Developmental, Phytochemical and Enzymatic Changes in Pot Marigold (Calendula officinalis L.) cvs. Hybrid and French with Salicylic Acid (SA) and Polyamine Spermidine (SP) Foliar Spray

Marigolds (Calendula officinalis L.) are valuable in ornamentation, human food, and other uses; to enhance productivity, plant growth regulators produce stimulatory effects, including salicylic acid (SA) and spermidine (SP), but there is a lack of scientific evidence about such effects in marigolds. The study assessed, under greenhouse conditions, changes in physico-chemical parameters, enzymatic activity, and bioactive compounds of marigold cvs. Hybrid and French marigolds were sprayed of SA (1 and 2 mM) and SP (2 and 3 mM) and compared to control (pure water). The SA at 2 mM improved leaf length (8.20 cm), flower height and diameter (5.32, 8.28 cm), flower fresh and dry weight (14.30, 1.5 g), and the maximum number of flower petals (55) in ‘Hybrid’. Similarly, 2 mM SA gave the maximum number of leaves (40.71) and stem thickness (5.76 mm) in ‘French’, but 3 mM SP promoted the maximum plant height in ‘Hybrid’. Superoxide dismutase, peroxidase, and catalase activities increased in ‘Hybrid’ with 2 mM SA; with this SA dose, ‘Hybrid’ had higher contents of total phenolic compounds (68.34 mg GAE g−1), antioxidants (77%), carotenoids (110 mg 100 g−1), and flavonoids (67.5 mg RE g−1) than the control. The best dose for improving growth in both marigold varieties was 2 mM SA.