Acid rain is leading to soil potassium (K) loss. A hydroponic experiment was conducted to determine the growth and physiological and stoichiometrical traits of Mosla hangchowensis (an endangered plant) and M. scabra (a weed) in response to three K concentrations. For M. hangchowensis, compared with standard Knop's solution (K₃), low K (K₀) induced reductions in net photosynthetic rate, soluble protein content, superoxide dismutase activity, total biomass, tissue K concentration, and leaf nitrogen (N) concentration and induced increases in malondialdehyde content and tissue sodium (Na) concentration (P < 0.05). In contrast, none of these factors were affected by K₀ for M. scabra. Critical values of N/K and K/P (phosphorus) ratios in shoots indicated that limitation in K uptake occurred for M. hangchowensis but not M. scabra under K₀. The continuing decline of K content due to acid rain may worsen the situation of M. hangchowensis, whereas M. scabra can maintain vigorous growth and develop well in the future.