Drought is a main stressor affecting plant production worldwide. Safflower (Carthamus tinctorius L.) is known to exploit biochemical strategies to tolerate drought stress. However, the little so far known about these strategies does not guarantee safflower yield stability in future. To fill the gap, changes in the biochemical traits and antioxidant activities of safflower were monitored using 100 genotypes under the two non-stress and drought-stress field conditions in two subsequent years (2017 and 2018). While drought stress was observed to give rise to reversible increases in total phenolics (TPC), total flavonoids (TFD), total flavonols (TFL), total anthocyanin (Ant), proline, malondialdehyde (MDA), and antioxidant activity, it decreased total chlorophyll (ChlT) and total carotenoid (Car) contents in safflower. Under drought stress, the highest values for TPC (21.55 16.07 mg GAEg −1 fresh weight [FW]), Car (0.08 mg g −1 FW), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) activity (98%) were measured in G 16 , averaged over the two study years. Also the highest values for TFD (5.17 mg QEg −1 FW), TFL (1.99 mg QEg −1 FW), Ant (234.1 µmol g −1 FW), ChlT (0.67 mg g −1 FW), and proline (851 μmol g −1 FW) were recorded for G 80 , G 60 , G 23 , G 62 , and G 33 . The least MDA content (2.8 µmol g −1 FW) was denoted to G 91 under drought stress. The results of both principal component and correlation analyses demonstrated the effective role of total flavonoids in safflower drought tolerance. The high genetic variance was seen to result in the high heritability of biochemical traits under drought stress, thereby improving drought tolerance in safflower cultivated in drought prone regions. The significant genetic variations in all the biochemical traits indicated that these traits, especially TPC and TFD, could be used as screening criteria for genotypic selection in arid climates.
<p>Improvement of elite safflower genotypes for drought-tolerance is hampered by a deficiency of effective selection criteria. The present study evaluated 100 genotypes of safflower in terms of their drought tolerance over a period of three years (2016–2018) under both non-stress and drought-stress conditions. The eight drought-tolerance indices of tolerance index (TOL), mean productivity (MP), geometric mean productivity (GMP), stress susceptibility index (SSI), stress tolerance index (STI), yield stability index (YSI), drought resistance index (DI), and harmonic mean (HARM) were calculated based on seed yield under drought (Y<sub>s</sub>) and non-drought (Y<sub>p</sub>) conditions. A high genetic variation was found in drought tolerance among the genotypes studied. The MP, GMP, and STI indices were able to discriminate between tolerant and drought-sensitive genotypes. Plots of the first and second principal components identified drought-tolerant genotypes averaged over the three study years. Cluster analysis divided the genotypes into three distinct groups using the drought tolerance indices. Ultimately, eight genotypes (namely, G<sub>3</sub>, G<sub>11</sub>, G<sub>13</sub>, G<sub>24</sub>, G<sub>33</sub>, G<sub>47</sub>, G<sub>58</sub>, and G<sub>61</sub>) from different origins were detected as more tolerant to drought stress suitable for use in safflower breeding programs in drought-affected areas. The most tolerant and susceptible genotypes could be exploited to produce mapping populations for drought tolerance breeding programs in safflower.</p>
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