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The relationship between exogenous melatonin applied into cucumber seeds during osmopriming and modifications of their antioxidant defense was studied. Accumulation of hydrogen peroxide, antioxidant enzyme activities and glutathione pool were investigated in embryonic axes isolated from the control, osmoprimed, and osmoprimed with melatonin seeds. Germinating cucumber seeds are very sensitive to chilling. Temperature 10°C causes oxidative stress in young seedlings. Seed pre-treatment with melatonin seemed to limit H2O2 accumulation during germination under optimal condition as well as during chilling stress and recovery period. Melatonin affected superoxide dismutase (SOD) activity and its isoforms during stress and recovery period but did not influence CAT and POX activities. Thus it is possible that in cucumber this indoleamine could act mostly as a direct H2O2 scavenger, but superoxide anion combat via SOD stimulation. The GSH/GSSG ratio is considered as an indirect determinant of oxidative stress. When the cells are exposed to oxidative stress GSSG is accumulated and the ratio of GSH to GSSG decreases. In our research pre-sowing melatonin application into the cucumber seeds caused high beneficial value of GSH/GSSG ratio that could be helpful for stress countering. Glutathione reductase (GSSG-R) activity in the axes isolated from these seeds was two fold higher than in those isolated from the control and from the osmoprimed without melatonin ones. Additional isoforms of GSSG-R in melatonin treated seeds were also observed. It explains high and effective GSH pool restoration in the seeds pre-treated with melatonin. We confirmed that melatonin could protect cucumber seeds and young seedlings against oxidative stress directly and indirectly detoxifying ROS, thereby plants grown better even in harmful environmental conditions. This work is the first that investigated on plant in vivo model and documented melatonin influence on redox state during seed germination. This way we try to fill lack of information about melatonin-regulated pathways involved in antioxidant strategy of plant defense.

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Melatonin, due to its pleiotropic effects plays an important role improving tolerance to stresses. Plants increase endogenous melatonin synthesis when faced with harsh environments as well as exogenously applied melatonin limits stress injuries. Presented work demonstrated that single melatonin application into the seeds during pre-sowing priming improved oxidative stress tolerance of growing seedlings exposed to paraquat (PQ). PQ is a powerful herbicide which blocks the process of photosynthesis under light conditions due to free radicals excess production, when O2 is rapidly converted to O2•– and subsequently to other reactive oxygen species. The parameters of chlorophyll fluorescence [Fv/Fm, Fv/Fo, Rfd, ΦPSII, qP, and non-photochemical quenching (NPQ)] in all variants of pea leaves (derived from control non-treated seeds – C, and those hydroprimed with water – H, and hydroprimed with melatonin water solution 50 or 200 μM – H-MEL50 and H-MEL200, respectively) were analyzed as a tool for photosynthetic efficacy testing. Moreover stability of the photosynthetic pigments (chlorophylls a, b, and carotenoids) was also monitored under oxidative stress conditions. The results suggest that melatonin applied into the seed significantly enhances oxidative stress tolerance in growing seedlings. This beneficial effect was reflected in reduced accumulation of O2•– in leaf tissues, preservation of photosynthetic pigments, improved functioning of the photosynthetic apparatus and higher water content in the tissues during PQ-mediated stress. Our findings provide evidence for the physiological role of this molecule and serve as a platform for its possible applications in agricultural or related areas of research.

Stresses provoked by adverse living conditions are inherent to a changing environment (climate change and anthropogenic influence) and they are basic factors that limit plant development and yields. Agriculture always struggled with this problem. The survey of non-toxic, natural, active substances useful in protection, and stimulation of plants growing under suboptimal and even harmful conditions, as well as searching for the most effective methods for their application, will direct our activities toward sustainable development and harmony with nature. It seems highly probable that boosting natural plant defense strategies by applying biostimulators will help to solve an old problem of poor yield in plant cultivation, by provoking their better growth and development even under suboptimal environmental conditions. This work is a concise review of such substances and methods of their application to plants.