Differential Effects of a Legume-Derived Protein Hydrolysate and Seaweed Extract on Yield and Leaf Quality of Cardoon Grown in a Floating System

Cardoon (Cynara cardunculus var. altilis DC.) is a Mediterranean crop valued for biomass production and bioactive compounds; however, information on the use of biostimulants in soilless systems remains limited. This study evaluated the effects of two biostimulants, a legume-derived protein hydrolysate (PH) and an Ascophyllum nodosum seaweed extract (SW), applied as weekly foliar sprays, on growth, physiological performance, mineral composition, and phytochemical traits of cardoon cultivated in a floating system. Biostimulant application significantly affected plant performance, inducing distinct treatment-dependent responses. Both PH and SW increased fresh and dry biomass compared with untreated plants. SW predominantly promoted vegetative growth, chlorophyll content, and nutrient accumulation, whereas PH markedly enhanced nutraceutical quality by increasing total phenolic content and antioxidant activity, reaching 64.4 mg GAE g−1 dry extract and the lowest IC50 value (172 µg mL−1). Harvest timing modulated the magnitude of biostimulant effects, with maximum biomass yield observed at intermediate developmental stages (up to 8.17 kg m−2), while phenolic concentration and antioxidant capacity declined at later stages. Multivariate analyses confirmed that PH and SW induced complementary metabolic strategies. Overall, the biostimulant type emerged as the primary driver of plant response, with harvest timing acting as a modulating factor. Targeted biostimulant management, therefore, represents a promising strategy for optimizing the productivity and phytochemical quality of cardoon in soilless cultivation systems.