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

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, mitigating the harmful effects of abiotic stress. Effects of pretreatment of soybean (Glycine max (L.) Merr.) seeds with 10, 30, 60 and 90 mu M 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 mu M ML (189%); 5th-day root protrusion by 290%, 214%, 185% and 85% (10, 30, 60, and 90 mu M ML respectively). The application of 10 (328%), 30 (224%), 60 (265%), 90 mu M ML (129%) increased root length, followed by root DM in stressed seedlings. Drought stress reduced photosynthetic efficiency, increased ROS, and reduced antioxidant enzymatic activities in soybean. In a second experiment, well-watered (100% field capacity) (WW) and drought stressed (DS) soybean plants received 30 and 50 mu M ML. Melatonin increased rate of CO2 assimilation and net photosynthetic rate at 30 mu M ML (WW) in 28% (2 d after application) and 50 mu M ML (DS) in 48% (5 d after application). Melatonin increased antioxidant enzymes activity (catalase and peroxidase) in plants under drought stress. The ML application enhanced leaf area 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 total seeds mass (27%) with 30 mu M ML. These results demonstrated that ML mitigated negative effects produced by drought on germination and growth of soybean seedlings, and acted as a drought tolerance inducer, improving yield under stress condition.