Various strategies are needed to reduce the negative impacts of drought stress on plant productivity. A study was conducted to investigate the effects of using seaweed extract (SE) and phosphate-solubilizing bacteria (PSB) biofertilizers, either alone or in combination, on the morphophysiological and biochemical traits, as well as the essential oil content of Calendula officinalis under drought stress. The study used a randomized complete block design with three replications, and each replication included five plants. It was designed as a factorial experiment. The treatments included 1% SE, 4% PSB, and the combination of both (CB) under three levels of drought stress, which were 100%, 70%, and 40% of the field capacity. The study found that the CB treatment at 40% of field capacity resulted in the highest increase in relative water content (39.80%), chlorophyll a (45.19%), chlorophyll b (46.62%), total chlorophyll (45.49%), flavonoids (13.33%), flavonols (4.35%), proline (6.87%), catalase (127.27%), and ascorbate peroxidase enzyme activity (37.93%) when compared with the control group. The CB treatment at 100% and 70% field capacity showed the highest improvement in essential oil content, with an increase of 118.18% and 116.25%, respectively, compared to the control. All treatments resulted in significant differences in carotenoid, anthocyanin, lignin, and soluble protein content. However, the CB treatment at 100% field capacity showed the highest distinction, achieving 71.83%, 159.23%, 223.63%, and 200.13% promotion, respectively, compared to the control. Although morpho-physiological and biochemical traits and essential oil content were decreased with increasing drought severity, using CB biofertilizer mitigated the adverse effects of drought stress and proved to be the most effective strategy for the sustainable agriculture of C. officinalis.
