Coêlho, Ester dos SantosRibeiro, João Everthon da SilvaSilva, Elania Freire daLima, John Victor LucasGomes, Ingrid JustinoOliveira, Pablo Henrique de AlmeidaSilva, Antonio Gideilson Correia daFernandes, Bruno Caio ChavesRodrigues, Ana PaulaSilveira, Lindomar Maria daBarros Júnior, Aurélio Paes2026-02-052026-02-052025-11-072073-4395PURE: 152117102PURE UUID: 6f5d11e3-5300-460f-a8a6-4ddc8bad98e5Scopus: 105023070166WOS: 001623452000001http://hdl.handle.net/10362/200026Publisher Copyright: © 2025 by the authors.Water stress is the main abiotic factor that limits the development and commercial quality of ornamental plants, such as Zinnia elegans. This study aimed to evaluate the ability of exogenous melatonin (MEL) to attenuate the deleterious effects of water deficit by modulating stomatal physiological, biochemical and structural parameters. Z. elegans plants were subjected to four water regimes (80% FC, 20% FC, early stress and late stress) with (1.0 mM) and without MEL application. Severe water stress (20% FC) drastically reduced the rate of CO2 assimilation (A) by 43.81% and stomatal conductance (gs) by 68.96%. However, the application of MEL significantly mitigated this damage, resulting in an increase in A of 26.99% gs of 43.75%, and relative water content of 28% in plants under severe stress compared with those in untreated stressed plants. The mechanism of action of MEL involves the modulation of stomatal motion and, as suggested, the promotion of osmotic fit and the protection of membrane integrity and photochemical efficiency. Exogenous melatonin acts as an effective bioregulator, improving the tolerance of Z. elegans to water deficit and sustaining its physiological performance and ornamental potential under stress conditions.246375834engBiostimulantGas exchangeOrnamental plantOsmotic adjustmentPhotochemical efficiencyWater stressAgronomy and Crop Sciencejournal article10.3390/agronomy15112571https://www.scopus.com/pages/publications/105023070166https://www.webofscience.com/wos/woscc/full-record/WOS:001623452000001