Changes in whole grain polyphenols and antioxidant activity of six sorghum genotypes under different irrigation treatments
Introduction
Sorghum (Sorghum bicolor (L.) Moench) is the fifth most valuable global cereal crop after wheat, rice, maize and barley. Due to its adaptability to drought and high temperatures, sorghum is widely grown in semi-arid and arid regions, where sorghum porridge is consumed as a major part of the diet (Stefoska-Needham, Beck, Johnson, & Tapsell, 2015). Although sorghum has been a significant component of the diet for populations in developing areas of the world for centuries, there has been less interest in sorghum as a human food source in developed countries, such as Australia and the United States, where it has been primarily used as livestock feed and biofuel production (Lemlioglu-Austin, 2015). However recently, more attention has been paid to the consumption of sorghum as a human food source, in developed countries, as a gluten free alternative to wheat and due to its high content health-promoting components, such as phenolic antioxidant compounds (Althwab et al., 2015, Dykes et al., 2011).
Phenolic compounds possess a benzene ring with one or more hydroxyl groups, and could be negative in terms of reducing starch, protein and minerals digestibility (Stefoska-Needham et al., 2015, Wu et al., 2016). However, recently, phenolic compounds have gained increased interest because of their antioxidant activity. The consumption of these compounds is thought to have potential health benefits, such as reducing oxidative stress and providing anti-inflammatory and anti-carcinogenic properties (Stefoska-Needham et al., 2015). A wide array of phenolic compounds, including phenolic acids, flavonoids, and condensed tannins, has been found in sorghum grain (Stefoska-Needham et al., 2015). Sorghum genotypes with pigmented testa have been found to have the highest phenolic content and antioxidant activity (Dykes, Rooney, Waniska, & Rooney, 2005). Most sorghum flavonoids are in the outer layers of the grain, and consequently their concentrations and profiles are related to pericarp colour, pericarp thickness and the presence of testa; these profiles being under genetic control (Taleon, Dykes, Rooney, & Rooney, 2012). For example, the white-grained sorghum genotypes have lower phenolic content and simpler phenolic profiles than coloured ones (Wu et al., 2016). It has long been known that the antioxidant activity of plant foods is affected not only by total amount of phenolic compounds but also by specific individual phenolic compounds (Rice-Evans, Miller, & Paganga, 1996). Therefore, it is important to investigate both total and individual phenolic compounds of sorghum genotypes to better understand which may be most suited to particular food applications.
Water deficit is one of the principal factors restricting crop performance in arid and semiarid regions, due to irregular annual rainfall during the growing season (Alderfasi, Selim, & Alhammad, 2016). Irrigation treatments can improve the quality of the crop in areas of low rainfall where there is access to water for irrigation purposes. Pernice et al. (2010) planted tomato with three irrigation treatments and found that no irrigation and reduced irrigation could increase the flavonoid concentrations and antioxidant activity of tomato fruits, when compared to standard irrigation. Romero, Tovar, Girona, and Motilva (2002) found that water stress had significant effects on phenolic profile and content of fruit from young olive trees, which in turn was related to increase oxidative stability, but could enhance bitterness index of virgin olive oils, which may negatively affect consumer acceptance. Servili et al. (2007) also reported that the phenolic profiles of virgin olive oil were affected by water stress during plant growth, which in turn influenced its sensory characteristics, such as increase“bitter” and “pungent” characteristics. However, no information is available on how water stress can affect the phenolic compounds in sorghum grain, which will be of particular importance in selection of genotypes and irrigation regimes to provide polyphenolic levels in the grain for food products, targeted at those with different nutritional and health status.
Therefore, the purpose of this work was to investigate both the effects of water stress on content of phenolic compounds and antioxidant activity of sorghum grain, and the response of six different sorghum genotypes. In addition, selected individual phenolic compounds were quantified, in order to gain preliminary understanding of the extent to which water stress might affect the biosynthesis of individual phenolic compounds in sorghum grain.
Section snippets
Plant material and treatments
Experiments were carried out at Curtin University Field Trials Area, Perth, Western Australia (latitude 32°00/S, longitude115°53/E, altitude 20 m) during 2014–2015. Fig. 1 presents the daily values of rainfall and air temperature, which were adopted from the nearest Bureau of Meteorology weather station (Perth Airport WA) (BOM (Bureau of Meteorology), 2014). Six sorghum genotypes: black pericarp ‘Shawaya’, brown pericarp ‘IS131C’, white pericarp ‘QL12’ and three red pericarp genotypes
Grain physical characteristics
The ANOVA main effects of genotype and irrigation treatment and their interaction for grain physical characteristics, phenolic contents and antioxidant capacity are presented in Supplementary Table S2. Grain weight (Fig. 2a) and diameter (Fig. 2b) were significantly influenced by genotype and irrigation regime (P ⩽ 0.05). The deficit irrigation treatment significantly decreased both grain weight and diameter. Sorghum B923296 had the highest values of grain weight and diameter under full and
Discussion
Grain physical characteristics are principle quality contributions for grain processing. The values of grain weight and diameter measured in the present study were in agreement with previous reports (Liu et al., 2013, Wu et al., 2016). However, Liu et al. (2013) found that no significant differences were found for kernel weight and kernel diameter among all sorghum samples under different irrigation levels. This difference may be due to the higher daytime temperature during growing season in
Conclusion
In conclusion, this study provided new findings that genotype, irrigation and their interaction significantly affected phenolic compounds and antioxidant activity in sorghum whole grain. Deficit irrigation treatment was shown to increase the level of these compounds and the antioxidant activity in the grain. This study provided new knowledge on the variations in levels of phenolic compounds, flavonoids and antioxidant activity of whole grain sorghum of different genotypes under water deficit
Conflict of interest
The authors declare no conflict of interest
Acknowledgements
The author GW would like to thank Curtin International Postgraduate Research Scholarships (CIPRS), Curtin University, Australia, for support of this research, and to Mr John Jackson in Department of Environment and Agriculture, School of Science, Curtin University for helping us to plant sorghums.
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2022, Scientia HorticulturaeCitation Excerpt :A similar response is reported by Sałata et al. (2022). Wu et al. (2017) found that a deficit irrigation increased the polyphenols content and the antioxidant activity in sorghum. It is likely that our results could be explained by the early phenological stage of seedlings on one side, and by the genetic background on the other side.