The protective effect of nitric oxide (NO) on the antioxidant system under paraquat(PQ) stress was investigated in leaves of 8-week-old lettuce (Lactuca sativa L.) plants. PQ stress caused a decrease of leaf growth including leaf length, width and weight. Application of NO donor, sodium nitroprusside (SNP), significantly alleviated PQ stress induced growth suppression. SNP permitted the survival of more green leaf tissue preventing chlorophyll content reduction and of higher quantum yield for photosystem Ⅱ than in non-treated controls under PQ exposure, suggesting that NO has protective effect on chloroplast membrane in lettuce leaves. Flavonoids and anthocyanin were significantly accumulated in the leaves upon PQ exposure. However, the rapid increase of these compounds was alleviated in the SNP treated leaves. PQ treatment resulted in lipid peroxidation and induced accumulation of hydrogen peroxide (H2O2) in the leaves, while SNP prevented PQ induced increase in malondialdehyde (MDA) and H2O2. These results demonstrate that SNP serves as an antioxidant agent able to scavenge H2O2 to protect plant cells from oxidative damage. The activities of two antioxidant enzymes that scavenge reactive oxygen species, superoxide dismutase (SOD) and catalase (CAT) in lettuce leaves in the presence of NO donor under PQ stress were higher than those under PQ stress alone. Application of 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3- oxide (PTIO), a specific NO scavenger, to the lettuce leaves arrested SNP mediated protective effect on leaf growth, photosynthetic pigment and antioxidant systems. However, PTIO had little effect on lettuce leaves under PQ stress compared with that of PQ stress alone. The obtained data suggest that the damage caused by PQ stress is in part due to increased generation of active oxygen by maintaining increased antioxidant enzyme activities and SNP protects plants from oxidative stress. From these results it is suggested that NO might act as a signal in activating active oxygen scavenging system that protects plants from oxidative damage induced by PQ stress and thus confer PQ tolerance.

Abstract
 1. Introduction
 2. Materials and Methods
  2.1. Plant material and growth conditions
  2.2. Growth measurements
  2.3. Determination of photosynthetic pigments
  2.4. Determination of flavonoids and anthocyanincontents
  2.5. Chlorophyll fluorescence measurement
  2.6. Determination of lipid peroxidation
  2.7. Determination of H2O2 content
  2.8. Determination of antioxidant enzyme activity
  2.9. Statistical analysis
 3. Results and Discussion
  3.1. Effect of NO on leaf growth under paraquatstress
  3.2. Effect of NO on photosynthetic responses underparaquat stress
  3.3. Effect of NO on antioxidant system under PQstress
 4. Conclusions
 References

• Jee-Na Lee(Department of Biological Sciences, Pusan National University)
• Jung-Hee Hong(Department of Biological Sciences, Pusan National University) Corresponding Author