ROLE OF SALICYLIC ACID IN TERMINAL HEAT STRESS TOLERANCE OF WHEAT (Triticum aestivum L.)

Wheat (TriticumaestivumL.) is very sensitive to high temperatureand experiences heat stress to varying degrees at different phenological stages, but heat stress during the reproductivephase is more detrimental than during the vegetative phase.In wheat, grain filling also relies on mobilisation of reserve carbohydrates particularly under stress condition from the stem to the developing grains. SA is a natural phenolic compound and acts as an endogenous signal molecule that plays a crucial task in the regulation of plant growth, development, interactions with other organisms, and responses to environmental stress. Therefore, in order to test the possibilities of foliar application of salicylic acid in improving the phenology, photosynthesis, remobilisation of carbohydrates and yield under normal and late sown high temperature condition and to analyse the SA mediated change in physiological, biochemical, molecular alternations in relation to high temperature stress. In this regard extensive studies were doneand three independent experiments were formulated.The first experiment was performed for identification of SA concentration for wheat plants under study. The wheat seedling (10 days old) grown in the incubator was treated with different concentrations (6) of SA and it was observed that onlylower concentration (0.1mM and 0.25 mM ) of SA was able to reinstate growth and significantly counter the detrimental effects of heat stress (39±2c) on wheat seedlings. In second experiment biochemical, physiological as well as a few gene expression patterns were analysed in leaves of 30 days old (vegetative stage) wheat genotype (HD3086) and it was observed that the exogenous application of SA (0.1mM) efficiently guarded thephysiological traits, by causing an increase in antioxidant enzyme activities and inductionof gene expression of Hsps and photosynthesis genes (TaHSP70, TapsbA and TapsbO) while decreasing the H2O2 and TBARS contents in wheat plants subjected to heat stress (39±2c) during vegetative stage. Finally, the SA concentration 0.1mM were used in the third experiment to authenticate the response of SA under prolonged high temperature stress at reproductive stage wheat genotypes and observation were recorded at two and ten days after foliar spray of SA. A pot culture experiment was conducted with four wheat genotypes, two relatively tolerant (HD2985 and HD3086) and two susceptible (HD3076 and HD3043) and staggered sowing were done to induce high temperature stress at reproductive stage. Observations were recorded as TBARS content, MSI, photosynthetic pigments, photosynthetic rate, photochemical efficiency, antioxidant enzymes activity (catalase, peroxidase, superoxide dismutase, glutathione reductase and ascorbate peroxidase), non-enzymatic metabolite (oxidised glutathione and reduced ascorbate),endogenous level of ABA, total SA, free SA, non-structural carbohydrates, remobilisation efficiency (RE), grain yield, test weight and harvest index were recorded two and ten days after foliar spray of SA. Values of all above parameters were reduced under late sown high temperature stress condition while some other parameters like lipid peroxidation (TBARS content), endogenous level of SA and ABA, RE of NSC and activities of antioxidant enzymes increased. Interestingly, the application of SA in general maintained the higher values of all these parameters in tolerant genotypes compared to susceptible genotypes and reduced the lipid peroxidation and catalase activity. In present study gene expression analysis of transcription factors (TahsfA2b and TahsfA1a), small and large HSPs (Tahsp101, Tahsp16.9 and Tahsp17.8) and expression of biosynthesis pathways of SA and ABA (TaICS, TaNCED) fructan hydrolysing enzyme (Ta6-FEH) were done in leaves two days after foliar spray of SA. Results of gene expression study clearly revealed exogenous application of SA plus high temperature enhanced the expression of both transcription factors and large HSP and it was higher in HD2985,HD3086 and HD3043 compared to HD3076. The expression of TaICS, TaNCED, Ta6-FEH and small HSps were not affected significantly by application of SA. However high temperature stress induced expression of all the genes compared to normal sown plants and expression was higher in tolerant genotypes. From present study, it can be concluded that the exogenous application of SA (0.1mM) positively modulated the physiological and biochemical traits and transcription level of Hsfs and large molecular weight HSP analysed two days after foliar application. However the positive effect on most of physiological and biochemical traits were not consistent as it decreased when analysed ten days after foliar spray. This clearly shows the response of SA is not long lasting specifically in our experiment or is transient in nature and it is also evident from yield data, which is not significantly enhanced.