Effects of Growth Phases and Intensification of Light on Secondary Metabolites and Agro-Morphological Traits of the St. John’s Wort (Hypericum perforatum L.)

Light regime and growth phase are significant determinants of agro-morphological traits and secondary metabolite accumulation in plants. This study evaluated the effects of two light conditions on agro-morphological and phytochemical traits of two <i>Hypericum perforatum</i> genotypes (Topas and Mariana). Natural daylight and an extended 19 h photoperiod with supplemental white fluorescent light were tested at two growth phases (vegetative versus reproductive (flowering)), based on leaf sampling at the respective phases. Analysis of variance showed significant effects of growth phase, light treatment, and genotype on most traits, with growth phase exerting the most decisive influence (<i>p</i> < 0.01). Significant genotype × growth phase interactions were observed for most traits, whereas genotype × light interactions mainly affected phytochemical parameters. Leaf area, gland number, and gland area increased during the reproductive phase, especially in Topas, and were further enhanced under the 19 h photoperiod. Leaf area increased markedly during the reproductive phase, reaching 118.81 mm² in Topas under the 19 h photoperiod compared with 68.40 mm² under natural light. Prolonged light exposure increased hypericin, pseudo-hypericin, hyperforin, flavonoids, and total phenolics. The highest sum of hypericins (4.67 mg g^-1 DW), flavonoids (143.09 mg QE g^-1 DW), and phenolics (242.74 mg GA g^-1 DW) was observed in the Topas in the reproductive phase under the 19 h photoperiod, whereas hyperforin content peaked in vegetative Mariana (55.65 mg g^-1 DW). In contrast, the lowest sum of hypericins (1.80 mg g^-1 DW) occurred in vegetative Mariana under natural light, while the minimum flavonoids (74.45 mg QE g^-1 DW) and phenolics (133.22 mg GA g^-1 DW) contents were recorded in the vegetative Topas under natural light regim, and the lowest hyperforin concentration (19.65 mg g^-1 DW) was found in the Mariana genotype under natural light regime and in the reproductive phase. Principal component analysis associated PC1 with agro-morphological traits and hypericin-related metabolites, and PC2 with hyperforin and white gland traits. Heatmap and correlation analyses supported these patterns. Overall, extended photoperiod and growth phase are critical drivers of medicinal compound accumulation in <i>H. perforatum</i>.