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Not AvailableHigh temperature is an important environmental stress that influences the growth and development of crop plants thus adversely affecting their production and productivity. An experiment was carried out to investigate the protective role of bioregulators viz., Putrescine (Put), Thiourea (TU) and Hydrogen Peroxide (H2O2) against high temperature stress in maize at early vegetative stage. Maize genotypes earlier identified as heat tolerant (NSJ 221 and NSJ 189) and heat sensitive (RJR 270 and PSRJ 13099) were chosen for the present study. Independent foliar spray of Put (4 mM), TU (20mM) and H2O2 (1.2 mM) was observed to have significant difference (P>0.01) between genotypes, treatments and their interaction when applied 72 hrs prior to heat exposure. High temperature stress led to disruption of cellular membrane by increasing cell membrane injury, lipid peroxidation and H2O2 contents. It led to decrease in total chlorophyll content, soluble proteins, quantum yield and POD activity. Increased SOD activity in heat stressed seedlings was recorded with genotypes NSJ 189 and NSJ 221. Foliar application of Put, TU and H2O2 ameliorated heat-induced damages by stimulating the antioxidant enzyme system through decrease in lipid peroxidation, membrane injury and H2O2 contents in all the genotypes when compared with untreated heat stressed seedlings. Spray with these chemicals resulted in an increase in chlorophyll content, quantum yield and activities of anti-oxidative stress enzymes. Genotypes RJR 270 and PSRJ 13099 recorded improved heat tolerance with spray either of these chemicals by enhancing their biochemical potential. Spray of Put or TU was observed to be more effective of improving heat stress tolerance of maize seedlings.Not Availabl