Antioxidant defense system in lettuces tissues upon various As species exposure

Characterization of antioxidant response is essential to elucidate the mechanism for plants tolerating arsenic (As) stress. Ten-day old lettuces were exposed to 50, 100, and 200 μg L^-1 of arsenite (As(III)), arsenate (As(V)) or dimethylarsinic acid (DMA) for 50 days in hydroponic culture. The activities of superoxide dismutase, catalase, peroxidase, glutathione peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase, as well as the glutathione concentration in tissues, were monitored. And the speciation and occurrence of As were concurrently analyzed in roots and leaves. The results showed that As(III) was the predominant As species in lettuces upon inorganic As exposure, while DMA was the primary As species upon DMA exposure. DMA presented higher mobility than inorganic As. The reduction of As(V) in roots upon As(V) exposure and in leaves upon As(III) exposure were suggested. The alterations of enzymatic antioxidant activities and non-enzymatic antioxidant contents showed that the antioxidant responses were As species-dependent, dose-dependent and tissue-dependent. And upon As(V) and DMA exposures, antioxidant responses were more intense than that upon As(III) exposure. Further the results indicated that the antioxidant responses in lettuce were associated with the conversion and transport of As species.