Chitosan and Microalgae Nanoparticles: Synergistic Role in Enhancing Drought Stress Tolerance in Wheat Seedlings

Drought stress is one of the most severe abiotic constraints limiting wheat productivity worldwide, particularly during early developmental stages that determine crop establishment and yield potential. Sustainable, biologically based strategies that enhance drought tolerance without environmental cost are therefore urgently needed. In this study, we evaluated the individual and combined effects of chitosan (Cs), microalgae (Ma) (<i>Nostoc linckia</i>, MACC-612), and a chitosan–microalgae nanoparticle formulation (Cs-Ma) on germination performance, early seedling growth, and molecular stress responses in two wheat (Mehregan and MV Nádor) cultivars with contrasting drought sensitivity under polyethylene glycol (PEG)-induced osmotic stress (−2 and −4 MPa). Drought stress significantly reduced germination percentage, germination rate, and radicle and coleoptile development in both cultivars, especially at −4 MPa. Application of Cs and microalgae individually partially alleviated these negative effects; however, the combined Cs-Ma treatment consistently produced the strongest improvements in seedling vigor and biomass accumulation under both moderate and severe drought stress. Evaluation of drought tolerance using tolerance index (TOL), stress tolerance index (STI), and stress intensity (SI) demonstrated that Cs-Ma markedly increased STI and reduced SI across most germination traits, indicating enhanced drought tolerance and lower stress sensitivity, particularly in MV Nádor. These physiological responses were supported by transcriptional reprogramming in radicle tissues, including upregulation of genes involved in polyamine biosynthesis (<i>TaSPDS</i>, <i>TaSAMDC</i>), phenylpropanoid metabolism (<i>TaPAL</i>), and protein protection (<i>TaHSP70</i>), along with moderated induction of polyamine catabolism (<i>TaPXPAO</i>). Overall, the results reveal a synergistic interaction between chitosan nanoparticles and microalgae biomass, highlighting Cs-Ma as an effective, eco-friendly biostimulant for improving early-stage drought tolerance in wheat.