Nitric oxide alleviates lead toxicity by inhibiting lead translocation and regulating root growth in watermelon seedlings

Lead (Pb) is one of the most abundant toxic heavy metals, which have a serious impact on the growth and yield of crop plants. Nitric oxide (NO) is a natural signaling molecule that regulates the growth and productivity of plants. Here, exogenous NO was found to enhance Pb tolerance in watermelon, which resulted in more Pb restriction in roots and less up-translocated Pb to aerial parts. Pb stress, however, led to an increase in shoot dry weight, root biomass, root relative water content, leaf malondialdehyde (MDA) content, and the total soluble protein content in leaves and roots. By contrast, shoot height and fresh weight, leaf biomass, and root MDA content were decreased under Pb stress. NO treatments alleviated Pb toxicity by decreasing Pb translocation, enhancing root growth (elongation and biomass), inducing antioxidant enzymes activities, and reducing root MDA contents in watermelon seedlings. In conclusion, our results provide useful insights into the mechanism of Pb tolerance in cucurbit crops and information for the cultivation management of watermelon in the presence of this heavy metal (Pb).