Integrated Nutrient Management Boosts Inflorescence Biomass and Antioxidant Profile of Carlina diae (Asteraceae)—An Endangered Local Endemic Plant of Crete with Medicinal and Ornamental Value

Due to the combined climate and biodiversity crisis, the sustainable utilization of phytogenetic resources stands as a one-way alternative, while nutrient management strategies are gaining an increasing role in agriculture. Building on previous studies regarding the Endangered local endemic of Crete (Greece) Carlina diae (Asteraceae), with medicinal and ornamental value, this investigation focused on its pilot cultivation and fertilization (foliar or soil application). Foliar application comprised inorganic fertilization (conventional) or integrated nutrient management (INM). Soil application consisted of conventional inorganic fertilizers, biostimulants, or INM with biostimulants. Above-ground biomass content of nutrients, leaf chlorophyll fluorescence, and color parameters (SPAD meter, DA meter, Chroma Meter) were estimated. The leaf chlorophyll content, three key antioxidant compounds, and nutrient titers were also determined. The fertilization scheme did not influence plant growth and visually perceived quality (leaf color and shape). Notably, foliar INM fertilization increased biomass partitioning to inflorescences (harvestable organs for either medicinal or ornamental purposes) and decreased tissue water content (facilitating processing). Considering all three antioxidants together, INM with biostimulant appeared the optimum scheme, being associated with the highest (carotenoids, phenolics) or the second highest (flavonoid) content. In C. diae , therefore, INM fertilization was optimal for upgrading yield (foliar) and herbal quality in terms of antioxidant profile (INM with biostimulant), which might be embraced as an eco-friendly approach for high-quality yields.