Exogenous melatonin induces tolerance to drought stress damage in seedlings and soybean plants

Bruna Oliveira Spolaor1, 2, Suzana Chiari Bertoli2, Daniele Silva Sukert2, Hélida Regina Sala2, Bruno Felipe Picoli de Oliveira2, Ítalo Rafael de Freitas2, and Adriana Lima-Moro1, 2*
Melatonin (ML) is an indolamine that regulates a wide range of physiological processes as growth of roots and shoots,antioxidant enzyme activity, seed germination, photosynthetic efficiency promoting stress tolerance in plants, mitigatingthe harmful effects of abiotic stress. Effects of pretreatment of soybean (Glycine max (L.) Merr.) seeds with 10, 30, 60 and 90 μM ML were evaluated in seedlings under osmotic stress measuring germination speed index, and root protrusion, growth, and DM. Under osmotic stress, ML increased germination speed index at 10 (239%), 30 (194%) and 60 μ M ML (189%); 5th-day root protrusion by 290%, 214%, 185% and85% (10, 30, 60, and 90 μ M ML respectively). The application of 10 (328%), 30 (224%), 60 (265%), 90 μM ML (129%) increased root length, followed by root DM instressed seedlings. Drought stress reduced photosynthetic efficiency, increased ROS, and reduced antioxidant enzymaticactivities in soybean. In a second experiment, well-watered (100% field capacity) (WW) and drought stressed (DS) soybean plants received 30 and 50 μM ML. Melatonin increased rate of CO2 assimilation and net photosynthetic rate at30μ M ML (WW) in 28% (2 d after application) and 50μ MML (DS) in 48%(5 d after application). Melatonin increased antioxidant enzymes activity (catalase and peroxidase) in plants under drought stress. The ML application enhanced leafarea in WW plants, however decreased in stressed plants. An increased number of seeds, pod numbers, total seed mass,root DM were observed under WW. Drought stress increased numbers of seeds (27%), pod numbers (33%) and totalseeds mass (27%) with 30 μM ML. These results demonstrated that ML mitigated negative effects produced by droughton germination and growth of soybean seedlings, and acted as a drought tolerance inducer, improving yield under stresscondition.
Keywords: Antioxidant, drought stress, gas exchange, Glycine max, melatonin, osmotic regulation, root growth, ROS, scavenging system, stress tolerance.
1Universidade do Oeste Paulista (UNOESTE), Departamento de Produção Vegetal, Presidente Prudente, São Paulo, Brasil. 2Universidade do Oeste Paulista (UNOESTE), Centro de Estudos em Ecofisiologia Vegetal do Oeste Paulista (CEVOP), 19067-175 Presidente Prudente, São Paulo, Brasil. *Corresponding author (adrianamoro@unoeste.br).