Anthocyanins have a higher antioxidant activity than other flavonoids, due to their positively charged oxygen atom (Kong et al., 2003; Figure 3). The antioxidant activity of anthocyanins depends on the degree of hydroxylation at the B-ring as well as the type and extent of acylation and glycosylation (Sadilova et al., 2006). Hydroxylation at the B-ring enhances antioxidant capacity (−OH > −OCH3 >> −H), therefore the antioxidant capacity of anthocyanidins decreases in the order of Dp > Pt > Mv = Cy > Pn > Pg (Pojer et al., 2013). Furthermore, glycosylation reduces the free radical scavenging ability of anthocyanins compared to their aglycone forms, by decreasing their hydrogen-donating, metal-chelating and electron delocalizing abilities (Zhao et al., 2014). The more sugar units at C3 and C5 position, the lower the antioxidant activity is (Sadilova et al., 2006). Finally, acylation of glycosyl moieties may partly circumvent the negative effect of glycosylation (Lachman and Hamouz, 2005). In summary, antioxidant activity increases with the number of hydroxyl groups in the B-ring and decreases with the number of glycosyl groups attached to the A and C ring. The latter effect is less severe when the glycosides are acylated.

Anthocyanins play an important role in facilitating plant reproduction as they attract pollinators and seed dispersers by imparting bright colors (Harborne and Williams, 2000; Hoballah et al., 2007). In addition to their colorful characteristics, anthocyanins protect plants from several biotic and abiotic stresses (Chalker-Scott, 1999; Ahmed et al., 2014), which may provide them a better adaptation to climate change. Anthocyanins are photoprotective agents which shade and protect the photosynthetic apparatus by absorbing excess visible and UV light and scavenging free radicals (Guo et al., 2008). For instance, red pear fruits (cv. Anjou) and purple pepper leaves (cv. Huai Zi) rich in anthocyanins showed a more stable PS II photosynthetic capacity and a higher photo-oxidation tolerance compared to non-anthocyanin tissues (Li et al., 2008; Ou et al., 2013). Besides, anthocyanins often accumulate in young vegetative tissues and sun-exposed side of fruits to protect them from photoinhibition and photobleaching under light stress without significantly compromising photosynthesis (Steyn et al., 2002; Gould, 2003; Li and Cheng, 2008; Zhu et al., 2017). Moreover, the existence of colored anthocyanins can reduce the infestation of insects and pathogens. For example, anthocyanin-rich tobacco leaves were not preferred by the Helicoverpa armigera larvae. The mortality of H. armigera larvae was significantly increased and pupation of Spodoptera litura was significantly delayed by feeding anthocyanin-pigmented leaves, compared to controls fed with green ones (Malone et al., 2009). Anthocyanin-enriched tomato fruits exhibited lower susceptibility to gray mold (Zhang et al., 2013). Furthermore, transgenic tomato plants with higher anthocyanin content displayed an enhanced tolerance to heat stress (Meng et al., 2015). Wounded anthocyanin-rich leaf tissue showed faster recovery from oxidative stress caused by mecha nical injury (Gould et al., 2002).

Besides the protective effects during plant growth, anthocyanins may also play an important role to improve the postharvest performance of vegetables. For example, acting as antioxidants, anthocyanins prevent lipid peroxidation, and maintain membrane integrity to decelerate cell senescence (Jiao et al., 2012). Tomato fruits, enriched in anthocyanins, showed less over-ripening and a longer shelf-life (Bassolino et al., 2013; Zhang et al., 2013). The latter proposed a model explaining the extended shelf life of anthocyanin-rich tomatoes, as follows. Firstly, anthocyanins increase the antioxidant capacity of the fruit, which leads to suppression of both the activity and the signaling function of reactive oxygen species (ROS) and consequently may delay the processes of over-ripening. Secondly, anthocyanins increase fruit resistance to botrytis by altering the dynamics of the ROS burst generated by Botrytis cinerea infection, thereby limiting the induction of cell death required for growth and spreading of the fungus.

Numerous in vitro and in vivo studies, including animal models, suggest that anthocyanins have health-promoting properties and may play a role in reducing chronic and degenerative diseases (Joseph et al., 2003; Lee et al., 2005; Achterfeldt et al., 2015; Charepalli et al., 2015).

