Abstract: Fluorescence induction kinetics was used to investigate the relationship between electrolyte leakage and light energy dissipation of PSII in the leaves of Puccinellia tenuiflora seedlings under Na₂CO₃ stress. The maximal photochemical efficiency of PSII (Fv/Fm) and potential photochemical efficiency (Fv/Fo) decreased as electrolyte leakage increased. Actual photochemical efficiency of PSII in the light (ΦPSII), electron transfer rate (ETR), the photochemical quenching coefficient(qP), photochemical efficiency (PR), heat dissipation (HD) and heat dissipation rate (HDR) increased when electrolyte leakage was less than 0.2. However, ΦPSII, ETR, qP and PR decreased when electrolyte leakage was more than 0.2. The dividing line for effectual control of P. tenuiflora appeared to be an electrolyte leakage of 0.2. The non-photochemical quenching coefficient (qNP) increased when electrolyte leakage was less than 0.15, while qNP decreased when electrolyte leak was over 0.15. Thus the inflexion of qNP appeared to be an electrolyte leakage of 0.15, whereas that for ΦPSII, ETR, qP and PR was 0.2. This could explain the regulation ability of effectual heat dissipation when electrolyte leakage was less than 0.15 and why the ability of heat dissipation reduced when electrolyte leakage was over 0.15. The results showed that P. tenuiflora improved the function of PSII by increased heat dissipation and light energy absorbion by light-harvesting pigment, resulting in the augmentation of active oxygen elimination by antioxidant enzyme systems when electrolyte leak was in a certain range. However, when electrolyte leakage exceeded this range, the functioning of PSII was inhibited or the PSII system suffered from irreversible damage.