Abstract: Effects of KNO₃ under salt stress on the biomass and photosynthetic capacity of two varieties of Helianthus tuberosus seedlings (Nanyu No.1 and Qingyu No.2) treated with NaCl (0 and 150 mmol/L) and different levels of KNO₃ (2.5, 5, 10, 20 and 30 mmol/L) were studied by pot/sand experiments in a greenhouse. 1) Compared with the control (NaCl 0 mmol/L), the biomass, chlorophyll (Chl) content, and net photosynthetic rate (Pn) of two varieties of H. tuberosus seedlings declined as the KNO₃ concentrations increased. 2) Under 150 mmol/L NaCl stress, the K⁺/Na⁺ (shoot and root) ratio and chlorophyll content of different varieties of H. tuberosus seedlings increased with increasing concentrations of KNO₃. 3) Sensitivities of KNO₃ for alleviating salt injury differed between varieties. Effects of KNO₃ (20 and 10 mmol/L) significantly alleviated salt injury caused by 150 mmol/L NaCl stress for Nanyu No.1 and Qingyu No.2. 4) Under the same treatment, KNO₃ alleviated injury caused by 150 mmol/L NaCl stress better in Nanyu No.1 than in Qingyu No.2. In conclusion, suitable exogenous KNO₃ can alleviate salt injury and promote photosynthetic capacity and biomass accumulation in varieties of H. tuberosus seedlings.