Delphinidin derivatives, the main type of anthocyanins found in Solanaceous vegetables, have been associated with reduction of vascular inflammation and prevention of thrombosis (Watson and Schönlau, 2015). They may also protect the human skin from UV-B irradiance by inhibiting keratinocyte apoptosis. Potato anthocyanins repressed the reproduction of cell lines for human erythrocyte leukemia, stomach cancer and prostate cancer (Zhao et al., 2009). In addition, potato anthocyanins decreased the incidence of breast cancer in rats. Consumption of transgenic anthocyanin-rich tomatoes led to a 25% extension of the lifespan of the p53 mouse, a cancer mouse model (Butelli et al., 2008), and a reduction in the development of atherosclerosis in a cardiovascular disease mouse model (Gonzali et al., 2009; Achterfeldt et al., 2015). The proliferation of human colon and ovarian cancer cell lines was significantly inhibited by peel extracts from purple tomatoes in a dose-dependent manner (Mazzucato et al., 2013). Although it is suggested that the antioxidant properties of anthocyanins form the basis for health benefits (Noda et al., 2000; Roleira et al., 2015), there is no evidence for health effects of antioxidants (Bast and Haenen, 2013; Carocho and Ferreira, 2013; Watson, 2013).

In contrast to the above-mentioned positive effects of anthocyanins, several studies reported no effect or even a negative effect of anthocyanins on health-related parameters (Tsuda, 2012; Pojer et al., 2013; Smeriglio et al., 2016). This apparent discrepancy may be due to differences in the anthocyanin composition and doses used and/or differences in experimental setup and methodology. Therefore, there is no unequivocal proof for the health benefits of anthocyanins.

EBGs—CHS, CHI, and F3H are the common flavonoid pathway genes which are involved in the biosynthesis of all downstream flavonoids. In general, the reported expression profile of EBGs varies and there is no consistent correlation between their expression levels and anthocyanin content in Solanaceous vegetables. In the anthocyanin-pigmented tomato Aft/Aft mutant (accession number LA1996), the expression of SlCHS, SlCHI, and SlF3H genes did not differ from that in the non-pigmented control genotype (Povero et al., 2011). In a different study, however, with fruits of the same LA1996, SlCHS was found to be substantially upregulated compared to red-fruited varieties (Sapir et al., 2008). A similar contrast has also been found in pepper fruits. Stommel et al. (2009) reported the upregulation of CaCHS in anthocyanin-pigmented fruits (Capsicum annuum, breeding line 06C59), while, Borovsky et al. (2004) and Aza-Gonzalez et al. (2013) found that the expression levels of CaCHS, CaCHI, and CaF3H during ripening of anthocyanin-pigmented fruits (C. annuum inbred line 5226, cv. Arbol and cv. Uvilla) were comparable to those of non-pigmented fruits (C. chinense PI 159234 and C. annuum cv. Tampiqueño 74). In eggplant, the expression level of SmCHS was reported to be significantly upregulated in black (cv. Black Beauty) or violet (cv. Classic) fruits compared to the green (genotype E13GB42) or white (cv. Ghostbuster) mutants (Stommel and Dumm, 2015; Gisbert et al., 2016). In addition, the transcript levels of SmCHS and SmCHI, but not SmF3H, correlated well with the anthocyanin accumulation pattern in cv. Lanshan Hexian (Jiang et al., 2016a). In potato tubers, the association of expression of EBGs and anthocyanin accumulation is more consistent. All EBGs were highly expressed in red (cv. AmaRosa and cv. Sullu) and purple tubers (cv. Guincho Negra, cv. W5281.2 and cv. Hei Meiren) and correlated with anthocyanin content (André et al., 2009; Jung et al., 2009; Payyavula et al., 2013; Liu Y. et al., 2015). Even in the same tuber, StF3H was found to be upregulated in anthocyanin-pigmented flesh compared to non-pigmented flesh (Stushnoff et al., 2010).

