Preharvest application of ethephon and postharvest UV-B radiation improve quality traits of beetroot (Beta vulgaris L. ssp. vulgaris) as source of colourant

Abstract

Background: Betanins have become excellent replacers for artificial red-purple food colourants. Red beet (Beta vulgaris L. spp. vulgaris) known as beetroot, is a rich source of betalains, which major forms are betanin (red to purple) and vulgaxanthin (yellow). Betalains and phenolic compounds are secondary metabolites, accumulation of which is often triggered by elicitors during plant stress responses. In the present study, pre-harvest applications of ethephon (an ethylene-releasing compound) and postharvest UV-B radiation were tested as elicitors of betalains and phenolic compounds in two beetroot cultivars. Their effects on quality parameters were investigated, and the expression of biosynthetic betalain genes in response to ethephon was determined.

Results: Ethephon was applied as foliar spray during the growth of beetroot, resulting in increased betanin (22.5%) and decreased soluble solids contents (9.4%), without detrimental effects on beetroot yield. The most rapid accumulation rate for betanin and soluble solids was observed between 3 and 6 weeks after sowing in both untreated and ethephon-treated beetroots. Overall, the expression of the betalain biosynthetic genes (CYP76AD1, CYP76AD5, CYP76AD6 and DODA1), determining the formation of both betanin and vulgaxanthin, increased in response to ethephon treatment, as did the expression of the betalain pathway activator BvMYB1. In the postharvest environment, the use of short-term UV-B radiation (1.23 kJ m^{- 2}) followed by storages for 3 and 7 days at 15 °C resulted in increased betanin to vulgaxanthin ratio (51%) and phenolic content (15%).

Conclusions: The results of this study provide novel strategies to improve key profitability traits in betalain production. High betanin concentration and high betanin to vulgaxanthin ratio increase the commercial value of the colourant product. In addition, lowering soluble solids levels facilitates higher concentration of beetroot colour during processing. Moreover, we show that enhanced betanin content in ethephon-treated beetroots is linked to increased expression of betalain biosynthetic genes.

Keywords: Beetroot; Betalain biosynthetic pathway; Betanin; Ethephon; UV-B radiation; Vulgaxanthin.

Fig. 1

Cross sections of roots of untreated and 360 g ha^− 1 ethephon-treated red beet plants at 16-weeks after sowing

Fig. 2

a, b Root weight and c, d transversal diameter monitored in untreated and 360 g ha^− 1 ethephon-treated red beet plants (3–18 weeks after sowing). Different letters indicate statistical significance according to Tukey’s test (p ≤ 0.05). Data represent the mean ± SE, n = 3

Fig. 3

a, b Betanin (Bn) content, c, d vulgaxanthin (Vx) content, e, f betanin to vulgaxanthin ratio (Bn:Vx), total phenolic content (g, h), and (i, j) total soluble solids content (TSS) monitored in roots of untreated and 360 g ha^− 1 ethephon-treated red beets (3–18 weeks after sowing). Different letters indicate statistical significance according to Tukey’s test (p ≤ 0.05). Data represent the mean ± SE, n = 3

Fig. 4

‘Fold changes in target gene expression in roots of 360 g ha^− 1 ethephon-treated red beet plants of ‘Monty Rz’ (a) and ‘Belushi Rz’ (b) relative to untreated plants (dashed horizontal line) at 16 weeks after sowing. The relative expression of target genes is determined according to the 2^(−ΔΔCt) method. Threshold cycles (Ct) for target genes are standardised to the BvActin Ct (ΔCt). Expression levels of target genes in untreated carrots were assigned an arbitrary value of 1. Data represent mean ± SE, n = 3

Fig. 5

Betalain and total phenolic contents in 16 weeks old beetroots of ‘Monty Rz’ (a, c, e) and ‘Belushi Rz’ (b, d, f) treated with (+UV-B) or without (-UV-B) 1.23 kJ m^− 2 for 70 s, followed by storage at 15 °C and 98–100% relative humidity in darkness, for 3 and 7 days. Bn: betanin content; Vx: vulgaxanthin content; Bn:Vx: betanin to vulgaxanthin ratio; TPC: total phenolic content. a, b, e, f: Different letters denote statistical significance according to Tukey’s test (p ≤ 0.05). c, d: Different letters denote statistical significance according to Nemenyi’s test (p ≤ 0.05). Data represent the mean ± SE, n = 5

Fig. 6

a, b Dry matter (DM), and (c, d) total soluble solids content (TSS) in 16 weeks old beetroots of ‘Monty Rz’ and ‘Belushi Rz’ treated with (+UV-B) or without (-UV-B) 1.23 kJ m^− 2 UV-B radiation for 70 s, followed by storage at 15 °C and 98–100% relative humidity in darkness, for 3 and 7 days. Different letters denote statistical significance according to Tukey’s test (p ≤ 0.05). Data represent the mean ± SE, n = 5