Thiamin-induced variations in oxidative defense processes in white clover (Trifolium repens L.) under water deficit stress

Vitamins are regarded as vital growth substances. They act as cofactors of various key enzymes involved in metabolism and also have vital influence on metabolic activities of plants. They are believed to play a significant role in plants against abiotic stresses, particularly during water stress. The current experiment was conducted to evaluate the role of exogenously applied thiamin on two white clover cultivars, Super Late Fsd and Layalpur Late, at varying [100% (control), 80%, and 60% field capacities] water regimes. A significant suppression was noted in plant biomass, plant height, chlorophyll a and b contents, total soluble proteins, and shoot and root P, Ca2+, and K+ contents particularly at 60% field capacity in both cultivars. Drought stress had no significant effect on phenolic contents, but MDA concentration was increased in white clover cultivar Layalpur Late at both 80% and 60% drought levels. However, in Super Late Fsd, it was increased at 80% and decreased at 60% field capacity. Drought stress significantly enhanced levels/activities of ascorbic acid and antioxidant enzymes (CAT, SOD, and POD). Foliar spray of 50 mM thiamin improved the shoot and root fresh weights and shoot dry weight. Varying levels of thiamin were found significantly effective for shoot length, root length, and photosynthetic pigments (chlorophyll a and b). However, no considerable change was observed in ascorbic acid, total soluble proteins, and activities of antioxidants (SOD, CAT, and POD) due to exogenous spray of thiamin on two white clover cultivars. Of the two white clover cultivars, Super Late Fsd was better in shoot and root P and Layalpur Late in root dry weight and root length. Overall, 100 mM of thiamin was found effective in improving growth of both white clover cultivars, which can be attributed to thiamin- induced improvement in photosynthetic pigments and phenolic contents.