Water stress
Water shortages are already a serious problem for much of the world’s agriculture and horticulture (Lobell & Gourdji 2012). The predicted increase in severe climate events due to climate change, including drought, is likely to present even greater challenges for growers. Of particular concern is the potential impact on global food supply chains of reduced yield of major grain crops (Araus et al. 2002). Predicting the outcomes of different water deficiency scenarios and how these will affect crops is complex (Tramblay et al. 2020). For essential photosynthetic pigments in leaves (carotenoids and chlorophylls), water deficiency can result in reduced growth and yield (Jaleel et al. 2009) and in some fruits, water deficiency can reduce both anthocyanin and carotenoid concentrations, reducing fruit colour (Jiang et al. 2020). However, for other fruits, water deficiency outcomes are not necessarily deleterious. This may seem counter-intuitive since drought is likely to decrease leaf photosynthesis and, therefore, the distribution of primary metabolites to the fruit. These primary metabolites provide the precursors for all secondary/specialised metabolites and so, logically, may reduce the pool of substrates for pigmented compounds. However, since both the carotenoid and phenylpropanoid pathways are partly regulated by stress-induced ROS production, there are also likely to be positive effects of water deficiency on fruit pigments. The outcomes can be species-dependent and, given this tension between pathways, highly variable owing to the timing of fruit development at which the water deficiency occurs (cell division, cell expansion or ripening phases), and the severity and duration of the deficit. In terms of overall fruit quality, there are possible effects on other major sensory attributes such as aromas, the sugar/acid balance and texture (Ripoll et al.2014).
In some cropping situations, water deficiency is intentionally used to improve colour. In red grape, anthocyanin is an important determinant of wine quality. When water is deficient, grape berry size is reduced, creating a higher skin:flesh ratio which, when coupled with increased anthocyanin production, can improve wine quality (Gambetta et al.2020). Increases in grape berry anthocyanin concentrations following exposure to water deficit have been reported in Cabernet Sauvignon (Deluc et al. 2009), Merlot (Bucchetti et al. 2011) and Tempranillo (Santesteban, Miranda & Royo 2011). Anthocyanin composition can also change in a cultivar-specific manner, such as the drought-induced increase in acylated anthocyanins in Cabernet Sauvignon, while the same conditions led to a reduction in these compounds in Syrah (Hochberg et al. 2015). Further compositional changes can be created by a shift towards more tri-hydroxylated anthocyanins (darker purples and blues) via the up-regulation of flavonoid 3’5’ hydroxylases (Castellarin et al. 2007).
Phytohormones are intrinsically linked with drought perception and the resultant transcriptional cascade (Ullah et al. 2018), particularly ABA (Yamaguchi-Shinozaki & Shinozaki 2005). For example, in strawberry fruit, drought conditions increased ABA, which was correlated with elevated anthocyanin concentration (as well as AsA) without a reduction in fruit yield (Perin et al. 2019). A previous study in strawberry clearly demonstrated how the presence of ABA regulated the expression of the anthocyanin-regulating MYB TF,FaMYB10, which, in turn, elevated the anthocyanin biosynthetic genes and fruit colour (Medina-Puche et al. 2013). In apple, an alternative molecular model of drought-induced anthocyanin production has been proposed, whereby the ethylene response factor ERF38 partners with the homologous MYB1 to drive the anthocyanin biosynthetic pathway genes (An et al. 2020c).
For fruit colours derived from carotenoids the picture is less clear cut than for anthocyanins. In some cases, water deficit has been shown to reduce fruit carotenoid concentrations (De Pascale et al. 2007; Jiang et al. 2020). However, in tomato, there is strong evidence that concentrations of lycopene (and β-carotene) increase with water deficiency (Favati et al. 2009; Klunklin & Savage 2017; Patanèet al. 2021; Zushi & Matsuzoe 1998).