Melatonin Applied by Osmopriming as a Biostimulator Improving Cucumber (Cucumis sativus L.) Seedling Growth at Abiotic Stress Conditions

Cucumber (Cucumis sativus L. cv. Odys) seed osmopriming effects on germination and subsequent seedling growth at suboptimal conditions (chilling temperatures or toxic copper concentration) were investigated. Biochemical stress markers, chlorophyll and phenolic compound contents were estimated. The seeds were osmoprimed (O) in polyethylene glycol (PEG) with water (4.5 MPa at 25 degrees C) or with melatonin (Mel) water solutions (OMel) at concentrations : 25, 50, 100 and 500 mu M. Mel contents in the seeds before and after priming were determined using HPLC-EC. Being sensitive to the chilling stress, the seeds that germinated well (99%) at 25 degrees C, showed only 30% germination at 15 degrees C, and almost no germination (4%) at 10 degrees C. Osmopriming in PEG increased germination at 15 degrees C to 78%, and to 98% when combined with 50 mu M of Mel (OMel 50). The osmoprimed seeds germinated even at 10 degrees C and reached 43%, and 83% when 50 mu M of Mel was added during the treatment. Toxic copper ion (Cu(+2)) concentration 2.5 mM did not affect seed germination but drastically reduced seedling growth and blocked their regeneration. It was noted that only in the case of seedlings grown from the seeds previously osmoprimed with Mel 50 mu M (OMel 50) regeneration processes after chilling and copper stresses went well. There is still lack of information clearly explaining the role of melatonin in plant physiology. This molecule acts multidirectionally and usually is allied to other compounds - its potential as a phytobiostimulator will be discussed.