LBGs—F3′H, F3′5′H, DFR, ANS, and UFGT are required for the biosynthesis of specific classes of flavonoids, including anthocyanins. Positive correlations between expression levels of LBGs and anthocyanin content have been consistently observed in many Solanaceous vegetables (Borovsky et al., 2004; André et al., 2009; Povero et al., 2011; Aza-Gonzalez et al., 2013). In tomato fruits, SlF3′5′H, SlDFR, and SlANS showed a high expression in anthocyanin-pigmented mutants (Aft/Aft, atv/atv and Aft/Aft atv/atv) compared to their red-fruited controls (Sapir et al., 2008; Povero et al., 2011). During fruit development of anthocyanin-pigmented peppers (C. annuum), CaF3′5′H, CaDFR, CaANS, and CaUFGT were upregulated at a young fruit stage, reaching a maximum at the late unripe stage prior to ripening, and were downregulated afterwards, which corresponded to the transient anthocyanin accumulation pattern of these fruits (Borovsky et al., 2004; Stommel et al., 2009; Aza-Gonzalez et al., 2013). For eggplant, the expression levels of SmDFR and SmANS were significantly higher in black (cv. Black Beauty) or violet (cv. Classic) fruits compared to green (genotype E13GB42) or white (cv. Ghostbuster) fruited genotypes, respectively, at all fruit developmental stages, up to commercial ripeness (Stommel and Dumm, 2015; Gisbert et al., 2016). In anthocyanin-pigmented potato tuber skin, the StF3′H, StF3′5′H (Jung et al., 2005), StDFR (De Jong et al., 2003; Zhang et al., 2009), StANS and StUFGT were highly expressed (André et al., 2009; Jung et al., 2009; Liu Y. et al., 2015). StDFR was also upregulated in red and purple tuber flesh (Stushnoff et al., 2010).

Transcription of structural genes involved in the anthocyanin biosynthetic pathway has many similarities in Solanaceous vegetables. In summary, the EBGs are expressed to a sufficient level in both anthocyanin-pigmented and non-pigmented tissues. Generally, there is no consistent correlation between their expression levels and anthocyanin content, which is most likely due to the fact that expression of EBG's is not only required for the production of anthocyanins, but also for that of other flavonoids, such as flavonols or flavanones. In contrast, the transcript level of LBGs coincides well with anthocyanin content and is significantly higher in pigmented compared to non-pigmented tissues, suggesting that variations in LBG expression determine the quantitative variation of anthocyanins in Solanaceous vegetables (Table 1). In fruits of pepper, eggplant and tomato, the expression of LBGs have a very similar, ripening dependent pattern, suggesting the presence of a conserved regulatory machinery that coordinates their expression.

The anthocyanin biosynthetic pathway is transcriptionally regulated by a MBW complex. The MYB transcription factors primarily determine the activation or repression role of the MBW complex, by binding to the promoters of structural genes, together with the common bHLH and WD40 factors. The MYB activators are mainly from the R2R3-MYB family. Known repressors consist of both R2R3-MYB and R3-MYB transcription factors. The expression of the R2R3-MYB and bHLH regulatory genes, is specific for pigmented tissue in most cases (Koes et al., 2005), while that of WD40s, which are involved in stabilizing the MBW complexes, is generally similar between anthocyanin-pigmented and non-pigmented tissues (Koes et al., 2005; Ramsay and Glover, 2005). Regulatory genes encoding MYB, bHLH, and WD40 transcription factors in tomato, pepper, eggplant, potato, and petunia are summarized in Table 2. Genes encoding MYB repressors have not been identified in tomato, pepper, eggplant, and potato yet.

It is important to note that the nomenclature of orthologous genes in the different species is not consistent. For example, the potato AN1 gene encodes an R2R3 MYB transcription factor, while its orthologs in petunia and tomato are called AN2. Furthermore, in the latter two species AN1 encodes a bHLH transcription factor. Since petunia is the best-studied model for anthocyanins, the petunia nomenclature is used when discussing general principles.

Anthocyanin levels are the net result of biosynthesis and degradation. A shift toward degradation, which is caused by downregulation of anthocyanin biosynthesis, leads to a decrease in anthocyanin content and, eventually, disappearance. In fruits of tomato, eggplant, and pepper, anthocyanins often accumulate in the skin of unripe fruits and afterwards their levels decrease during ripening (Borovsky et al., 2004; Povero et al., 2011; Mennella et al., 2012). In purple pepper fruits (Figure 6), anthocyanin discoloration was accompanied with a decline in expression of positive regulatory genes, such as CaMYB_A, and most of its downstream structural genes, leading to a reduced anthocyanin biosynthesis relative to its degradation (Borovsky et al., 2004). When positive transcription factors of the anthocyanin pathway were constitutively overexpressed, as in transgenic Del/Ros1 tomato, anthocyanins accumulated during all ripening stages resulting in a deep purple ripe fruit (Maligeppagol et al., 2013). Kiferle et al. (2015) separately overexpressed two similar tomato MYB genes, SlANT1 and SlAN2, under control of the constitutive CaMV 35S promoter. In both cases, this led to intense anthocyanin pigmentation in immature fruits. However, this intense pigmentation was only maintained in 35S:ANT1 mature fruits, whereas anthocyanins partially degraded in 35S:SlAN2 mature fruits. Thus, a decrease in the expression of anthocyanin activators plays an important role in reducing anthocyanin biosynthesis. Even upon constitutive overexpression of anthocyanin activators, the final anthocyanin concentration in ripe fruits may depend on which regulatory gene is overexpressed and their abilities to activate downstream structural genes.

