Responses of Soilless-Cultivated Golden Thistle to the Total Salt and Nitrogen Concentrations in the Nutrient Solution

Golden thistle (Scolymus hispanicus L.) is a wild edible green of high nutritional value, used in the traditional Mediterranean diet. Nowadays, there is an increasing demand from consumers for golden thistle and concomitantly an increasing interest in integrating it into modern cultivation systems. Soilless culture is a promising cultivation option that can maximize yield and quality of golden thistle. The aim of this study was to examine the combined effect of electrical conductivity (EC) and nitrogen (N) supply level on growth and nutritional quality of golden thistle grown on a substrate in a soilless cropping system. The two experimental factors were examined in a 2-factorial experiment with two EC levels, a low (2.2 dS m−1) and high (2.8 dS m−1), combined with two total-N (NO3− + NH4+) supply levels, low (13.30 mmol L−1) and high (17.30 mmoL L−1), in the supplied nutrient solution. Root fresh and dry weight (commercial yield) were unaffected by treatments; however, high EC significantly reduced shoot fresh and dry biomass by 21 and 28% compared to low EC. High EC increased K+ concentrations in shoots and roots but decreased shoot Ca2+ level. Nitrate concentration in the drainage solution and plant tissues was primarily driven by N supply, with high N increasing leaf NO3− by up to 45% without surpassing the regulatory safety limit. Water productivity did not differ among treatments, but low EC improved agronomic efficiency of K+, Ca2+, Mg2+, and S, while low N enhanced N agronomic efficiency by 44%. Overall, low EC promoted vegetative growth and nutrient use efficiency, while increasing N above 13.3 mmol L−1 offered no yield benefit and raised tissue nitrate levels. For optimal yield and quality, a nutrient solution with low EC and N supply is recommended for the soilless cultivation of golden thistle.