Plant Growth Regulation 31: 215–224, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 215 Relationship between polyamines and paraquat toxicity in sunflower leaf discs Mar´ ıa P. Benavides*, Susana M. Gallego, Mar´ ıa E. Comba & Mar´ ıa L. Tomaro C´ atedra de Qu´ ımica Biol´ ogica Vegetal, Departamento de Qu´ ımica Biol´ ogica, Facultad de Farmacia y Bioqu´ ımica, Universidad de Buenos Aires, Jun´ ın 956 (1113) Buenos Aires, Argentina (*author for correspondence: Facultad de Farmacia y Bioqu´ ımica, Universidad de Buenos Aires, Jun´ ın 956 (1113) Buenos Aires, Argentina, e-mail: mbenavi@huemul.ffyb.uba.ar) Received 20 December 1999; accepted 4 January 2000 Key words: antioxidants, Helianthus annuus L., paraquat, polyamines Abstract Polyamines have been reported as efficient antioxidant compounds in plants. Sunflower leaf discs, treated with 100 μM 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% respec- tively) 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. Abbreviations: ADC – arginine decarboxylase; ODC – ornithine decarboxylase; LOX – lipoxygenase; SOD – superoxide dismutase; PQ – paraquat; Put – putrescine; Spd – Spermidine; Spm – Spermine; TBARS – thiobarbituric acid reactive substances 1. Introduction Polyamines (Pas) are ubiquitous nitrogen compounds which are present in plant cells at up to millimolar concentrations and are considered to participate in a variety of events related to plant growth and develop- ment [3, 18]. Despite extensive studies on polyamine metabolism, the exact role that these compounds play in plant physiology remains unclear [34]. The most important changes in Pas levels are related to plants under stress [16], e.g. osmotic stress [15, 2], ozone fumigation [24], UV radiation [22], atrazine treat- ments [37] and salt stress [15]. Paraquat (methyl viologen, 1,1 ′ -dimethyl-4,4 ′ - bipyrydinium dichloride, PQ) is a contact foliar herbi- cide, which causes rapid membrane damage by gener- ating highly toxic superoxide anions [11]. It is well known that the herbicide exerts this effect by cata- lyzing the transfer of electrons from photosystem I of chloroplast membranes to molecular oxygen, pro- ducing free radicals that cause lipid peroxidation and membrane damage [11]. Protective mechanisms against the production of reactive oxygen species do exist in plants. They include low molecular weight antioxidants, such as ascorbate and glutathione, as well as enzymatic components, e.g. superoxide dis-