MELATONIN (N-acetyl-5-methoxytryptamine) is receiving a considerable interest due to its prospected role in alleviating the heavy metals stress in plants. Thus a step toward the profitable use of melatonin (ME) in agriculture, the present study investigated the exogenous application of ME in Eurica vesicaria grown in contaminated soil amended with lead nitrate Pb(NO3)2 to produce Pb levels of 350mg/kg. Melatonin was applied by two methods: priming of seeds for 12hrs [0.0μM (P0); 25μM (P1) and 50μM (P2)] and by foliar spray at seedling foliage [0.0μM (F0); 25μM (F1) and 50μM (F2)].
Rocket plants affected by Pb toxicity exhibited significant reduction in the growth characteristics (fresh and dry weights of shoot and root, and leaf area/plant), photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoids), photosynthetic activity (14CO2-assimilation), and photosynthetic enzymes (phosphoenolpyruvate carboxylase and ribulose-1,5-bisphosphate-carboxylase/oxygenase). Meanwhile, exogenous application of ME appeared to ameliorate toxic effects of lead by improving the above criteria as compared to the control or Pb treated plants. These results showed that ME can promote the growth of Eurica vesicaria growing in Pb-contaminated soil by stimulating the photosynthetic activity, and the foliar spray method is the optimum at a dose of ME (50μM).