Role of salicylic acid and hydrogen sulfide in promoting lead stress tolerance and regulating free amino acid composition in Zea mays L.

Lead toxicity is a threat to growth and production of crop plants. In this study, effect of salicylic acid and sodium hydrosulfide pretreatments was investigated on the alteration in the Pb accumulation, protein content, nitrate reductase activity and metabolism of selected free amino acids in maize plants subjected to lead stress. Maize seeds were soaked in 0.5mM salicylic acid and sodium hydrosulfide for 12h and then exposed to 2.5mM Pb (NO3)(2) for 9days. Results demonstrated that lead stress significantly reduced the plant growth, shoot glutathione content and protein content, nitrate reductase activity in the shoots and roots while increased glutathione content in roots of maize plants. We also observed accumulation of most free amino acids composition except tyrosine and tryptophan under lead stress condition. Plants pre-treated with salicylic acid and sodium hydrosulfide exhibited improvement of plant growth, decrease of Pb accumulation, increase of protein and glutathione contents and nitrate reductase activity. Salicylic acid and sodium hydrosulfide pretreatments decreased most amino acids content in shoot of lead stressed plants. In case of roots, salicylic acid and sodium hydrosulfide had different effects on the content of amino acids. Pretreatment of sodium hydrosulfide resulted in greater levels of free amino acids in roots. These results indicate that moderate lead stress can be attributed to effect of salicylic acid and sodium hydrosulfide by improvement of nitrate reductase activity and glutathione content and regulation of amino acids metabolism.