Abstract: To investigate the effects of calmodulin (CaM) antagonists on potassium accumulation and photosynthetic characteristics of seedlings under low potassium stress and the mechanisms involved, two potassium levels and two calmodulin antagonists (chlorpromazine and trifluoperazine) treatments were set up in an indoor hydroponic culture experiments using tobacco variety K326 as the test material, and the leaf calcium ion content, calmodulin content, potassium content in various parts of the plant, relative expression of genes related to root potassium uptake, physiological characteristics were measured. The plant physiological characteristics, leaf antioxidant enzyme activities and photosynthetic characteristics were determined. The results showed that the potassium content, leaf antioxidant enzyme activity, chlorophyll content and photosynthetic characteristics (except water vapour pressure saturation deficit) of all parts of the plant were lower under low K stress compared to normal potassium levels, in which the above-ground and root potassium contents of the plant were significantly reduced by 23.80% and 55.50%, respectively. The relative expression of NKT2, NtKC1 and NtHAK1 genes in the root system was increased by 4.05, 2.98 and 12.74 times, respectively. The potassium content of all plant parts, antioxidant enzyme activity, chlorophyll content and six indicators of photosynthetic characteristics (except water vapour pressure saturation deficit) were significantly reduced after the application of calmodulin antagonists at two potassium levels. Under low potassium stress, leaf calmodulin content, potassium content in all parts of the plant, root potassium uptake related gene expression, total chlorophyll content and photosynthetic characteristics (except water vapour saturation deficit) were significantly lower in the calmodulin antagonist treatments compared to the non-calmodulin antagonist treatment, with an average reduction of 23.69% and 60.00% in above-ground and root potassium content. The relative expression of NKT2, NtKC1 and NtHAK1 genes in the root system was reduced by 82.84%, 94.18% and 52.44% on average. The addition of calmodulin antagonists under low potassium stress can inhibit calmodulin content, block the formation of Ca²⁺-CaM complex, and affect signalling, resulting in reduced antioxidant enzyme activity and photosynthesis in leaves, reduced expression of K⁺ absorpt-related genes in roots, and inhibited K+ absorption and accumulation in plants.