Abstract <p>The effect of the presowing treatment of spring barley seeds with Mn, Zn, and Cu nanoparticles (NPs) on the morphophysiological parameters of plants under osmotic stress is studied. The experiment is conducted under controlled conditions. The effect of NPs on the growth, development, yield elements, and biochemical parameters of barley plants under various irrigation regimes in the tillering and earing phases is evaluated. The results show that the treatment of seeds with NPs has a complex effect on plants, depending on water availability and the phase of development. Under conditions of sufficient moisture, the NPs contribute to an increase in the number of stems and grains per ear by 20 and 16%, respectively, as well as development of the root system. However, when there is a shortage of moisture, the effect of NPs is more complex, manifesting itself in changes in plant height, the number of grains, and the grain weight per ear. Biochemical analysis reveals that under conditions of limited irrigation, the level of lipid peroxidation (LPO) increases relative to the control by 20 and 16%. The accumulation of polyphenols and flavonoids increases similarly (by 50.7 and 52.3%, respectively). The presowing treatment of seeds with NPs contributes to a change in these indicators, which affects the antioxidant protection of plants under stress. Thus, the presowing treatment of spring barley seeds with Mn, Zn, and Cu NPs can have a positive effect on plant growth and productivity under conditions of both sufficient and limited moisture. However, the effect of NPs depends on the phase of plant development and the degree of water deficiency, which underscores the need for further research to optimize the use of nanopreparations in agriculture. The observed effect shows that this method is promising for increasing the drought resistance and yield of crops. Further research is needed to develop optimal application schemes based on agroclimatic conditions.</p>