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.