Zinnia elegans L. (Asteraceae) is a warm-season ornamental species that can be susceptible to low-temperature stress during early growth stages. This study evaluated the effects of exogenous proline on the morphological, physiological, and biochemical responses of Z. elegans seedlings to chilling conditions. A factorial experiment based on a completely randomized design was conducted using foliar proline applications at 0, 5, and 10 mM combined with two temperature regimes. At the four-true-leaf stage, seedlings assigned to the chilling treatment were exposed to 3 ± 1 °C for 7 h per night for five consecutive days, while control seedlings remained under normal greenhouse conditions. Chilling significantly decreased plant height, root volume, root dry weight, relative leaf water content, and photosynthetic performance, while increasing oxidative damage and the activities of several stress-related enzymes, including peroxidase, catalase, polyphenol oxidase, γ-aminobutyric acid transaminase, and phospholipase D. Chilling and proline application also altered total phenolic content, whereas PAL activity was not significantly affected. Exogenous proline generally alleviated several adverse effects associated with chilling, improving vegetative growth, water status, root development, photosynthetic performance, and antioxidant responses. The 10 mM treatment frequently showed the most favorable responses, although its superiority was not consistent across all measured traits. Overall, exogenous proline was associated with improved post-chilling recovery status of Z. elegans seedlings through coordinated changes in water status, photosynthetic performance, oxidative stress responses, antioxidant defense, and stress-related metabolism.
