Investigation of the role of exogenous melatonin application on the antioxidant defense system in the seeds of wheat (Triticum aestivum L.) exposed to ionising radiation stress

In this study, ameliorative effects of two types of melatonin treatments, pre-sowing priming and application during growth, in roots and shoots of wheat (Triticum aestivum L. 'Bezostaya-1') seedlings exposed to different radiation doses (100, 200, 300, and 400 Gy) of radioactive cobalt (60Co) gama rays were investigated at physiological and biochemical levels. For this purpose, morphometric parameters (root and shoot length, fresh and dry weight, leaf area), osmolyte content (proline, sucrose, fructose, glucose and fructan contents, osmotic potential), photosynthetic parameters (chlorophyll and carotenoid contents, Fv/Fm ratio), enzymatic (SOD, POX, CAT, APX, GR, MDHAR, DHAR) and non-enzymatic (TFM, phenolic acids, GSH) antioxidant defense system components were determined. Both melatonin applications, being more apperent in pre-sowing melatonin, improved IR stress induced decreases in growth parameters, shoot osmotic potential and photosynthetic pigment content in the wheat seedling. The effects of melatonin on the antioxidant enzymes varied depending on the IR dose, enzyme type and method of melatonin application. The TFM content and PAL activity, which increased with IR stress, enhanged with post-sowing melatonin application while decreased with pre-sowing melatonin application. Melatonin and IR applications changed the phenolic acids and GSH content depending on the dose. Both of pre and post-sowing melatonin applications decreased proline and sugar contents which increased with IR stress. In conclusion, both melatonin applications, particularly pre-sowing melatonin, provide high tolerance to the wheat seedlings against to IR stress. Taken all together, it has been demonstrated that exogenous applied melatonin, especially pre-sowing melatonin application, provides the wheat seedlings high tolerance against IR stress.