Effect of Fertilization, Irrigation and Microbial Biostimulant on the Antioxidant Profile of Some Sweet Pepper Genotypes

L.) are an important dietary source of antioxidants. Optimizing fruit antioxidant quality under reduced inputs is essential to valorize sustainable pepper production. Here, we evaluated seven Spanish genotypes (traditional/local, derived experimental hybrids and commercial hybrids) across six treatments combining two fertilization (100% and 50%) and irrigation (100% and 75%) regimes, with plant growth-promoting rhizobacteria (PGPR) applied under reduced fertilization treatments. Vitamin C and flavonoids were quantified by HPLC at the green-ripe and fully ripe stages, and carotenoids were determined spectrophotometrically at the fully ripe stage. Several genotypes largely maintained antioxidant content under stress treatments, whereas specific genotype × ripening stage combinations showed maximum increases in vitamin C (+102%), flavonoids (+86% for kaempferol) and carotenoids (+67% for yellow-orange carotenoids) under certain low-input treatments compared to the control. The PGPR effects on vitamin C and carotenoids were generally small, with occasional reductions. However, the PGPR increased total and some individual flavonoids by up to 96% (luteolin) in green-ripe Piquillo and 128% (quercetin) in fully ripe Isabel F1 fruits compared to the corresponding non-inoculated treatments. This multi-genotype, two ripening-stage evaluation identifies Spanish traditional germplasm and derived hybrids with stable or improved antioxidant profiles under low-input conditions and provides insight into PGPR effects. These results support the use of traditional genotypes in breeding for sustainable production.