Exogenous Melatonin Application Enhances Growth and Floral Traits of Zinnia elegans Under Drought Stress

Zinnia (Zinnia elegans) is a widely cultivated ornamental plant whose growth and floral traits can be compromised by abiotic stresses, especially water deficit. Melatonin (MEL) has stood out as a plant growth regulator with antioxidant potential, capable of mitigating the adverse effects of water stress. This study aimed to evaluate the effects of foliar MEL application on the growth and floral characteristics of Z. elegans under different water regimes. The experiment was carried out in a greenhouse using a randomized block design in a 4 × 2 factorial scheme with five replications. The first factor consisted of four water conditions: 80% of field capacity (FC) (no stress), 20% of field capacity (severe stress), early water restriction (20% of FC followed by 80% of FC), and late water restriction (80% of FC followed by 20% of FC). The second factor corresponded to the foliar application of MEL at two concentrations (0.0 and 1.0 mM). Growth variables (plant height, stem diameter, number of leaves, leaf area, and dry mass of different organs) and floral characteristics (number of petals, area, perimeter, and diameter) were evaluated. Water deficit, especially under severe stress (20% FC), significantly reduced plant growth and floral traits, decreasing the total dry mass by 60.27% and total floral area by 47.57% compared to the control. However, the application of 1.0 mM MEL attenuated the deleterious effects of water deficit, increasing total dry mass by 50.26% and total floral area by 25.56% under severe stress (20% FC) compared to untreated plants, making it a promising strategy for zinnia production in environments with limited water availability.