Exposure to Lead (Pb) Induced Changes in the Metabolite Content, Antioxidant Activity and Growth of Jatropha curcas (L.)

Abstract

Lead (Pb) is a rapidly increasing heavy metal due to different human activities. It is highly toxic to plants, and its toxicity can impair various morphological, physiological and biochemical functions in plants. In the present study, an experiment was designed to investigate the response mechanisms to counteract Pb in the seedlings of Jatropha curcas. For this purpose, Pb accumulation, metabolite contents, antioxidant activity and accumulation of antioxidants such as phenolics and flavonoids were evaluated. The transcription levels of two genes, metallothioneins (JcMT2a) and phenylalanine ammonia-lyase (JcPAL), involved in the metal detoxification mechanism were also analyzed. The seedlings of J. curcas were hydroponically treated with different doses of Pb (0, 0.5, 5, 10, 20 and 40 mg/L). With increasing Pb dose in the nutrient solution, its accumulation was in the following pattern: roots> stems> leaves. The seedlings showed higher tolerance index (TI) in the highest Pb concentration. The dry weights of roots, stems and leaves, and the total content of proteins were significantly decreased with increasing Pb concentration. However, the photosynthetic pigments, total phenolic and flavonoid compounds were increased in response to elevated Pb concentrations. This was associated with increasing of total antioxidant capacity and radical scavenging activity. Relative transcription of JcMT2a and JcPAL showed distinct pattern in roots and leaves. Integral mechanism for Pb stress tolerance was likely accomplished by activation of antioxidant defense system which was evidenced by up regulation of JcPAL. Our study provides biochemical and genetic basis of Pb-induced defense mechanisms for metal detoxification in J. curcas.