The influence of terminal heat stress on physiological and yield characteristics of promising sunflower cultivars in Ahvaz climate condition

IntroductionClimate change and global warming which are the consequences of rising temperatures, can affect agriculture in various ways. Heat stress is a serious threat to crop production in the world. Higher mean temperatures generally result in increased heat stress. This abiotic stress can adversely affect crop productivity and quality and weaken global food security. With current increased average temperatures, the occurrence of heat stress could become more intense. Terminal heat stress is the most widespread heat stress, limiting crop yields. Terminal heat stress during the reproductive stages will have a negative influence on grain yield and oil quality traits of oil seed crops. Sunflower is an annual oilseed crop with adaptation to dry land or irrigated cropping systems. It is suited to both spring and summer planting in most regions and it will be exposed to major climate change and potentially impacted by high mean temperatures. Therefore, climate change and heat stress is a major production constraint for sunflower worldwide and it may be severely damaged when exposed to stresses above its tolerance threshold.Materials and methodsIn order to study of agronomic and physiological traits of sunflower cultivars under terminal heat stress, a field experiment was carried out during 2017-2018 growing season in Ahvaz conditions in a split plot arrangement in randomized complete blocks design with three replications. Main plots consisted of three sowing date including; 11th Nov, 11th Dec and 11th Jan, and sub plots consisted of five sunflower cultivars including; Qasem, Fantasia, Shams, Lakomka and Progress. Sowing date with desired temperature was considered as control, delayed and late sowing dates were considered as heat stress so that the flowering and grain filling periods of cultivars exposed to normal condition, moderate and severe terminal heat stress, respectively. Sunflower cultivars were evaluated under the optimum temperature (sown in the 11 November) and terminal heat stress condition (sown in 11 December and 11 January to expose the sunflower to higher temperatures than the optimum at the grain filling stage).Results and discussionTerminal heat stress in delayed and late sowing date caused a decrease in biological yield, grain yield, number of seeds per head, 1000-seed weight, head weight, oil yield, chlorophyll index, relative water content, chlorophyll content and stomatal conductance. There was a significant difference between cultivars