Melatonin priming manipulates antioxidant regulation and secondary metabolites production in favor of drought tolerance in Chenopodium quinoa Willd

Melatonin can be considered a physiological protective agent against environmental stress because of its natural antioxidant ability. The main goal of this study was to investigate the possible melatonin-positive effects on the growth enhancement and drought tolerance of quinoa (Chenopodium quinoa Willd) cultivars. To accomplish this objective, we compared the effects drought stress (100 % field capacity as control and 40 % field capacity), priming (melatonin priming (MP), hydro priming (HP), non-priming (NP), and cultivars (Titicaca, Giza1). Thus, the obtained results indicated higher oxidative damage, lipid peroxidation, and reduced chlorophyll content, which decreased the seed yield more in Giza1 than in Titicaca in comparison with the control. MP remarkably enhanced endogenous melatonin content, superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) activity in Titicaca (1.57, 2.50, 1.40, 1.90, and 2 times, respectively) and Giza1 (1.68, 4.50, 4, 2 and 2.6 times, respectively), compared with NP, under drought stress. MP effectively protected Titicaca and Giza1 chlorophylls and carotenoids, while decreasing the malondialdehyde (MDA) content through antioxidant enzyme activation, facilitation of soluble protein, as well as sugar synthesis and osmolyte accumulation. Therefore, MP improved photosynthesis in Titicaca and Giza1 under drought stress compared with NP. Overall, Titicaca showed a relatively higher drought stress tolerance. Furthermore, MP ameliorated the adverse consequences of drought stress and facilitated the recovery of susceptible cultivars such as Giza1 through the enhancement of photosynthetic pigments, antioxidant systems, osmotic adjustment, and secondary metabolite production. The main novelty of the current study is the elucidation of the role of melatonin priming in the induction of drought tolerance in quinoa.