PROTECTIVE EFFECTS OF EXOGENOUS NITRIC OXIDE AGAINST LEAD TOXICITY IN LEMON BALM (Melissa officinalis L.)

This research investigated the effects of exogenous sodium nitroprusside (SNP) supplementation as nitric oxide (NO) on alleviating Pb-induced oxidative damage in lemon balm (Melissa officinalis L.) plants. Pb (100, 300 and 500 mu M) alone and in combination with SNP (100 and 200 mu M) were given to hydroponically grown Melissa officinalis L. plants. The findings suggested that in Pb-treated plants, height, total dry weight, and chlorophyll content of leaves markedly decreased while application of 100 mu M SNP alleviated the inhibitory effect of Pb on plant growth and chlorophyll content. Pb exposure caused oxidative stress by elevating lipid hydroperoxide contents of the seedlings. Application of 100 mu M SNP counteracted Pb toxicity by reducing lipid hydroperoxide contents of Pb-treated seedlings. Furthermore, it was found that the activities of antioxidant enzymes such as polyphenol oxidase, catalase, guaiacol peroxidase, and ascorbate peroxidase were increased in Pb-treated plants. The use of NO especially at low concentrations reversed Pb-induced negative effects whereas high concentrations of NO had no obvious alleviating effect on Pb toxicity in Melissa officinalis L. On the other hand, application of 100 mu M SNP could function as a defense mechanism of the plant against Pb toxicity and mitigate Pb stress.