The effect of nitric oxide (NO) on antioxidant system and protective mechanism against oxidative stress under UV-B radiation was investigated in leaves of maize (Zea mays L.) seedlings during 3 days growth period. UV-B irradiation caused a decrease of leaf biomass including leaf length, width and weight during growth. Application of NO donor, sodium nitroprusside (SNP), significantly alleviated UV-B stress induced growth suppression. NO donor 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 UV-B stress, suggesting that NO has protective effect on chloroplast membrane in maize leaves. Flavonoids and anthocyanin, UV-B absorbing compounds, were significantly accumulated in the maize leaves upon UV-B exposure. Moreover, the increase of these compounds was intensified in the NO treated seedlings. UV-B treatment resulted in lipid peroxidation and induced accumulation of hydrogen peroxide (H2O2) in maize leaves, while NO donor prevented UV-B induced increase in the contents of malondialdehyde (MDA) and H2O2. These results demonstrate that NO serves as antioxidant agent able to scavenge H2O2 to protect plant cells from oxidative damage. The activities of two antioxidant enzymes that scavenge reactive oxygen species, catalase (CAT) and ascorbate peroxidase (APX) in maize leaves in the presence of NO donor under UV-B stress were higher than those under UV-B stress alone. Application of 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3- oxide (PTIO), a specific NO scavenger, to the maize leaves arrested NO donor mediated protective effect on leaf growth, photosynthetic pigment and free radical scavenging activity. However, PTIO had little effect on maize leaves under UV-B stress compared with that of UV-B stress alone. Nω -nitro-L-arginine (LNNA), an inhibitor of nitric oxide synthase (NOS), significantly increased H2O2 and MDA accumulation and decreased antioxidant enzyme activities in maize leaves under UV-B stress. This demonstrates that NOS inhibitor LNNA has opposite effects on oxidative resistance. From these results it is suggested that NO might act as a signal in activating active oxygen scavenging system that protects plants from oxidative stress induced by UV-B radiation and thus confer UV-B tolerance.

Abstract
 1. Introduction
 2. Materials and Methods
  2.1. Plant material and growth conditions
  2.2. Ultraviolet-B treatment
  2.3. Growth measurements
  2.4. Determination of photosynthetic pigments
  2.5. Determination of flavonoids and anthocyanincontents
  2.6. Chlorophyll fluorescence measurement
  2.7. Determination of lipid peroxidation
  2.8. Determination of H2O2 content
  2.9. Determination of antioxidant enzyme activity
  2.10. Statistical analysis
 3. Results and Discussion
  3.1. Effect of NO on leaf biomass under UV-Bradiation
  3.2. Effect of NO on photosynthetic responses underUV-B irradiation
  3.3. Effect of NO on the antioxidant system underUV-B radiation
 4. Conclusions
 References

• Jung-Hee Hong(Department of Biological Sciences, Pusan National University) Corresponding Author
• Myung-Hwan Jo(Department of Biological Sciences, Pusan National University)
• Tae-Yun Kim(Department of Biological Sciences, Pusan National University)