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).