1. Introduction
Crop waterlogging is increasingly a global problem due to increased
frequencies of extreme climate events (Wollenweber, Porter, &
Schellberg, 2003). Globally, excessive water and poor soil drainage
constraints adversely affect ~10% of arable land area
(Setter & Waters, 2003), with average annual economic losses caused by
crop waterlogging amounting to tens of billions of US dollars from 2004
to 2013 (Hirabayashi et al., 2013). With climate change, more than 10%
of agricultural regions will have greater risk of waterlogging due to
higher frequencies and greater magnitudes of extreme rainfall events
(Chang-Fung-Martel, Harrison, Rawnsley, Smith, & Meinke, 2017;
Hirabayashi et al., 2013).
Waterlogging is a ‘wicked problem’ in that it is highly complex and
multi-faceted. In field crop experimental trials, waterlogging driven by
excessive rainfall or subsurface or lateral flooding may have poor
reproducibility, because waterlogging-prone environments have
considerable complexity, including variable dimensions of time, space,
biology and chemistry. Thus, methods with which such events are analysed
and quantified in a farming systems context requires careful
consideration (Harrison, Cullen, & Armstrong, 2017; Harrison, Cullen,
& Rawnsley, 2016).
Barley crops (Hordeum vulgare L. ) are currently cultivated in
more than 100 countries for use as animal feed and human consumption
(Zhou, 2009). Global barley production has diminished over the last two
past decades, decreasing from 155 Mt tons in 2008-9 to 142 Mt in 2017-18
(Statista, 2020). Part of this decline is due to increased frequency of
waterlogging and susceptibility of barley to waterlogging stress damage
(Setter & Waters, 2003). In many contexts, improving crop tolerance to
minor waterlogging is generally cost effective, however under severe
waterlogging, combined agronomic, engineering and genetic solutions are
more effective (Manik et al., 2019).
Defined physiologically, waterlogging tolerance is the survival or
maintenance of growth under waterlogging relative to non-waterlogged
conditions (Gibbs & Greenway, 2003; van der Moezel, Pearce-Pinto, &
Bell, 1991). Oxygen deficiency in soil pores caused by waterlogging
reduces root growth, leading to premature leaf senescence and tillering,
inhibition of dry matter accumulation and production of sterile florets.
In combination, such effects stun kernel number and weight, ultimately
penalising potential grain yield (de San Celedonio, Abeledo, Brihet, &
Miralles, 2016; de San Celedonio, Abeledo, & Miralles, 2014, 2018;
Masoni, Pampana, & Arduini, 2016).
Defined agronomically, waterlogging tolerance is less grain loss
relative to susceptible control genotype (Setter & Waters, 2003). Past
studies have measured yield declines of 40–79% in waterlogged barley,
depending on genotype, growth stage, soil type and duration of
waterlogging (de San Celedonio et al., 2014). Yield loss in barley is
also likely to be sensitive to the phenological stage with which
waterlogging occurs (de San Celedonio et al., 2014). One of the few
reports that examined the relationship between yield loss and
phenological stage reported that barley was most susceptible during
grain filling, moderately susceptible during tillering and least
susceptible during seedling stage (Setter & Waters, 2003). However,
there are few reports confirming this observation. It is also likely
that post-waterlogging growth recovery is a function of genotype of
environmental interactions, analogous to crop recovery following
defoliation (Harrison, Evans, Dove, & Moore, 2011a, 2011b).
Waterlogging tolerance is likely to be a complex trait related to many
morphological and physiological traits that are under strong
environmental influence (Zhou, Li, & Mendham, 2007). Lack of oxygen
causes roots to shift energy metabolism from an aerobic to anaerobic
mode, resulting in cellular energy crises (Gibbs & Greenway, 2003).
Apart from tolerance to secondary metabolic compounds associated with
anaerobic soil conditions (Huang et al., 2015; Pang et al., 2007),
tolerant genotypes of barley may adapt to transient waterlogging via
development of morphological mechanisms allowing plants to cope with the
stress (Herzog, Striker, Colmer, & Pedersen, 2016; Hossain, Araki, &
Takahashi, 2011; Kreuzwieser & Rennenberg, 2014). Morphological
adaptations include development of adventitious roots with well-formed
aerenchyma (Pang, Zhou, Mendham, & Shabala, 2004; Zhang et al., 2015).
An aerenchyma is a continuous gas filled channel that occurs under
flooded or hypoxic conditions. This enhances internal diffusion of
atmospheric and photosynthetic oxygen from the aerial parts to the
flooded roots, allowing roots to maintain aerobic respiration
(Armstrong, 1979). Waterlogging tolerant barley genotypes such as the
wild barley TAM407227 show not only higher adventitious root porosity
than sensitive barley genotypes (e.g. Franklin, Naso Nijo), but also
have faster development of aerenchyma (Zhang et al., 2015) under
waterlogging conditions. Metabolically, tolerance mechanisms in barley
include enhanced activities of glycolytic and fermentative enzymes that
increase availability of soluble sugars, and involvement of antioxidant
defence mechanisms (e.g. superoxide radicals, hydroxyl radicals and
hydrogen peroxide) against post-stress oxidative damages under anaerobic
conditions (Armstrong, Brandle, & Jackson, 1994; Davies, 1980; Drew,
1997; Mittler, Vanderauwera, Gollery, & Van Breusegem, 2004; Pan et
al., 2019; Setter et al., 1997). Contemporary crop breeders are
targeting genetic tolerance mechanisms including aerenchyma formation
using molecular marker assisted selection. In barley, a major QTL for AF
under waterlogging conditions was identified from several waterlogging
tolerance genotypes (Broughton et al., 2015; Zhang et al., 2017; Zhang
et al., 2016). This QTL was located in the same position as a QTL for
waterlogging tolerance on chromosome 4H (Li, Vaillancourt, Mendham, &
Zhou, 2008; Zhang et al., 2017; Zhou, 2010; Zhou, Johnson, Zhou, Li, &
Lance, 2012). However, allelic differences exist in different parents,
with the contribution of AF to field waterlogging tolerance ranging from
around 5% to over 80%. A prospective allele from a wild barley
identified in past work (Zhang et al., 2016) has been introgressed to a
commercial variety, Macquarie, and this new line, Macquarie+, will be
used in this study.
In this study, we imposed four waterlogging treatments on six barley
genotypes differing in waterlogging tolerance; two genotypes (Macquarie+
and TAMF169) had the allele for AF under waterlogging stress from the
wild barley. The objectives of this study were to examine (1) the impact
of timing and period of waterlogging on grain yield and yield
components, and (2) the contribution of the QTL for AF under
waterlogging stress to mitigate yield loss.