The salt stress triggered by sublethal, 100 mM and lethal, 250 mM NaCl induced ethylene production as well as rapid accumulation of superoxide radical and H2O2 in the root tips of tomato (Solanum lycopersicum cv. Ailsa Craig) wild type and ethylene receptor mutant, Never ripe (Nr/Nr) plants. In the wild type plants superoxide accumulation confined to lethal salt concentration while H2O2 accumulated more efficiently under sublethal salt stress. However, in Nr roots the superoxide production was higher and unexpectedly, H2O2 level was lower than in the wild type under sublethal salt stress. Nitric oxide production increased significantly under sublethal and lethal salt stress in both genotypes especially in mutant plants, while peroxynitrite accumulated significantly under lethal salt stress. Thus, the nitro-oxidative stress may be stronger in Nr roots, which leads to the programmed death of tissues, characterized by the DNA and protein degradation and loss of cell viability under moderate salt stress. In Nr mutants the cell death was induced in the absence of ethylene perception. Although wild type roots could maintain their potassium content under moderate salt stress, K(+) level significantly declined leading to small K(+)/Na(+) ratio in Nr roots. Thus Nr mutants were more sensitive to salt stress than the wild type and the viability of root cells decreased significantly under moderate salt stress. These changes can be attributed to a stronger ionic stress due to the K(+) loss from the root tissues.

Salicylic acid (SA) is a key component of plant defence, which exerts a concentration-dependent effect on photosynthesis under multi-faceted influence of light. Photosynthetic activities and chloroplast morphology were studied in tomato plants after treatment with a sublethal, 0.1 mM, and a cell death-inducing, 1 mM concentrations of SA under normal photoperiod during light phase and after a prolonged dark phase. SA (1 mM) decreased the maximal (Fv/Fm) and effective quantum yields of PSII [Y(II)] and PSI [Y(I)] under both environmental conditions, however, the photoprotective processes were not significantly different between light and dark samples. Decrease in grana height, thylakoid dilation and deformation of lumen were also observed in the light. In contrast to illuminated samples, 0.1 mM SA decreased Y(II) and Y(I) after dark incubation, but nonphotochemical energy dissipation and cyclic electron flow increased, suggesting that the photoprotective mechanisms could be activated in plants exposed to prolonged darkness. fluorescence yield in dark-adapted state; FM -fresh mass; Fv/Fm -maximum quantum yield of PSII; HR -hypersensitive response; LEF -linear electron flow; PN -net photosynthetic rate; qL -estimates the fraction of open PSII centres; qP -photochemical quenching; RC -reaction centres; ROSreactive oxygen species; SA -salicylic acid; TEM -transmission electron micrograph; Y(I) -effective quantum yield of PSI; Y(II)effective quantum yield of PSII; Y(CEF) -yield of cyclic electron flow around PSI; Y(NA) -the quantum yield of nonphotochemical energy dissipation in PSI due to acceptor side limitation; Y(ND) -quantum yield of nonphotochemical energy dissipation in PSI due to donor side limitation; Y(NO) -quantum yield of nonregulated nonphotochemical energy dissipation; Y(NPQ) -quantum yield of regulated nonphotochemical energy dissipation.

The presence of solar radiation is one of the most important environmental factors, which is required for the optimal growth and development of plants. The absence of it (e.g. in the night period or artificially prolonged darkness) can alter the light-dependent signalling and regulation pathways and may induce new defence responses. Antioxidant enzymes as components of the plant defence system play a crucial role in the detoxification of reactive oxygen species (ROS) induced by several stressors; however, their regulation can be different in the light or in the dark. In this review we summarize the current knowledge about the physiological and molecular aspects of dark-modulated key antioxidant enzymes (superoxide dismutase, catalase, and ascorbate peroxidase) in different plant species and discuss their roles in different developmental processes (seedling growth and development or senescence) and in responses to environmental stresses (cold, chilling, heat, and biotic stress). Moreover, the hormonal regulation of respective gene transcription and the changes in activity of various isoenzymes at subcellular level are also summarized. Based on this knowledge, modification of these antioxidant enzymes may be used to increase the yield and stress tolerance of cultivated plants in the changing environment.Additional key words: ascorbate peroxidase, catalase, reactive oxygen species, superoxide dismutase. 

Highlights  ABA-deficient sitiens mutant of tomato was more sensitive to salt stress than WT  Sitiens mutant exhibited severe osmotic and moderate ionic stress under salt stress  Mutants displayed higher decrease in net CO2 assimilation rate under high salinity  Cyclic electron transport was severely reduced under salt stress in sitiens mutants  Proline could alleviate salt stress injury at sublethal salt stress in the mutants 3

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