Abstract
High temperatures may cause scorching of the twigs and leaves along with visual symptoms of sunburn, leaf senescence, growth inhibition, and ultimately decreased plant growth and biomass. Sunflower (Helianthus annuus L.) is one of the important oil seed crops and it potentially fits in agricultural system and oil production sector of Pakistan. Hence, it is important to get the best variety which may able to tolerate high temperatures in hot and humid environments. For this purpose, we have conducted the present study using ten different cultivars of (Hysin-33, T-40, H-OI, Hysin-39, Suncross, Gulshin, FH-825, FH-797, FH-784, AGSUN-5270) of H. annuus grown in the growth chambers in two different temperatures, i.e. control (25 degrees C) and heat stress (45 degrees C). A pot experiment was conducted and contains nutritional sand medium in the controlled environment in the growth chamber for 4 weeks. Results from the present study showed that heat stress induced a significant (p < 0.05) decrease in shoot length, root length, shoot fresh weight, root fresh weight, shoot dry weight, root dry weight, chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid content while increasing the production of reactive oxygen species (ROS) by increasing the concentration of malondialdehyde (MDA) and hydrogen peroxide (H2O2), which is manifested by increasing content of flavonoid and phenolic. Results also showed that Hysin-39, FH-825, and Hysin-33 showed better growth and development in heat-stressed environment and considered a heat-tolerant cultivar while Gulshin, Suncross, and AGSUN-5270 showed poor growth and development in the same stressed environment and were considered heat-sensitive cultivars. The overall trend of H. annuus cultivars grown under heat stress is as follows: Hysin-39 > FH-825 > Hysin-33 > T-40 > H-OI > FH-784 > FH-797> AGSUN-5270 > Suncross > Gulshin.