Anthocyanin discoloration may occur as a result of active enzyme-driven breakdown processes. The active enzymatic in planta degradation of anthocyanins was first suggested in Brunfelsia calycina (Solanaceae) flowers whose color changed rapidly from dark purple to complete white after opening (Vaknin et al., 2005). Later, a vacuolar class III peroxidase, BcPrx01, was suggested to be responsible for this anthocyanin degradation (Zipor et al., 2015).

Additional evidence for active enzymatic anthocyanin degradation was obtained from a petunia mutant whose petal color completely faded after bud opening (Quattrocchio et al., 2006). Color fading in petunia has a strong substrate specificity for anthocyanidin-3-(p-coumaroyl-rutinoside)-5-glucoside, which is the most common anthocyanin in Solanaceous vegetables (De Vlaming et al., 1982). This color fading only occurs in a genetic background containing a dominant FADING (FA) gene, which has not been cloned yet. However, the FA gene is not the only precondition for fading, since the fading effect of FA is restricted to certain petunia backgrounds with bluish petal colors (Quattrocchio et al., 2006). The bluish anthocyanin color is due to an increased vacuolar pH, suggesting that the FA action might be pH-dependent. In petunia, vacuolar pH is regulated by an MBW complex consisting of PhPH4 (R2R3-MYB)-PhAN1 (bHLH)-PhAN11(WD40) plus the WRKY transcription factor PhPH3. The PhPH4-PhAN1-PhAN11-PhPH3 complex regulates the expression of two proton pumps (PhPH1 and PhPH5) responsible for acidification of the vacuole, leading to a red hue of the anthocyanins. Mutant analysis revealed that the fading effect was not dependent on the vacuolar pH, since Phph1 and Phph5 mutants in an FA background had an increased vacuolar pH, but did not show any fading phenotype (Verweij et al., 2008, 2016; Faraco et al., 2014). In contrast, mutations in the PhPH4-PhAN1-PhAN11-PhPH3 complex regulating vacuolar pH (Phph3, Phph4 and Phan1) revealed a clear fading phenotype (Quattrocchio et al., 2006; Passeri et al., 2016), suggesting that misregulation of unidentified downstream genes of the PhPH4-PhAN1-PhAN11-PhPH3 complex is an essential component for color fading. The unknown target genes might counteract the FA action to protect anthocyanins from degradation, or, alternatively, might actively repress expression of the FA gene to ensure anthocyanin stability (Figure 7).

In blood orange and litchi fruits, β-glucosidase and polyphenol oxidase and/or peroxidase have been suggested to be involved in anthocyanin degradation during the final ripening stage (Zhang et al., 2001, 2005; Barbagallo et al., 2007). Oren-Shamir (2009) proposed three candidate enzyme families: polyphenol oxidase, peroxidase and β-glucosidases, to be involved in anthocyanin degradation. There are two presumed anthocyanin degradation pathways. One is the direct oxidation by peroxidase. The other is comprised by a two-step degradation, deglycosylation by β-glucosidase and oxidation by polyphenol oxidase or peroxidase (Barbagallo et al., 2007; Oren-Shamir, 2009).

