RESPONSES OF ANTIOXIDANTS TO PARAQUAT IN PEA LEAVES

Differential sensitivity to the oxidant paraquat was observed in pea ( Pisum sativum L.) based on cultivar and leaf age. To assess contributi ons of inductive responses of the antioxidant enzymes in short-term re sistance to oxidative damage, activities of glutathione reductase (CR) , superoxide dismutase (SOD), and ascorbate peroxidase (APX) and trans cript levels for plastidic CR, Cu,Zn SOD, and cytosolic APX were deter mined. Responses to paraquat exposure from three different leaf age cl asses of pea were studied. Resistance was correlated with leaf age, ph otosynthetic rates, enzyme activities, and pretreatment levels of plas tid GR and plastid Cu,Zn SOD transcripts. In response to paraquat, sma ll increases in activities of CR and APX were observed in the more res istant leaves. These changes were not reflected at the mRNA level for the plastidic CR or Cu,Zn SOD. Paraquat-mediated increases in cytosoli c APX mRNA occurred in all leaf types, irrespective of resistance. Dev elopmentally controlled mechanisms determining basal antioxidant enzym e activities, and not inductive responses, appear to be critical facto rs mediating short-term oxidative stress resistance.