Antioxidant enzymatic activities and gene expression associated with heat tolerance in a cool-season perennial grass species

► This study determined major antioxidant enzymes associated with heat tolerance in a cool-season grass species. ► Different enzymes or isoforms and genotypes exhibited differential responses to heat stress at both enzymatic and gene expression levels. ► The enzymes, including SOD, CAT, POD, and APX were involved in oxidative protection from heat stress in Kentucky bluegrass. The objectives of this study were to examine antioxidant enzyme responses to heat stress at both enzymatic activity and transcript levels and to determine the predominant antioxidant processes associated with heat tolerance in a cool-season grass species, Kentucky bluegrass (Poa pratensis) using two genotypes differing in heat tolerance. Plants of heat-tolerant ‘Midnight’ and heat-sensitive ‘Brilliant’ were exposed to optimal temperature conditions (20/15°C, day/night) or heat stress (35/30°C) in growth chambers for 28d. ‘Midnight’ exhibited significantly higher photochemical efficiency (Fv/Fm), chlorophyll content, and lower electrolyte leakage compared with ‘Brilliant’ under heat stress. After long-term heat stress (21 and 28d), ‘Midnight’ maintained significantly higher activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), and glutathione reductase than ‘Brilliant’. Transcript levels of chloroplast (chl) Cu/Zn SOD, Fe SOD, CAT, POD and cytosolic (cyt) APX were significantly higher in ‘Midnight’ than in ‘Brilliant’ under long-term heat stress. Two antioxidant enzymes, monodehydroascorbate reductase and dehydroascorbate reductase, did not exhibit significant genotypic variations in enzymatic activity or transcript level. The differential responses of antioxidant enzymes to heat stress between heat-tolerant ‘Midnight’ and heat-sensitive ‘Brilliant’ at both enzymatic and gene levels indicated that SOD (chl Cu/Zn SOD and Fe SOD), CAT, POD, and cytosolic APX could play predominant roles in antioxidant protection against oxidative damages from long-term heat stress in Kentucky bluegrass.