Sunlight Induces Black Color and Increases Flavonoid Levels in the Grain of Sorghum Line Tx3362

Abstract

The grain sorghum [Sorghum bicolor (L.) Moench.] line ‘Tx3362’ has a uniformly black pericarp color when produced under summer production conditions and it also contains high levels of 3‐deoxyanthocyanidins (3‐DOAs) in the bran layers of the grain. Consequently, Tx3362 has high levels of antioxidant activity and is a source of natural pigmentation that can be used as natural food coloring. However, prior research indicates that the black color is not fully penetrant in all environments. Specifically, black sorghum panicles shaded from sunlight between flowering and physiological maturity are dark red in color. The objective of this study was to determine if the time of light exposure affects pericarp color, 3‐DOA production, and other grain composition traits. Three experiments were conducted that shaded developing Tx3362 black sorghum panicles for varying time intervals in two Texas environments. After harvest, samples were visually phenotyped, quantitatively measured for color via colorimeter, and analyzed using near‐infrared spectroscopy (NIR) to estimate grain composition values. Across both environments, increased shading reduced and, in some cases, eliminated the black color, resulting in red grain. In addition, increased shading reduced the concentration of 3‐DOA, total phenols, tannins, and fiber but increased fat concentrations. Because of the strong association between visual score and 3‐DOA concentration, field selection for the darkest pericarp should result in selection for the highest 3‐DOA content. Additionally, maximum production of these beneficial compounds is environment‐specific. Therefore, environments with high sunlight intensity are conducive to the production of darker grain and higher 3‐DOA, tannin, and phenol content.

Inheritance of Pericarp Color, Nutritional Quality, and Grain Composition Traits in Black Sorghum

“…A relatively large number of genes were predicted to be involved with fiber content (Table 8). Fiber could be overshadowed, or overshadowing, total phenolic concentration, as seen in previous reports (Pfeiffer and Rooney, 2015). The specialty compounds of interest are located in the bran layer—the same region where fiber is found in the caryopsis.…”

“…This would include grain composition parameters such as fat, protein, and starch composition, but it would not impact estimates of 3‐DOA, phenol, or tannins (Dykes et al, 2014). Pollen flow could not be controlled in this experiment, because shading from pollination bags inhibits the expression of the black pericarp trait (Pfeiffer and Rooney, 2015).…”

“…Whole panicles from different plants were visually phenotyped on a 1 (red) to 5 (black) color scale (Pfeiffer and Rooney, 2015). After visual scoring, panicles were threshed and cleaned using a Wintersteiger LD180 (Wintersteiger Ag).…”

“…Although genetic factors are the primary control for this trait, environment also affects specific concentrations of 3‐DOAs, tannins, and total phenols as well as the physical appearance of the grains (Dykes et al, 2009; Hayes and Rooney, 2014; Pfeiffer and Rooney, 2015). As the developing grain receives increased exposure to sunlight, there is increased production of these compounds as well as a darker grain appearance.…”

Phenotypic, Phytochemical, and Transcriptomic Analysis of Black Sorghum (Sorghum bicolor L.) Pericarp in Response to Light Quality

“…A recent genetic study of black sorghum demonstrated that additional genes (including recessive alleles) beyond the dominant alleles at the R and Y loci function to control the black-grain phenotype. 16 Unlike 3-DOA accumulation in pericarp tissue, 3-DOA production in sorghum leaf tissue is a dominant and common trait irrespective of grain color and is stimulated by pathogen attack or mechanical wounding. 2 Sorghum leaves are characterized as either tan (nonpigmented), red, or purple; a trait referred to as "secondary plant color".…”