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).