INTRODUCTION
The ability of some plants to produce and release into the environment harmful or beneficial secondary metabolites for target species is called Allelopathy (Rice 1984). These chemical phytotoxic compounds, commonly known as allelochemicals, belong to different chemical classes such as phenols and terpenoids, and play a defensive role for the donor plant (Scavo, Restuccia & Mauromicale 2018a). In recent years, the increasing concerns about the environmental impact of syntyhetic herbicides and the dramatically development of weed-resistant ecotypes, has led to a greater attention by the scientific community on the application of allelopathy as a chemical-free tool for weed management. Indeed, Macias et al. (Macias, Molinillo, Varela & Galindo 2007) suggested how plant allelochemicals could represent a good candidate for the future production of bioherbicides.
The Asteraceae is one of the most studied botanical families comprising allelopathic species, both weeds and crops (Chon & Nelson 2010). Cultivated cardoon (Cynara cardunculus L. var. altilisDC.) is a C3 herbaceous perennial species, originated from the Mediterranean basin together with the globe artichoke [var.scolymus (L.) Fiori], which evolved separately as a result of different selection criteria from their common ancestor wild cardoon [var. sylvestris (Lamk) Fiori] (Rottenberg & Zohary 1996). Cultivated cardoon is known since ancient times as a vegetable for its enlarged bleached petiole. However, in the rainfed farmlands of Mediterranean climates, it is cultivated mainly as an industrial crop for the production of energy (biomass for direct combustion, biomethane, bio-ethanol, oil from seeds) and paper pulp thanks to its high lignocellulosic biomass, as well as for the extraction of bioactive (Pesce, Negri, Bacenetti & Mauromicale 2017; Mauromicale et al.2019). The allelopathic activity of cultivated cardoon has been investigated in deep on seed germination (Scavo, Restuccia, Pandino, Onofri & Mauromicale 2018b) and seedling growth of several weed species (Scavo et al. 2019b), as well as against bacterial species of agricultural and food interest (Mazzaglia et al. 2018; Scavoet al. 2019c). It was also found that C. cardunculuscropping significantly reduced the number of seeds in the soil seed bank, while showing a positive effect on some bacteria involved in the soil N-cycle (Scavo, Restuccia, Abbate & Mauromicale 2019d). Allelochemicals involved in C. cardunculus phytotoxicity were discussed in previous researches and indicated as sesquiterpene lactones (STLs) and polyphenols (Rial, Novaes, Varela, G. Molinillo & Macias 2014; Rial et al. 2016; Scavo, Pandino, Restuccia & Mauromicale 2020). STLs (i.e. cynaropicrin, deacylcynaropicrin, 11,13-dihydro-deacylcynaropicrin, aguerin B, grosheimin, 11,13-dihydroxi-8-deoxygrosheimin and cynaratriol), the most abundant terpenic compounds of C. cardunculus leaves, possess a very wide range of biological activities (Picman 1986; Zhang, Won, Ong & Shen 2005; Chaturvedi 2011), being widely reported as phytotoxic compounds (Rial et al. 2014; Scavo et al. 2019f). For instance, cynaropicrin, aguerin B and grosheimin have shown phytotoxic activity on two weed species of economic importance, i.e., barnyardgrass (Echinochloa crus-galli L.) and brachiaria [Urochloa decumbens (Stapf) R.D. Webster] (Rial et al. 2014). Moreover, the phytotoxic activity of C. cardunculus leaves have been correlated with the joint action of this STLs, being identified synergistic and antagonistic interaction between them (Rial et al. 2016). Also cynaropicrin, the major compound found in Cynaraspecies, has been reported as anti-inflammatory (Mizuno & Usuki 2018; Hayata et al. 2019), cytotoxic against several types of cancer cells (Shimoda et al. 2003; Cho et al. 2004; Yasukawa, Matsubara & Sano 2010) or antispasmodic (Emendörfer et al.2005). Scavo et al. (Scavo et al. 2019e) developed the best procedure for extracting the secondary metabolites from cultivated cardoon leaves based on costs and phytotoxic activity. Nevertheless, studying the influence of genotype and harvest time on the C. cardunculus STL profile and phytotoxicity, Scavo et al. (Scavo et al. 2019f) indicated that wild cardoon had the highest amount of STLs among the three C. cardunculus botanical varieties, with globe artichoke showing the lowest concentration. The amount of STLs and the inhibitory activity of the six C. cardunculus genotypes under study was closely affected by climatic conditions, which were favorable in April.
However, the allelopathic behaviour of a plant is the result of different abiotic and biotic stress factors, which influence the quantity of allelochemicals released by the donor plant and the effect of an allelochemical on the target plant (Inderjit & del Moral 1997; Scavo et al. 2018a). According to the “stress hypothesis of Allelopathy” formulated by Reigosa et al. (Reigosa, Souto & González 1999), a stress condition generally increases the production of allelochemicals in the donor plant, as well as the sensitivity of the target plant. Quantity and quality of light are reported to affect the allelopathic potential in plants (Kato-Noguchi 1999; Li, Pan, Liu & Wang 2009; Scavo et al. 2018a). While long photoperiods have been commonly indicated to increase the production of secondary metabolites, including allelochemicals, in several plants (Pramanik, Nagai, Asao & Matsui 2000; Chaves Lobón, Alias Gallego, Sosa Diaz & Escudero Garcia 2002), there is no evidence in literature about the increasing of allelochemicals biosynthesis in response to short days.
Given these findings, we hypothesised that cultivated cardoon plants under light stress increased both their STL leaf concentrations and phytotoxic activity. Therefore, the aim of the present study was to evaluate the effect of light stress (by plant shading) in field conditions on the quali-quantitative composition of STLs in cultivated cardoon leaf extracts and their phytotoxic potential expressed as inhibition of wheat coleoptile elongation, weed germination and growth.