RESEARCH ARTICLE The role of endogenous nitric oxide in melatonin-improved tolerance to lead toxicity in maize plants Mustafa Okant 1 & Cengiz Kaya 2 Received: 5 December 2018 /Accepted: 5 February 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Melatonin (MT) and nitric oxide (NO) are known as scavengers of free radicals and an antioxidant against biotic and abiotic stresses in plant defense systems. However, whether NO interplays role in MT-induced antioxidant defense remains to be determined in the plants exposed to lead (Pb) toxicity. So, two experiments were designed to evaluate the role of NO in MT- mediated tolerance of maize plants to Pb stress. In the initial experiment, prior to starting different treatments, a solution of 0.05- or 0.10-mM MT was sprayed every other day for a period of 10 days to the leaves of maize plants exposed to Pb stress (0.1-mM PbCl 2 ). Pb toxicity significantly caused reduction in plant biomass (both fresh and dry), PSII maximum efficiency (F v /F m ), total chlorophyll, leaf potassium (K), calcium (Ca), and leaf water potential, but it resulted in increased levels of proline, hydrogen peroxide (H 2 O 2 ), malondialdehyde (MDA), electron leakage (EL), leaf Pb, and endogenous NO. An addition experiment was set up to further understand whether NO played role in mitigation of Pb toxicity in maize plants by MT using scavengers of NO and cPTIO combined with the MT treatments. MT-induced tolerance to Pb toxicity was totally eliminated by cPTIO by reversing endogenous NO. The present results clearly indicated that MT mediated the endogenous NO to improve tolerance of maize plants to Pb toxicity. This evidence was also supported by the increases of H 2 O 2 and MDA and reduces some antioxidant enzyme activities tested as well as the plant growth inhibition and increased leaf Pb content by application of MT combined with cPTIO. Keywords Lead toxicity . Melatonin . Maize . Nitric oxide . Antioxidant system Introduction Maize (Zea mays L.) is one of the major crops grown widely in almost all parts of the world under various environmental conditions (Al-Tawaha et al. 2018). It is a multipurpose crop being used as foodstuff for human being and feed for poultry and livestock. It is used as raw material for industries due to its nutritional value (Afzal et al. 2009). Maize is the greatest concern of scientists because it can persist in harsh environmental conditions (Habiba et al. 2018). The heavy metal stress leads to detrimental effects on maize plants (Rizwan et al. 2017). Naturally, plants often have to struggle for life in environ- ments where different biotic and abiotic stress conditions pre- vail. Lead (Pb) is one of chief ecological contaminants, espe- cially in regions with high anthropogenic heaviness (Corpas and Barroso 2017; Surucu et al. 2018), and toxic level of Pb has shown harmful effects on crop biomass and yield (Kobylinska et al. 2017). Heavy metal concentrations in the soil are usually very low, but using phosphate fertilizer in a long time of period can result in hazardously high levels of these pollutants even if it is also used for remediation of Pb toxicity (Sonmez and Pierzynski 2005; Yan et al. 2015). In a wide cultivar of species, high Pb concentrations lead to irreg- ular phytomorphology, reduced cell growth, and ultimately cause cell death (Pourrut et al. 2011; Zhou et al. 2018). Furthermore, it leads to accumulation of ROS, which disturbs the ultrastucture of cellular organelles especially the cell mem- branes (Cuypers et al. 2010; Shahid et al. 2014). To mitigate the deleterious effect of many stresses, includ- ing Pb contamination, the reasonable approach could be the use of a bioregulator enhancing tolerance of plant to detrimen- tal stressors. Among numerous defensive compounds natural- ly available in plants, melatonin (MT) appears to be enormous Responsible editor: Gangrong Shi * Cengiz Kaya c_kaya70@yahoo.com 1 Field Crops, Agriculture Faculty, University of Harran, Sanliurfa, Turkey 2 Department of Soil Science and Plant Nutrition, Faculty of Agriculture, University of Harran, Sanliurfa, Turkey Environmental Science and Pollution Research https://doi.org/10.1007/s11356-019-04517-3