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.