Differential reponses to paraquat‐induced oxidative injury in a pea (Pisum sativum) protoplast system

Antioxidant responses to varying degrees of paraquat stress in freshly isolated photosynthesizing pea (Pisum sativum L.) protoplasts from cultivars Progress and Nugget were studied. Leaves of comparable maturity were used for protoplast isolation. Nugget protoplasts were more resistant to paraquat in the micromolar range under our conditions. In Nugget, a non-bleaching paraquat concentration (10 µM) inhibited CO₂-dependent O₂ evolution ca 50% during the first 40 min, remaining at that rate (“coping behavior”) for up to 100 min. In contrast, Progress protoplasts treated with the same concentration of paraquat did not exhibit coping behavior. Antioxidant enzyme activities were unaltered throughout the time course of the experiment in treated protoplasts from Nugget and in chloroplasts isolated from them. Thus, the coping behavior of Nugget protoplasts cannot be attributed to changes in activities of the three antioxidant enzymes tested. Paraquat treatment did not affect antioxidant enzyme activities in Progress protoplasts nor in chloroplasts isolated from them. When higher doses of paraquat were used (12 h, 0.1 mM paraquat), protoplasts from both cultivars were rapidly bleached and total protein decreased to ca 30% of pre-stress levels. Glutathione reductase (GR, EC 1.6.4.2) activity dropped in protoplasts from both cultivars under the severe stress conditions in concert with declines in protein levels. However, superoxide dismutase (SOD, EC 1.15.1.1) activity remained constant over the first 9 h of the time course, increasing to ca 150& of original levels by the final, 12-h time point. The activity of the plastid Cu,Zn isoform, expressed as a percentage of total SOD activity, declined over the time course of the experiment while that of mitochondrial MnSOD appeared to increase. This change in activity of MnSOD correlated with cell decline, therefore, and was not correlated with protection. These data are in agreement with some earlier reports and are compatible with the hypothesis that SOD activity levels increase in response to reactive oxygen species levels, even under conditions leading to cell death.