Low-input intensification of cultivated cardoon for biomass-derived total phenols: Plant density and mycorrhizal biostimulant effects across two mediterranean sites

Cultivated cardoon ( Cynara cardunculus L. var. altilis ) is a resilient, multipurpose Mediterranean crop capable of high biomass yields and phenolic accumulation even under low-input conditions in marginal lands. Multi-harvest field experiments were conducted for two growing seasons in two different Mediterranean sites (in Sicily and Apulia; Southern Italy) to assess the combined effects of plant density (D1 = 2 plants m⁻²; D2 = 4 plants m⁻²) and application of an arbuscular mycorrhizal fungi (AMF)-based biostimulant (Myco+ vs. Myco−) on leaf biomass dynamics and total phenols (TP) production in lamina and midrib under a repeated-cut system. The same factorial design, harvest schedule, and low-input irrigation and fertilization regime were adopted at both experimental sites. The Sicilian site promoted cumulative leaf fresh (283 t ha⁻¹ FW) and dry biomass (30.7 t ha⁻¹ DW), whereas the Apulian crop showed higher TP yield per hectare (660 vs. 479 kg ha⁻¹ lamina; 118 vs. 73 kg ha⁻¹ midrib). Lower plant density increased biomass per plant, whereas D2 maximized i) biomass per hectare under site-specific Sicilian conditions (+51% on FW and DW basis) and ii) TP yield (772.0 kg ha⁻¹). Root colonization and qPCR results showed enhanced AMF colonization and greater Glomeraceae and Gigasporaceae target-DNA abundance in Myco+ plants, reflecting a sustained overall biostimulant treatment effect on the root-associated AMF community. Nevertheless, the Myco+ effect on biomass yield was positive only in Apulia (30.3 vs 23.8 t ha⁻¹ DW). However, Myco+ offset the negative impact of D1 on TP yield. Overall, plant density was the main agronomic driver of TP yield, whereas the response to the AMF-based biostimulant depended on site and plant density.