Physiological and Metabolic Changes of Safflower (Carthamus tinctorius L.) Cultivars in Response to Terminal Heat Stress

Crop productivity in tropical areas is threatened primarily by high temperatures during reproductive phase which causes a drastic change in physiological and biochemical behavior of the major crops. In the present study, two field experiments were conducted during 2017-18 and 2018-19 growing seasons. Four safflower cultivars, namely, Goldasht, Faraman, Sofeh, and Parnian, were grown under normal, moderate, and severe terminal heat stress (THS) conditions. Sowing date with desired temperature was considered as control, delayed, and late sowing dates were considered as moderate and severe THS, respectively, so that the flowering and grain filling periods of cultivars were exposed to heat stress. H2O2 concentration and malondialdehyde (MDA) contents were increased in response to moderate and severe THS for all cultivars except for Goldasht in 2017-2018, indicating heat stress-induced oxidative stress. The highest MDA contents (27.3 and 17 mmol g(-1)Fw(-1)) were found in Sofeh under severe THS during 2017-2018 and 2018-2019, respectively. Moreover, the scavenging activity of DPPH radicals and activity of antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidise (APX), catalase (CAT), glutathione reductase (GR), and Glutathione S-transferase (GST) were increased in response to moderate and severe THS. Free amino acids accumulation including gamma-aminobutyric acid, lysine, leucine, isoleucine, taurine, and tyrosine was observed in response to THS. Further, we found that Faraman accumulated higher contents of fructose, xylose, and galactose in fraction under THS, whereas xylose content was significantly higher in Sofeh as compared to the other carbohydrates. By comparison, Faraman and Goldasht were most tolerant to heat stress due to significant increase in DPPH value, phenol and carbohydrate contents, free amino acid contents, and higher antioxidant activity compared to the other cultivars. Totally, these results underlay the enhanced tolerance to heat stress of the safflower cultivars, and changes in these traits in delayed and late sowing dates clarify that the perspective of oilseed crops cultivation in arid region is probably related to their potential adaptation to climate change.