Balancing Osmotic Protection and Oxidative Stress: Physiological and Biochemical Responses of Pot Marigold (Calendula officinalis L.) Plants to Water Stress

Water deficit is a widespread environmental constraint that disrupts plant metabolism, impairs growth, and compromises ornamental value. In this study, we examined the integrated morpho-physiological and biochemical responses of <i>Calendula</i> <i>officinalis</i> L. to moderate (MWS; 60% field capacity) and severe (SWS; 35% field capacity) drought, compared with well-watered plants, over a three-week period under controlled conditions. Drought stress triggered pronounced reductions in vegetative growth: leaf number decreased by 33.1% under MWS and 51.0% under SWS, and leaf length declined by 34.7% and 42.7%, respectively. Fresh and dry biomass decreased significantly, especially under SWS where it was accompanied by a decrease in leaf water loss capacity. Non-enzymatic antioxidant responses included a decline in carotenoid content and strong osmolyte accumulation, with proline increasing under SWS. Indicators of oxidative damage, hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) also rose, but only under SWS. In parallel, the enzymatic antioxidant system (catalase, peroxidase and superoxide dismutase) was significantly activated under drought. Our results demonstrate that <i>C. officinalis</i> uses a dual drought response, combining osmotic adjustment with an upregulation of antioxidant defenses to limit oxidative stress. However, under prolonged severe drought, these mechanisms are insufficient to prevent biomass loss, underscoring its vulnerability in water-limited environments.