Effect of Terminal Heat Stress on Some Agronomic, Physiological and Oil Yield Traits of Safflower Cultivars (Carthamus tinctorius L.) under Ahvaz Conditions

Terminal heat stress is a major abiotic stress especially in tropical and sub-tropical regions that severely reduces crop growth and yield. Safflower (Carthamus tinctorius L.) is an oilseed crop grown mainly for the production of high quality edible oil rich in polyunsaturated acids. This annual crop is native to a climate with a long dry season and a limited rainy season and compares to other annual crops has more resistance to drought stress. In addition, it's resistant to some diseases and susceptible to humidity. Since, to obtain the maximum yield of a crop, it is necessary to escape the development stages of (the/a) plant from unfavorable environmental factors and the full utilization of plant from favorable environmental conditions. Since the length of development stages more affected by temperature and day length, hence sowing date can be selected so that the different stages of plant growth were adapt with optimal temperature and day length. Yau (2006) indicated that later sowing of spring safflower in semi-arid and high elevation Mediterranean environment resulted in lower seed yield and later flowering does not allow an escape from the terminal drought and heat. Yau (2006) indicated that later sowing of spring safflower in semi-arid and high elevation Mediterranean environment resulted in lower seed yield and later flowering does not allow an escape from the terminal drought and heat. The aim of this study was to investigate the yield and yield components of safflower cultivars in response to different sowing data in Ahvaz. In order to study the effect of sowing date on yield and yield components of safflower cultivars, this experiment was carried out as split plot layout based on randomized complete blocks design with three replications in at the research farm of Shahid Chamran University of Ahvaz in 2017-2018. The main factor included three levels of sowing date (11 Dec, 31 Dec and 20 Jan), and a sub factor including four cultivars of safflower (Goldasht, Parnian, Faraman and Sofeh). Data collected on all parameters were analyzed statistically using SAS software and mean comparison was carried out using Duncan test at the 5% of probability level. The studied traits included grain yield, biological yield, harvest index, oil yield, total chlorophyll concentration, catalase and peroxidase enzyme activity and stress susceptibility index. A significant interaction was observed between the levels of sowing date and the cultivars in terms of plant height, weight of plant, biological yield and grain yield. Among the safflower cultivars, the highest weight per plant (143.3 g plant-1), biological yield (23352 kg ha-1), grain yield (4151.1 kg ha-1) and the lowest plant high (128.8 cm), was recorded in Faraman cultivar in sowing date of 11 December. In contrast, grain yield severely decreased in the 20 January sowing date with compared to the 11 December. So that, studied cultivars of Sofeh, Parnian, Faraman and Goldasht cultivar were decreased 53% (1277.3 kg ha-1), 44% (1668.9), 60% (1650.8) and 47% (1887.3), respectively. Under the heat stress, due to delayed planting date and decreased enzymatic activity of catalase, enzyme activity changed and the cultivars exposed to the terminal heat stress, but enzymatic activity of peroxidase increased. Stress susceptibility index (SSI), Parnian and Goldscht cultivars had a high tolerance with mean 0.85 and 0.92, while Faraman and Sofeh cultivars with an index of 1.16 and 1.03 had a high sensitivity to stress. This research appears the delay in optimal planting date leads to synchronize the plant's reproductive stages with a high temperature at the end of the season. As a result, shorter plants, lower biological yield, lower grain and oil yield will be observed, however, the response of the cultivars from each other was different depend on their enzymatic capacity and the ability to maintain chlorophylls. Planting date should be selected such a way that this process does not a coincidence with terminal heat season. So, under hot weather conditions in Khuzestan, selecting the optimum planting date can be an appropriate solution to reduce the negative aspects of the terminal heat stress on the grain yield and component yield of different cultivars of safflower.