Arbuscular mycorrhizal fungi inoculation and deficit irrigation mediate phytochemical quality and antioxidant activity of cardoon (Cynara cardunculus var. altilis) leaves

Cardoon ( Cynara cardunculus altilis ) is a Mediterranean crop of growing agro-industrial interest for its richness in phenolic compounds. Drought conditions impose substantial physiological constraints on cardoon, frequently leading to deterioration of key yield-quality traits. Inoculation with arbuscular mycorrhizal fungi (AMF) can mitigate this stress, a response that can be systematically evaluated by exposing plants to a controlled gradient of irrigation levels from optimal watering to severe deficit. This study evaluated the effects of AMF inoculation ( Rhizophagus irregularis ; a commercial consortium, Mykoflor; and a non-inoculated plants served as control) and four deficit irrigation (DI) levels (20, 40, 60 and 90% field capacity, F.C.) on root colonization efficiency, leaf phytochemical composition and antioxidant activity across two growing seasons. AMF inoculation significantly enhanced mycorrhizal colonization frequency under all DI levels, reaching up to 83% at 90% F.C., with no significant differences between the two inocula. Moderate-to-optimal irrigation (60–90% F.C.) supported the highest colonization rates, while the lowest rate (54%) was recorded under severe DI (20% F.C.). Inoculated plants accumulated higher levels of total phenolic acids (TPA), total flavonoids (TF), chlorogenic acid (CHL), cynarin (CYN), caffeic acid (CAF), protocatechuic acid (PRO) and apigenin-7- O -glucoside (API) compared to non-inoculated controls, with Mykoflor consistently outperforming R. irregularis for most compounds. Deficit irrigation conditions (20–40% F.C.) significantly increased TPA, TF and antioxidant activity (AA), while severe DI (20% F.C.) maximized CHL, CYN, luteolin-7- O -glucoside (LUT) and TF accumulation. Pearson correlation analysis revealed strong positive associations between TPA and CAF ( r = 0.98), AA ( r = 0.89), and CHL ( r = 0.89), while TF correlated strongly with PRO, API, LUT and CYN ( r ≥ 0.91). The ‘AMF × DI’ interaction was the most influential factor, affecting 9 out of 10 investigated compounds. The results indicate that combined AMF inoculation and moderate water stress can strategically enhance the phytochemical profile of leaf cardoon, supporting its valorization in nutraceutical and agro-industrial supply chains with reduced irrigation inputs.