Relationship between polyamines and paraquat toxicity in sunflower leaf discs

Polyamines have been reported as efficient antioxidant compounds in plants. Sunflower leaf discs, treated with 100 mu M paraquat (PQ), a well known oxidative stress inducer, showed decreased levels of putrescine (Put), spermidine (Spd) and spermine (Spm) (between 33% and 80% with respect to the controls). Arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) activities decreased 42% and 33% respectively. Among the markers of oxidative stress measured after PQ treatment, chlorophyll and glutathione content were reduced (30% and 49% respectively) and thiobarbituric acid reactive substances (TBARS) content increased (60%). Superoxide dismutase (SOD) activity declined 60% with respect to the control and lipoxygenase (LOX) increased 25% when leaf-discs were treated with the herbicide. Pretreatment with exogenous polyamines (1 mM) reversed paraquat toxicity to different degrees according to the polyamine and/or the tested parameter. Spermidine was able to inhibit chlorophyll loss, while Spm reverted the effect of PQ on the level of TBARS almost completely and also restored SOD activity close to control values. Putrescine was the least effective as an oxidant protectant. These results provide support for the argument that polyamines are effective antioxidants through their ability to act as radical scavengers.