Severe water shortage limits horticultural crop growth and development, thereby compromising plant quality. Novel tools to enhance stress tolerance in medicinal horticultural crops are crucial to cope with growing environmental challenges to world crop performance. In this study, water solutions of robinin (25, 100, and 200 ppm) and/or foliar sprays of chitosan (0, 50, and 200 ppm) were applied to Chrysanthemum morifolium Ramat subjected to a 2 (2DWI) or 6 day (6DWI) irrigation intervals for 6 weeks. Morphological, physiological, and genetic markers associated with plant-response mechanisms to water stress were explored. Robinin + chitosan-treated plants showed increased morphological performance associated with enhanced chlorophyll, carbohydrates, proline, K+, Ca+2, phenols, leaf water potential, antioxidants, and leaf water content. Superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) enzymes were more active in robinin + chitosan-treated plants, while H2O2 accumulation was diminished. Higher expression levels of the Chrysanthemum antioxidant gene of zinc-finger transcription factor gene (Cm-BBX24), Chrysanthemum roots fu (DREB1A-1), Chrysanthemum heat shock protein CgHSP70, pyrroline-5-carboxylate synthetases (P5CS), pyrroline-5-carboxylate reductase (P5CR), and proline dehydrogenase (ProDH) were found in robinin- and chitosan-treated plants. Robinin + chitosan treatment stimulated the accumulation of carbohydrates, K+, Ca+2, proline, and chlorophylls to achieve osmotic adjustment and maintain turgor pressure. Accumulation of reactive oxygen species was controlled by enzymatic and non-enzymatic means, as well as the overexpression of stress-related genes (Cm-BBX24, DREB1A-1, CgHSP70, P5CS, P5CR, and ProDH) in robinin + chitosan-treated plants. Plant-response mechanisms for enhanced drought resistance interacted under robinin + chitosan treatment to improve plant performance under stress conditions.