Besides enzymatic factors, non-enzymatic factors also affect anthocyanin color and stability, and may enhance their vulnerability to enzymes that degrade anthocyanins. The chemical structure of anthocyanin determines its color and stability. The higher the level of B-ring hydroxylation, the more purple the color, but the more unstable the anthocyanins are (Woodward et al., 2009). The effect of glycosylation varies depending on the number and the position of sugar moieties (Zhang et al., 2014a). Glycosylation at C3 elevates stability and shifts color slightly toward red. The stabilizing effect of diglycosides at C3 is stronger than that of monoglycosides. In contrast, glycosylation at C5 reduces pigment intensity. Acylation increases anthocyanin stability and an increasing number of acyl moieties causes a color shift from red to blue (Lachman and Hamouz, 2005). Co-pigmentation, normally with flavones, flavonols or anthocyanins, results in more stable and intensely colored anthocyanins that shift color toward blue (Zhang et al., 2014a). Metal ions, for example, iron and magnesium, improve anthocyanin stability by forming complexes with them (Oren-Shamir, 2009). Furthermore, anthocyanins show pH-dependent structural isoforms in acidic and neutral solutions, but degrade in alkaline environments (Woodward et al., 2009). In the acidic vacuole, the color of anthocyanins shifts from red to blue with increasing pH. For example, the color of petunia mutants with an increased vacuolar pH (from around 5.5 to 6.0) shifted from red to blue (Quattrocchio et al., 2006).

Light is one of the most important environmental factors affecting anthocyanin accumulation. High light intensity stimulates anthocyanin production in many plant species (Maier and Hoecker, 2015). For example, the part of the tomato fruit (Aft/Aft atv/atv) surface directly exposed to light showed a more intense anthocyanin pigmentation compared to the shaded parts (Mazzucato et al., 2013). In addition to intensity, light quality also affects anthocyanin biosynthesis. Poor anthocyanin pigmentation of eggplant fruits, growing in a greenhouse with low UV transmittance, was improved by providing UV-A irradiation (Matsumaru et al., 1971). Guo and Wang (2010) reported UV-A irradiation increased anthocyanin content in tomato seedlings compared to white light. They also suggested that UV-A radiation on tomato fruits increased their anthocyanin content. Blue and red light have also been reported to induce anthocyanin biosynthesis compared to darkness (Xu et al., 2014; Liu Z. et al., 2015). The amount of anthocyanin in tomato seedlings was elevated with an increased percentage of blue light (Hernández et al., 2016). For supplemental far-red light, contradictory effects on anthocyanin content have been reported (Li and Kubota, 2009; Liu Z. et al., 2015).

The effects of light intensity and spectrum on anthocyanin content are attributed to their influence on anthocyanin biosynthetic genes. Albert et al. (2009) suggested that high-light regulated anthocyanin production mainly through controlling R2R3-MYB transcription factors. Solanaceous R2R3-MYB activators such as SlAN2 and CaMYB_A, were upregulated by high light, whereas an R2R3-MYB repressor, PhMYB27, was downregulated (Lightbourn et al., 2007; Albert et al., 2011; Kiferle et al., 2015). Transcription levels of Solanaceous JAF13 and AN11 were not affected by high irradiance (Lightbourn et al., 2007; Albert et al., 2014; Kiferle et al., 2015). The reported effect of high light on transcription of Solanaceous AN1 was not consistent. The expression of SlAN1 in young tomato plants and PhAN1 in petunia plants was increased under high light exposure (Albert et al., 2014; Kiferle et al., 2015) while Lightbourn et al. (2007) did not observe any significant change in transcription of CaAN1 in pepper leaves after applying additional light. The effect of light quality on anthocyanin biosynthetic genes has hardly been studied in Solanaceous vegetables, only in petunia flowers, in which blue and red light were reported to induce the expression of CHS genes when compared to dark condition (Katz and Weiss, 1999). Studies in Arabidopsis and other plants provided more evidence for the stimulatory effect of blue and red light on anthocyanin production by increasing the transcription of R2R3-MYB activator genes and structural genes (Shi et al., 2014; Xu et al., 2014; Liu Z. et al., 2015).

