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.