Growth stage-based modulation in antioxidant defense system and proline accumulation in two hexaploid wheat (Triticum aestivum L.) cultivars differing in salinity tolerance

Changes in the antioxidant defense system and proline accumulation were examined at different growth stages (vegetative, boot and reproductive) in plants of two hexaploid spring wheat cultivars (S-24, salt tolerant; MH-97, salt sensitive), grown in hydroponics and salinity-affected with 0, 50, 100 and 150 mM of NaCl. Salt stress provoked a marked decline in plant dry mass and ascorbic acid contents, and increased proline, total soluble proteins and H₂O₂ contents in both wheat cultivars at all growth stages. However, higher accumulation of proline and H₂O₂ contents was recorded at the vegetative and boot stages, respectively, in both wheat cultivars. Salt stress caused a consistent rise in the activities of some key antioxidant enzymes (CAT, SOD, POD, and APX) at all growth stages only in the salt tolerant cultivar S-24, whereas such pattern of enhanced activities of enzymatic antioxidants in cv. MH-97 was found only at the vegetative stage under saline regimes. Maximum activities of various enzymatic antioxidants were observed at the vegetative stage in both wheat cultivars under varying external salt treatments. The results showed that high salinity tolerance of cv. S-24, as manifested by lower decrease in its dry matter under salt stress, was associated with higher activities of antioxidant enzymes, increased accumulation of proline, and lower levels of H₂O₂, as compared with cv. MH-97 at all growth stages under saline regimes.