Responses of Antioxidants to Paraquat in Pea Leaves (Relationships to Resistance).

Differnential sensitivity to the oxidant paraquat was observed in pea (Pisum sativum L.) based on cultivar and leaf age. To assess contributions of inductive responses of the antioxidant enzymes in short-term resistance to oxidative damage, activities of glutathione reductase (GR), superoxide dismutase (SOD), and ascorbate peroxidase (APX) and transcript levels for plastidic GR, Cu,Zn SOD, and cytosolic APX were determined. Responses to paraquat exposure from three different leaf age classes of pea were studied. Resistance was correlated with leaf age, photosynthetic rates, enzyme activities, and pretreatment levels of plastid GR and plastid Cu,Zn SOD transcripts. In response to paraquat, small increases in activities of GR and APX were observed in the more resistant leaves. These changes were not reflected at the mRNA level for the plastidic GR or Cu,Zn SOD. Paraquat-mediated increases in cytosolic APX mRNA occurred in all leaf types, irrespective of resistance. Developmentally controlled mechanisms determining basal antioxidant enzyme activities, and not inductive responses, appear to be critical factors mediating short-term oxidative stress resistance.