Sunflower oil and seed are considered healthy food due to rich source of unsaturated fatty acids and functional molecules to improve the immunity against various pathogens under the current pandemic conditions (Rauf et al. 2020). Tocopherol and sterol are important functional molecules that act as antioxidants to improve immunity against pathogens (Velasco et al. 2002; Belo et al. 2017; Belo et al. 2018). Sterol contents control plasma cholesterol levels and thus preventing various cardio vascular diseases (Belo et al. 2018). Tocopherols can neutralize the peroxy radical and thus prevent oil and lipids from peroxidation, thereby improving their shelf life (Sadiq et al. 2019).
Tocopherol and sterol contents was found to be affected by the seasons and locations. Generally, tocopherol contents (440–815 µg g− 1) were higher in the autumn season than in the spring season (330–747 µg g− 1) across Sargodha and Faisalabad; whereas sterol content ranged between 1334 and 2560 µg g− 1 in the autumn and from 1320 to 2942 µg g− 1 in the spring seasons. Bahawalpur and Multan had significant lower tocopherol and sterol contents when compared with Sargodha and Faisalabad (Fig. 1).This may be due to higher available reproductive degree days during spring season than autumn season, which may had negative impact on the accumulation of tocopherol content specifically. Moreover, Multan and Bahawalpur locations had higher reproductive degree days accumulation than Sargodha and Faisalabad, which had a negative impact on the accumulation of both tocopherol and sterol contents.
Tocopherol and sterol contents accumulation has been affected by various environmental factors including increased intercepted radiations, which also increased the tocopherol accumulation period at specific range in sunflower and other species such as almond (Nolasco et al. 2004; Velasco et al. 2010; Belo et al. 2017; Kodad and Alonso 2018). Heat and drought stresses were detrimental for tocopherol accumulation in almond. It was also identified that α and βtocopherol contents in the species was affected by the genotypes and environment (Fritsche et al. 2017; Kodad and Alonso, 2018). Results showed significant genotype × location effects affect the tocopherol or sterol production in sunflower (Fritsche et al. 2017; Belo et al. 2018; Kodad and Alonso, 2018). In this context, the stability parameters indicated that some of the new hybrids such as H4 and H5 had relatively stable tocopherol and sterol contents across locations. Hence, these hybrids may be recommended for cultivation to produce oil with high oleic acid.
Overall, the tocopherol content of high oleic acid in the hybrids was not higher than standard low oleic acid hybrids in the autumn season or in the spring season at locations with relatively low temperature. However, high oleic acid hybrids comparatively had 38% higher tocopherol content at warmer locations such as Multan and Bahawalpur. On the other hand, high oleic acid hybrids produced higher sterol content in bot seasons and all locations. This result suggests that high oleic acid oil hybrids favors the accumulation of tocopherol content under warm climate. High oleic acid content in sunflower is also known to be triggered under warm temperature, which may have induced oxidative stability to sunflower oil and provided advantage for tocopherol accumulation as compared to low oleic acid hybrids (Angeloni et al. 2017; Akkaya et al. 2019). Our previous finding also showed that oleic acid, sterol and tocopherol contents were positively correlated (Hussain, 2020).
The high magnitude of heterosis for various hybrids such as H8, H4 and H5 may be an evidence of overdominance type of gene action for tocopherol and sterol contents. Heterosis has been shown to result by interallelic interactions causing an overdominance type of gene action (Khotyleva et al. 2017). Combining ability analyses was done to select positive combiners for tocopherol and sterol contents. Breeding lines such as B.116.P, B.112.P and RH.365 were best combiners for tocopherol. Breeding with positive general combining ability may carried preponderance of positive alleles for these traits (Khalil et al. 2016). Hence, these breeding lines may be further use in breeding programs for developing hybrids with high tocopherol contents.