Enzymes of Calendula officinalis L. as Affected by Foliar Application of Nano-nitrogen and Potassium Fertilizers, Under Water Stress Conditions

Water stress is a major environmental factor that limits the growth and productivity of Calendula officinalis L. To alleviate its negative effects, recent approaches have increasingly focused on nano-fertilizers that enhance plant antioxidant defenses. This study therefore aimed to evaluate the effect of foliar application of nano-nitrogen (0, 2, and 4 mL L⁻¹) and nano-potassium (0, 2, and 4 g L⁻¹) fertilizers under two irrigation regimes (100% and 50% of field capacity) on the activity of key antioxidant enzymes, including catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD). The results revealed that irrigation at 50% field capacity significantly increased CAT, SOD, and POD activities compared with full irrigation. Enzyme activities decreased with increasing concentrations of nano-nitrogen and potassium fertilizers. The highest activities of all tested enzymes were observed under water stress without foliar application of nano-fertilizers (0 mL L⁻¹ nano-N + 0 g L⁻¹ nano-K + 50% irrigation), where CAT, SOD, and POD reached 0.42, 3.00, and 13.61 units mg⁻¹ fresh weight, respectively. In contrast, the lowest enzyme activities were observed under full irrigation combined with the highest fertilizer levels (4 mL L⁻¹ nano-N + 4 g L⁻¹ nano-K + 100% irrigation), with CAT, SOD, and POD values of 0.31, 2.15, and 6.92 units mg⁻¹ fresh weight, respectively. In conclusion, water stress enhanced the antioxidant defenses of Calendula officinalis L., while higher levels of nano-fertilizer slowed down enzyme activity.