Chlorosis at leaf margins is a typical symptom of potassium(K)deficiency,but inappropriate application of K with other nutrients often masks symptoms of K deficiency.A two-year field experiment was con-ducted to measure the interactive effects of N and K on leaf photosynthesis and dry matter accumulation and the resulting growth dilution effect on K concentration and leaf K deficiency symptoms.N application aggravated the imbalance of N and K nutrients and further exacerbated K deficiency symptoms under K limitation.Synergistic effects of N and K promoted plant growth,amplified the growth dilution effect,and reduced the critical K concentration in leaves.Using 90％of the maximum shoot biomass as a threshold,the critical K concentration was 0.72％at the recommended N(N180)fertilization level.The critical K con-centration increased by 62.5％owing to the reduced biomass under insufficient N(N90)supply.In con-trast,high N(N270)reduced the critical K concentration(0.64％),accelerating chlorophyll decomposition and exacerbating K deficiency symptoms.The basis of changing the critical K concentra-tion by magnifying growth dilution effect was the functional synergistic effect of N and K on photosyn-thetic characteristics.Under insufficient N,the low maximum carboxylation rate(Vcmax)limited the net photosynthetic rate(An)and necessitated more K to maintain high CO2 transmission capacity,to improve the total conductance gtot/Vcmax ratio.High N supply increased gtot and Vcmax,possibly mitigating the effect of K reduction on photosynthesis.In conclusion,it is unwise to judge K status of plants only by K concentration without accounting for crop mass(or dilution effect),critical K concentration and defi-ciency symptoms are affected by N fertilization,and the synergistic effect of N and K on leaf photosyn-thesis is the foundation of maximal growth of plants under diverse critical K concentrations.