Experimental site design, crop management and leaf sampling.
Plants of ’Bianco gigante’, an Italian cultivated cardoon commercial cultivar, were grown in the experimental station of Catania University [South Italy, 37° 25I N; 15° 30IE; 10 m a.s.l.]. The area is characterised by a typical semi-arid Mediterranean clime comprising mild-wet winters and warm-dry summers. The soil of the experimental field was typical vertical and/or xerochrept (Staff 1999) with 27% sand, 45% clay, 28% silt, 1% organic matter, 0.1‰ total nitrogen, 20 ppm available P2O5, 210 ppm exchangeable K2O, and pH 7.2.
A completely randomized design with four replications, each of which comprised 100 plants spaced 0.80 m apart, in rows separated apart by 1.25 m (1.0 plant m−2), was adopted to study two imposed shading levels, removal of 0% (control ) and 60% (light stress ) of sunlight. Shading was imposed by erection of a black polyethylene net (“Ombra 60”) at ~1.60 m aboveground level from middle October to April. The effectiveness of the shading was tested on a weekly basis, both inside and outside the field experiment, using a solarimeter (Licor Line Quantum LQA, One Meter Sensing Length; LI-Cor Inc., Lincoln, NE, USA). The netting was extended down 0.30 m aboveground level at each edge, in order to avoid lateral irradiations and to minimize the development of microclimate differences among main plots. Before planting, the experimental field soil was ploughed up to 30 cm and harrowed. The fertilisation program was: 50 kg ha−1 of urea (N), 80 kg ha−1of double perphosphate (P2O5) and 80 kg ha−1 of potassium sulphate (K2O) prior to awakening with two furthers 80 kg ha−1 of ammonium nitrate. Irrigation was supplied. Weed and pest control was performed by spraying oxyfluorfen and imidacloprid, respectively, when required. The effect of light stress was evaluated on two different harvest times: January 9th and April 14th, 2018. January harvest corresponds to 30% of the maximum leaf mass reached, code 43 according to the BBCH scale proposed by Archontoulis et al. (Archontoulis, Struik, Vos & Danalatos 2010), while in April 50% of the maximum leaf mass is reached (code 45). The choice of these harvest times is owed on the fact that they intercept very different meteorological conditions affecting STL concentration in C. cardunculus leaves, as suggested by Scavo et al. (Scavo et al.2019f). Fifty full-expanded leaves were randomly sampled in the central part of each plot at the two phenological stages described.