1 INTRODUCTION
Plants are usually exposed to various biotic and abiotic stresses under
the both natural and agricultural conditions (Inderjit and Einhellig,
1993; Maqbool et al., 2013). Allelopathy is sub-discipline of
chemical ecology and is one of the most important biotic stresses
affecting plant growth and developments (Einhellig 1995; Achigan-Dakoet al., 2014). This stress is a multidimensional stress and its
effects on plants have been observed at molecular, biochemical,
physiological, morphological, and even ecological levels (Inderjit and
Einhellig, 1993; Kohli et al., 2001; Gniazdowska and Bogatek,
2005).
The decline in leaf chlorophyll content is an early general response of
crops to allelopathic stress, which is probably the result of imbalance
in cell’s homeostasis (Borellaet al., 2014; Dehghaniet al., 2014; Singh and Sunaina, 2014). In addition, carotenoids’
content alters in response to the allelopathy as well (Kohliet al., 2001; Ahrabi et al., 2011; Dehghani et al.,2014). Furthermore, the plants’ anthocyanins and even flavonoids pigment
contents increase in response to the allelopathic stress. Anthocyanins
and flavonoids are the well-known low molecular weight antioxidant
compounds whose concentrations increase under many stress conditions
(Ahrabi et al., 2011).
Effects of allelopathy on photosynthetic pigments ultimately lead to
influence on the photosynthesis as one of the most important metabolic
pathways in the plants. The direct impacts of allelopathy on plant
photosynthesis are mainly inhibition and/or damage to the proteins
involved in photosynthesis apparatus, increasing the decomposition of
photosynthetic pigments, and change in expression of photosynthetic
genes. Moreover, alteration in chloroplasts’ structure leads to
reduction of photosynthetic pigment contents, decline in energy and
electron transfer due to reducing of ATP synthesis activity, and
decreasing in stomatal conductance and transpiration rate which can
indirectly influence plant photosynthesis (Meazza et al., 2002;
Yu et al., 2003; Wu et al., 2004; Yu et al., 2006;
Bakhshayeshan-Agdam et al., 2020). Influencing the function of
PSII is the main effect of allelopathic stress on photosynthesis (Wink
and Latzbruning 1995; Wasternack and Hause 2013; Sunmonu and Van Staden
2014; Achigan-Dako et al., 2014). Accordingly, the D1 subunit of
this photosystem is highly susceptible to stress, which are directly or
indirectly destructed by increasing free radicals’ concentration in the
chloroplasts (Gonzalez et al., 1997; Shao et al., 2009; Uddinet al., 2012). Interfering of allelopathic compounds with the
hormonal signal transduction pathways (such as ABA) can also involve in
allelopathic effects on plants’ photosynthesis (Inderjit and Einhellig,
1993; Weir et al., 2004).
The responsible compounds in allelopathy phenomenon are called
allelochemicals (Kohli et al., 2001; Singh et al., 2001;
Bakhshayeshan-Agdam et al., 2020). In plants, these compounds are
non-nutritive and species- or tissue- specific substances and are mainly
produced as secondary metabolites. In addition, decomposition of organic
materials by microbes can lead to releasing of some allelochemicals.
Other organisms such as fungi and algae can produce some allelopathic
compounds (Inderjit and Nilsen, 2003). Allelochemicals belonging to
different groups of plant compounds and various classifications for
these compounds were proposed by researchers; but none of suggested
classifications are inclusive (Inderjit and Einhellig, 1993; Chou, 1999;
Shao-Lin et al., 2004; Weir et al., 2004; Leslie, 2005).
Generally, phenolic compounds, alkaloids, non-protein amino acids,
terpenoids, saponins, and benzoxazinones are most important
allelochemicals in plants (Kohliet al., 2001; Khan et al.,2010; Razavi, 2011; Soltyset al., 2013).
Redroot pigweed (Amaranthus retroflexus L.) is one of the most
invasive weeds worldwide with well-known allelopathic effects. This
plant has a high invasion power and many crops are susceptible and
vulnerable to its invasion (Bhowmik and Doll, 1982; Chou, 1999; Ahrabiet al., 2011; Shahrokhi et al., 2012). Moreover, redroot
pigweed is in the list of resistant plants to herbicides. Extracts of
this plant have diverse allelopathic effects on receiver plants such as
growth inhibition of various plants species (Costea et al ., 2004;
Mlakar et al., 2012; Shahrokhi et al., 2012;
Bakhshayeshan-Agdam et al., 2015).
Although there have been many studies on allelopathy, especially redroot
pigweed’s allelopathy, none of these studies have specifically focused
on the effects of allelopathy on photosynthesis. Hence, current research
has aimed to investigate redroot pigweed’s allelopathic effects on
photosynthesis performance, photochemistry, and photosynthetic gene
expression including PsbA and PsbS of cucumber and wheat plants which
are identified as sensitive and resistant species to allelopathic
effects of redroot pigweed, respectively (Bakhshayeshan-Agdam et
al., 2015). Finally, in this study, it was attempted to investigate the
direct interaction of amaranth allelopathic compounds detected in the
shoots of studied plants (Table S1), by molecular docking.