Anthocyanin biosynthetic genes were upregulated under light and downregulated under darkness in tobacco leaves transiently overexpressing the potato StMYBA1gene, under control of the CaMV 35S promoter (Liu et al., 2017). This suggests that, in addition to the right genetic makeup, light is an important cue for anthocyanin production in Solanaceae. Application of light-impermeable bagging to eggplant during cultivation resulted in white fruits. Jiang et al. (2016b) investigated the role of several light-signal transduction components in the light-dependent regulation of anthocyanin biosynthesis in eggplant. They studied the protein-protein interactions of SmCOP1 (a repressor of photomorphogenesis and anthocyanin biosynthesis), SmHY5 (a BZIP transcription factor promoting expression of light-inducible genes, such as anthocyanin biosynthetic genes), SmCRY1 and SmCRY2 (two blue light photoreceptors) and SmMYB1, by yeast two-hybrid and bimolecular fluorescence complementation analyses. They identified interactions between SmCRYs and SmCOP1, between SmCOP1 and SmHY5 and between SmCOP1 and SmMYB1. Based on these interactions, Jiang proposed a model for light-induced anthocyanin biosynthesis in eggplant (Figure 8): in light, SmCRYs inhibited the activity of SmCOP1, which allowed SmHY5 and SmMYB1 to bind to the promoters of SmCHS and SmDFR genes resulting in anthocyanin biosynthesis in eggplant; in darkness, SmCRYs failed to inhibit the function of SmCOP1 and consequently, SmHY5 and SmMYB1 were targeted by SmCOP1 for ubiquitination and subsequent protein degradation through a 26S proteasome pathway, thus blocking the MYB1-dependent activation of anthocyanin biosynthesis. This model nicely demonstrates that, in addition to transcriptional regulation, post-translational control mechanisms also play an important role in regulating the anthocyanin pathway.

Temperature is another major environmental factor influencing anthocyanin metabolism. Low temperature induced anthocyanin accumulation in Solanaceae (Løvdal et al., 2010; Jiang et al., 2016a). Jaakola (2013) and Xu et al. (2015) proposed that the regulation of anthocyanin biosynthesis by low temperature and light might be through the same mechanism, as induction of anthocyanin biosynthesis at low temperature needed light. Nevertheless, the mechanism is not fully understood. Several transcription factors, including SlAN2, SlAN1, and SlJAF13 mediated anthocyanin biosynthesis under low temperature (Kiferle et al., 2015; Qiu et al., 2016). Structural genes SlCHS, SlF3H, SlF3′5′H, and SlDFR were upregulated in cold conditions (Løvdal et al., 2010; Kiferle et al., 2015; Qiu et al., 2016). In eggplant, EBGs (SmCHS, SmCHI, and SmF3H) have been reported to respond earlier than LBGs (SmF3′5H, SmDFR, and SmANS) under low temperature (Jiang et al., 2016a). The expression of SlAN11 was neither influenced by high light nor by low temperature, suggesting that SlAN11 expression is independent of light and temperature stimuli (Kiferle et al., 2015).

High temperature reduced anthocyanin accumulation occurs in plants by inhibiting the expression of anthocyanin activators and related structural genes and/or enhancing that of repressors (Yamane et al., 2006; Rowan et al., 2009; Lin-Wang et al., 2011). For example, from veraison to harvest stage, both the transcriptional and enzymatic levels of anthocyanin biosynthesis were restrained in grape berries (cv. Sangiovese) at high temperature (Movahed et al., 2016). In addition, the peroxidase activity in these berries increased. Movahed et al. (2016) overexpressed VviPrx31, encoding a grapevine class III peroxidase, in petunia and caused anthocyanin reduction in petunia petals under heat stress, indicating active anthocyanin degradation. It further indicated that VviPrx31 is responsible for anthocyanin degradation at high temperature. Therefore, the effect of high temperature reducing anthocyanin content in grape berries is not only contributed by impairing biosynthesis, but likely also by enhancing degradation. High temperature induced anthocyanin degradation was also suggested in plum fruits (Niu et al., 2017). The high temperature-dependent decrease in anthocyanin concentration was associated with an increased activity of a class III peroxidase and elevated H₂O₂ levels. However, by applying peroxidase inhibitors, anthocyanin content under both temperature treatments increased and the increasing extent was even higher at 35°C compared to 20°C, despite the higher H₂O₂ level at high temperature. Therefore, the increased peroxidase activity was indicated to contribute to reduced anthocyanin content at high temperature. In plum fruits, the concentration of protocatechuic acid, a product resulting from H₂O₂ mediated oxidation of anthocyanins in vitro, barely changed at 20°C, but significantly increased at 35°C. This suggests that protocatechuic acid could be an anthocyanin degradation product in vivo due to a class III peroxidase catalyzed anthocyanin degradation by H₂O₂. In conclusion, anthocyanin degradation might result from the increased activity of peroxidase enzymes in response to thermal stress.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer, GT, and handling Editor declared their shared affiliation.