Antioxidant Defense Mechanisms of Salinity Tolerance in Rice Genotypes

In order to elucidate the role of antioxidant responses in salinity tolerance in rice genotypesunder salt stress, experiments were conducted using four rice varieties, including salt-sensitive BRRIdhan 28 and three salt-tolerant varieties BRRI dhan 47, BINA dhan 8 and BINA dhan 10. Thirty-day-oldrice seedlings were transplanted into pots. At the active tillering stage (35 d after transplanting), plantswere exposed to different salinity levels (0, 20, 40 and 60 mmol/L NaCl). Salt stress caused a significantreduction in growth for all the rice genotypes. Growth reduction was higher in the salt-sensitive genotypethan in the salt-tolerant ones, and BINA dhan 10 showed higher salt tolerance in all measuredphysiological parameters. The reduction in shoot and root biomass was found to be minimal in BINA dhan10. Chlorophyll content significantly decreased under salt stress except for BINA dhan 10. Proline contentsignificantly increased in salt-tolerant rice genotypes with increased salt concentration, and the highestproline content was obtained from BINA dhan 10 under salt stress. Catalase and ascorbate peroxidaseactivities significantly decreased in salt-sensitive genotype whereas significantly increased in salt-tolerantones with increasing salt concentration. However, salt stress significantly decreased guaiacol peroxidaseactivity in all the rice genotypes irrespective of salt tolerance. K+/Na+ ratio also significantly decreased inshoots and roots of all the rice genotypes. The salt-tolerant genotype BINA dhan 10 maintained higherlevels of chlorophyll and proline contents as well as catalase and ascorbate peroxidase activities undersalt stress, thus, this might be the underlying mechanism for salt tolerance.