Nutrient stress
While fruit pigments are highly influenced by temperature, light and water, it is important to consider that they may also be altered by other environmental factors. For instance, salinity stress can either increase or reduce tomato anthocyanins depending on cultivar, while the pigmented carotenoids are reliably increased by as much as 2–3 fold across a range of cultivars (Borghesi et al. 2011). However, perhaps the most important consideration is nitrogen (N) status (Wang et al., 2018). In Arabidopsis , restricted N availability has been shown to shift metabolism towards flavonoid production, including anthocyanins, with evidence for increased transcription of the regulating TF (AtPAP1) and the biosynthetic pathway (Lillo, Lea & Ruoff 2008). If this is also true for fruits, it suggests that horticultural practices can elevate fruit anthocyanin concentrations, but that in planning nutrient applications, a balance is required to avoid detrimental yield effects on the overall crop (Jezek et al.2018). That the effects of N deficiency is not always so clear and in other crops, such as strawberry, and N application timing needs to be determined on a crop-by-crop basis (Jezek et al. 2018).
At the molecular level, studies in apple show a number of candidate genes linking N deficiency to an elevation in the anthocyanin pathway. Low N induces anthocyanin production, partly mediated by the mitogen-activated protein kinase (MAPK) signalling cascade, as shown for red-fleshed apples, where a MAPK family gene, MdMKK9, regulates the N status (Sun et al. 2022). Under low N conditions the autophagy nutrient recycling process can be activated, and studies in apple have demonstrated that the AuTophaGy (ATG) protein, MdATG18a, improves tolerance to nitrogen deficiency via upregulation of the anthocyanin pathway (Sun et al. 2018). In these cases, the increases in anthocyanin content are driven by an upregulation of the anthocyanin-regulating TF, MdMYB1/10. Conversely, the BTB/TAZ protein, BTB2, which is lowly expressed under low N but highly expressed under high N, negatively regulates the MYB, suppressing anthocyanin accumulation (An, Wang & Hao 2020b). It seems that fruit carotenoid contents, as shown in tomato, are either decreased or less affected by low nitrogen supply depending on the timing of the nitrogen supply reduction (Benard et al. 2009; Hernández et al. 2020).