ratio, the photochemical quantum yield of PS 2 and the photochemical fluorescence quenching, while the non-photochemical quenching increased. Above results indicate that NaCl affects photosynthesis through both stomata closure and non-stomatal factors.

The effect of salt stress (50, 100 and 150 mM of NaCl) on the activity of superoxide dismutase (SOD, EC. 1.15.1.1), ascorbate peroxidase (APX, EC. 1.11.1.11), glutathione reductase (GR, EC. 1.6.4.2) enzymes and also on the rate of lipid peroxidation in terms of thiobarbituric acid‐reactive substances (TBARS) content and photosynthetic capacity in two wheat (C3 plants) and two maize (C4 plants) varieties was studied. In the non‐salined control plants, the antioxidant enzymes activities were significantly higher for maize than for wheat. Adding salt to the nutrient solution increased the level of antioxidants in leaves of both maize and wheat. The first substantial response to salinity was found for SOD on the 2nd day, whereas changes occurred for APX on the 4th day and for GR on the 4th/5th day of salt treatment. Although SOD activity increased considerably more in wheat (C3), it never reached as high levels as in maize (C4) grown in the same treatment combinations. The total increase in APX activity was similar for wheat and maize, whereas GR activity was higher in leaves of maize. Lipid peroxidation analyses showed an increase in TBARS contents in both plants' species grown under salinity that corresponded to the damage that occurred in secondary oxidative stress. However, as a result of advanced antioxidant defense in maize, the TBARS quantities did not elevate to as high level as in wheat. Chlorophyll fluorescence measurements revealed a considerable decrease in the efficiency of PS II and electron‐transport chain (ETC). Assimilation rate of CO2 decreased in both plant groups; however, in C4 maize, we observed a much better capacity to preserve the photosynthetic apparatus against overproduction of ROS. Results suggest that efficient antioxidant defense plays an important role in maize, the C4 plant, resistance to environmental stresses like salinity or drought.

The effect of heavy metals on the modification of plasma membrane H+-ATPase (EC 3.6.3.14) activity in cucumber roots was studied. In plants stressed for 2 h with 10 μM or 100 μM Cd, Cu or Ni the hydrolytic as well as the transporting activity of H+-ATPase in the plasma membranes of root cells was decreased. Transcript levels of Cucumis sativus plasma membrane H+-ATPase in roots treated with 10 μM Cd, Cu, or Ni as well as with 100 μM Cu or Ni were similar to the control, indicating that the action of metals did not involve the gene expression level. Only in roots exposed to 100 μM Cd was the level of plasma membrane H+-ATPase mRNA markedly decreased. The inhibition of the plasma membrane proton pump caused by 100 μM Cd, Cu and Ni was partially diminished in the presence of cantharidin, a specific inhibitor of protein phosphatases. Western blot analysis with the antibody against phosphothreonine confirmed that decreased activity of plasma membrane H+-ATPase under heavy metals resulted from dephosphorylation of the enzyme protein. Taken together, these data strongly indicated that alteration of the enzyme under heavy metal stresses was mainly due to the post-translational modification of its proteins in short-term experiments.

The effects of Cu, Cd, and Pb toxicity on photosynthesis in cucumber leaves (Cucumis sativus L.) were studied by the measurements of gas exchange characteristics, chlorophyll (Chl) fluorescence parameters, and Chl content. Concentrations of metals in sequence of 20 µM Cu, 20 and 50 µM Cd, and 1 000 µM Pb decreased the plant dry mass to 50-60 % after 10 d of treatment whereas 50 µM of Cu decreased it to 30 %. The content of Cd in leaves of plants treated with 50 µM Cd was three times higher than the contents of Cu and Pb after plant treatment with 50 µM Cu or 1 000 µM Pb. Hence Cd was transported to leaves much better than Cu and Pb. Nevertheless, the net photosynthetic rate and stomatal conductance in leaves treated with 50 µM Cu or Cd were similarly reduced. Thus, Cu was more toxic than Cd and Pb for photosynthesis in cucumber leaves. None of the investigated metals decreased internal CO 2 concentrations. Also the effect of metals on potential efficiency of photosystem 2, PS2 (F v /F m ) was negligible. The metal dependent reduction of PS2 quantum efficiency (Φ PS2 ) after plant adaptation in actinic irradiation was more noticeable. This could imply that reduced demand for ATP and NADPH in a dark phase of photosynthesis caused a down-regulation of PS2 photochemistry. Furthermore, in leaves of metal-treated plants the decrease in water percentage as well as lower contents of Chl and Fe were observed. Thus photosynthesis is not the main limiting factor for cucumber growth under Cu, Cd, or Pb stress.

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In this study the role of the plasma membrane (PM) H(+) -ATPase for growth and development of roots as response to nitrogen starvation is studied. It is known that root development differs dependent on the availability of different mineral nutrients. It includes processes such as initiation of lateral root primordia, root elongation and increase of the root biomass. However, the signal transduction mechanisms, which enable roots to sense changes in different mineral environments and match their growth and development patterns to actual conditions in the soil, are still unknown. Most recent comments have focused on one of the essential macroelements, namely nitrogen, and its role in the modification of the root architecture of Arabidopsis thaliana. As yet, not all elements of the signal transduction pathway leading to the perception of the nitrate stimulus, and hence to anatomical changes of the root, which allow for adaptation to variable ion concentrations in the soil, are known. Our data demonstrate that primary and lateral root length were shorter and lower in aha2 mutant lines compared with wild-type plants in response to a variable nitrogen source. This suggests that the PM proton pump AHA2 (Arabidopsis plasma membrane H(+) -ATPase isoform 2) is important for root growth and development during different nitrogen regimes. This is possible by controlling the pH homeostasis in the root during growth and development as shown by pH biosensors.

No abstract

The strategies developed by plants to avoid the toxicity of cadmium (Cd) and other heavy metals involve active sequestration of metals into the apoplast and vacuoles. The protein systems excluding heavy metals from the cell cytosol localize to the plasma membrane and tonoplast and are energized either by ATP or by the electrochemical gradient generated by H+-ATPase or by V-ATPase and pyrophosphatase (PPase), respectively. In this work, a comparative study on the contribution of both the plasma membrane and tonoplast in the active detoxification of plant cells after treatment with Cd was performed. The studies using plants treated and untreated with Cd reveal that both, H+-coupled and MgATP-driven efflux of Cd across plasma membranes and tonoplast is markedly stimulated in the presence of Cd in the environment. Previous studies on plasma-membrane localized H+-coupled Cd efflux together with the present data demonstrating tonoplast H+/Cd2+ antiport activity suggest that H+-coupled secondary transport of Cd displays a lower affinity for Cd when compared with Cd primary pumps driven by MgATP. In addition, it is shown that MgATP-energized Cd efflux across both membranes is significantly enhanced by cysteine, dithiothreitol, and glutathione. These results suggest that Cd is excluded from the cytosol through an energy-dependent system as a free ion as well as a complexed form. Although both membranes contribute in the active exclusion of ionized and complexed Cd from the cytosol, the overall calculation of Cd accumulation in the everted plasma membranes and vacuolar vesicles suggests that the tonoplast and vacuole have a major function in Cd efflux from the cytosol in the roots of cucumber subjected to Cd stress.