The availability of water and nitrogen in the soil affect the metabolism of onion bulbs. The synthesis of metabolites and bioactive compounds are the most affected, along with the quality of the onion bulbs However, it is necessary to know the effects of different water levels and nitrogen fertilization to optimize the quality of the onion. The objective of this research was to study the effects of the different conditions of hydric stress and nitrogen fertilization during the development of onion (Allium cepa L.) crop, regarding its physicochemical and bioactive properties. Onions were grown using four available irrigation regimes (25, 50, 75 and 100%) and four doses of nitrogen fertilization (100, 150, 200 and 250 kg N ha⁻¹). Onion without any treatment was considered as a control. The treatments low in irrigation and nitrogen fertilization increased the pH level (5.7 to 5.9) and bulb coloration in bright white/yellowish tones. An increase was observed compared to control in titratable acidity (0.13%) just in the nitrogen content, ascorbic acid (46%) and antioxidant capacity with DPPH (12.3%) and ABTS (93.7%). A decrease was shown in soluble solids (14.6%), firmness (3.5 kg cm⁻²), dry matter (6.6%), total phenols (50%) and FRAP (33.2%) values. Pyruvic acid remained constant (1.5 µmol g⁻¹ FW). The onion bulb extracts showed an erythroprotective effect with a hemolysis inhibition percentage higher than 95%. Finally, the onions had low pungency, and were soft and extra sweet. The treatments with 25% usable humidity and nitrogen fertilization of 150 and 250 kg ha⁻¹, favored the physical, chemical and bioactive quality of the onion bulb.

Phycobiostimulants derived from macroalgae, microalgae, and cyanobacteria are increasingly used to improve crop performance. However, their effects vary across products, crops, and environments. In this review, we propose a biomass–process–composition–mechanism–efficacy (BPCME) framework to systematically trace this variability. Biomass provenance, including taxonomic identity, cultivation conditions, physiological state, and harvest timing, defines the initial biochemical potential, whereas processing and formulation determine the molecular profile delivered to the plant. The current findings indicate that phycobiostimulants act mainly by modulating endogenous signaling networks, hormone crosstalk, redox regulation, metabolic reprogramming, and rhizosphere and plant–microbiome responses rather than by direct nutrient or hormone replacement. Agronomic performance is further moderated by the genotype, dose, application route, developmental stage, soil conditions, and environmental stress. To improve reproducibility, we propose a phycobiostimulant quality-by-design (QbD) framework that integrates candidate critical biomass attributes and process parameters, chemical fingerprints, biological potency, and batch equivalency. Cascading algal biorefineries and precision application strategies can improve resource efficiency while helping to define the conditions for reliable field performance. Progress towards precision phycobiostimulants will require the experimental linking of product identity and mechanism to reproducible potency across batches and agronomic environments.