Abstract: By hydroponic experiments, the K absorption properties and physio-biochemical characteristics of K-efficient tobacco genotypes grown under normal or reduced K supply conditions were studied. The results showed that biomass and K content of the K-efficient tobacco genotypes were largely higher than that of K-inefficient genotypes under different K levels, with the biggest difference in biomass between two genotypes being 2.56 times. The chlorophyll contents of the K-efficient genotypes showed no difference but were significantly higher than that of K-inefficient genotypes under different K levels. SOD, Nr and INV activities of the K-efficient genotypes were also higher than that of the K-inefficient genotypes. The increase of SOD and INV activities of K-inefficient genotype was more than that of K-efficient genotypes with the reduction of K supply, while Nr activity of K-inefficient genotype decreased less. Under different K levels, the root activity and H⁺ secreting ability in K-efficient genotype were also remarkably higher than that of K-inefficient genotypes. Along with the reduction of K supply, root activity in both genotypes were increased, while H⁺ secretion capacity decreased. The root absorb areas of different genotypes under the normal K level showed no significant difference. However, root absorb areas of K-efficient tobacco genotype were significantly higher than that of K-inefficient genotypes under lower K application rate. In summary, great differences exist in K absorbing properties and physio-biochemical characteristics between K-efficient and K-inefficient genotypes. K-efficient tobacco genotypes have stronger mechanisms of response to low K stress.